Easily adhesive polyester film for foldable display

Abstract

To provide an easily adhesive polyester film used in a foldable display, and an optical film and an image display device using the same.SOLUTION: An easily adhesive polyester film for a foldable display is used in an image display device and is foldable, wherein a base material possessed by the easily adhesive polyester film is composed of a polyester film, has a resin layer A on a first surface side of the base material and a resin layer B on a second surface side opposite to the first surface in the base material, film thickness of the resin layer B is less than 30 nm, and the resin layer B is formed of a composition containing a polyester resin having a condensed polycyclic aromatic group and an isocyanate crosslinking agent.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an easily adhesive polyester film used for a foldable display, and to an optical film and an image display device using the same. [Background technology]

[0002] Image display devices such as smartphones and tablet terminals have been known for some time, and currently, foldable image display devices are being developed. Smartphones, tablet terminals, and the like are usually covered with a cover glass. Glass is generally hard but not bendable, so if a cover glass is used for an image display device, there is a high risk of it breaking when attempting to fold it. For this reason, the use of a foldable optical film including a polyester film as a bendable substrate, a hard coat layer, and an adhesive layer instead of a cover glass for a foldable image display device is being considered (see, for example, Patent Document 1).

[0003] In such foldable optical films, a polyester film is generally selected as a substrate that can be bent, and in order to improve the adhesion between the polyester film substrate and the hard coat layer and adhesive, a coating layer having high adhesive properties is often provided on the surface of the polyester film as an intermediate layer between them.

[0004] The hard coat film is required to have durability against temperature, humidity, light, etc., transparency, chemical resistance, scratch resistance, stain resistance, etc. Furthermore, since it is often used on the surface of a display or decorative material, visibility or design is required. Therefore, in order to suppress glare and iridescent colors caused by reflected light when viewed from any angle, it is common to provide an antireflection layer with a multilayer structure in which a high refractive index layer and a low refractive index layer are alternately laminated on top of the hard coat layer.

[0005] In recent years, hard coat layers with various compositions have been developed, and the adhesion between the substrate and the hard coat layer has been discussed depending on the composition. For example, when a film laminated with a hard coat layer is used in a liquid crystal television display, not only initial adhesion immediately after lamination but also performance such as resistance to humidity and heat and adhesion retention over time are required to ensure high reliability for long-term use.

[0006] The same is true for the adhesives, which are required to have durability against temperature, humidity, light, etc., transparency, chemical resistance, scratch resistance, stain resistance, etc. Furthermore, since they are often used on the surfaces of displays and decorative materials, visibility or design is also required. In recent years, adhesives with various compositions have been developed, and the adhesion between the substrate and the adhesive has been discussed depending on the composition. For example, when members bonded together with an adhesive are used in a liquid crystal television display, in order to ensure high reliability for long-term use, not only initial adhesion immediately after lamination but also performance such as resistance to humidity and heat and adhesion retention over time are required.

[0007] Furthermore, there is concern that the force applied by the impact and deformation caused by folding may reduce adhesion to hard coat layers and adhesives, and therefore, the performance required is not only to ensure adhesion in the free state as described above, but also to ensure adhesion in the harsh environment caused by folding. By providing a resin layer with high adhesive properties on both sides of the polyester film, adhesion to the hard coat layer and adhesive layer when constructing the optical film is improved, thereby improving the adhesion reliability of the optical film.

[0008] In the field of conventional highly adhesive polyester films, it has been reported that laminated polyester films having a coating layer containing a polyester resin using a naphthalenedicarboxylic acid component on at least one side of the polyester film have excellent adhesion to surface functional layers such as hard coat layers (see, for example, Patent Document 2).

[0009] Furthermore, it has been reported that an easily adhesive polyester film having a coating layer containing a polyurethane resin containing an aliphatic polycarbonate polyol, which has excellent flexibility and adhesion, on at least one side of the polyester film, has excellent adhesion to an optical functional layer under high temperature and high humidity conditions (see, for example, Patent Document 3). However, although adhesion has been confirmed in all of these, the adhesion to the hard coat layer has been taken into consideration, and no easily adhesive polyester film has been obtained that takes into consideration adhesion to the adhesive layer. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-125063 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-246663 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-168053 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention was made in response to the problems of the prior art. Specifically, an object of the present invention is to provide a foldable optical film having excellent optical properties, and an image display device including the same. The present invention aims to provide an easily adhesive polyester film having excellent adhesion not only to a hard coat layer but also to an adhesive, and to provide an easily adhesive polyester film for use in a foldable display. Another object of the present invention is to provide a foldable optical film having excellent optical properties obtained by using the film, and an image display device including the same. [Means for solving the problem]

[0012] The present inventors have conducted extensive research to achieve the above object and have arrived at the present invention. That is, the present invention has the following configuration.

[0013] [1] A foldable, easily adhesive polyester film for a foldable display used in an image display device, The substrate of the highly adhesive polyester film is made of a polyester film, a resin layer A on the first surface side of the substrate; a resin layer B on a second surface side of the base material opposite to the first surface, The resin layer B has a thickness of less than 30 nm, The resin layer B is formed from a composition containing a polyester resin having a condensed polycyclic aromatic group and an isocyanate crosslinking agent. [2] The highly adhesive polyester film for a foldable display according to [1], wherein the polyester resin contained in the resin layer B has a naphthalene skeleton in the molecule. [3] The highly adhesive polyester film for a foldable display according to [1] or [2], wherein the isocyanate crosslinking agent contained in the resin layer B is a pyrazole-type blocked isocyanate. [4] The highly adhesive polyester film for a foldable display according to any one of [1] to [3], wherein the resin layer A has a thickness of 10 nm or more and 200 nm or less. [5] The foldable display device has the easy-adhesion polyester film for foldable displays according to any one of [1] to [4] above, a functional layer provided on the resin layer A on the side opposite to the substrate; a pressure-sensitive adhesive layer provided on the opposite side of the resin layer B from the substrate, The optical film is characterized in that no cracks or breaks occur when a test in which the optical film is folded 180° with the functional layer facing inward and the distance between opposing sides of the optical film is 10 mm is repeated 10,000 times. Optical film. [6] [5] The optical film described in [5], wherein the optical film does not crack or break when subjected to a test in which the optical film is folded 180° so that the functional layer is on the outside and the distance between opposing sides of the optical film is 30 mm, repeated 10,000 times. [7] The optical film according to [5] or [6], which has a yellow index of 15 or less. [8] The method for producing an optical film according to any one of [5] to [7] above, and producing a functional layer on the first surface side of the polyester film as the substrate by a coating process. A method for manufacturing an optical film. [9] A foldable image display device comprising: a display element; and the optical film according to any one of [5] to [7], which is placed closer to a viewer than the display element.

[10] The image display device according to [9], wherein the display element is an organic light-emitting diode element. [Effects of the Invention]

[0014] According to one aspect of the present invention, there is provided a foldable optical film obtained by unfolding an easily adhesive polyester film having excellent optical properties and excellent adhesion. According to another aspect of the present invention, there is provided an image display device including such an optical film. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention provides an easily adhesive polyester film for a foldable display that can be used in an image display device, The substrate of the highly adhesive polyester film is made of a polyester film, a resin layer A on the first surface side of the substrate; a resin layer B on a second surface side of the base material opposite to the first surface, The resin layer B has a thickness of less than 30 nm, The resin layer B provides an easily adhesive polyester film for a foldable display, which is formed from a composition containing a polyester resin having a condensed polycyclic aromatic group and an isocyanate crosslinking agent.

[0016] The term "display" as used herein refers to a general display device, and examples of the display include LCD, organic EL display, inorganic EL display, LED, and FED. However, LCD, organic EL, and inorganic EL having a bendable structure are preferred. In particular, organic EL and inorganic EL, which can reduce the number of layers, are more preferred, and organic EL, which has a wide color gamut, is even more preferred.

[0017] The foldable display referred to in the present invention is a single continuous display that can be folded in half or the like when carried. By folding, the size can be reduced, improving portability and enabling a thinner display when folded. The bending radius when folding refers to the radius of the inside of the folded portion. It can be said that the smaller the bending radius, the better, but the smaller the bending radius, the more likely it is that fold marks will be left, and problems such as cracking of functional layers such as laminated hard coats can occur. The surface protection film described below may be located on the outside or inside of the folded foldable display. Furthermore, the foldable display may be a three-fold or four-fold type, or may be a rollable type, all of which are considered to fall within the scope of the foldable display of the present invention.

[0018] In another embodiment, the present invention may be an easy-adhesion polyester film for a foldable display, in which a resin layer having easy adhesion is provided on one side of a polyester film and a functional layer, such as a hard coat, is provided thereon. It is applied to a foldable display. It may be used in any part as long as it is a component 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 film of the present invention can be used are described.

[0019] The essential component of a foldable organic EL display is an organic EL module, but if necessary, a circular polarizer, a touch panel module, a surface protective film, a back protective film, etc. may also be provided.

[0020] The general structure of an organic EL module is composed of an electrode, an electron transport layer, an emitting layer, a hole transport layer, and a transparent electrode.

[0021] It is preferable that the mobile terminal device has a touch panel. When an organic EL display is used, it is preferable that a touch panel module is disposed above the organic EL display or between the organic EL module and the circular polarizer. The touch panel module has a transparent substrate such as a film and a transparent electrode disposed thereon. The laminated polyester film obtained by adding a functional layer to the highly adhesive polyester film for a foldable display 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.

[0022] Circular polarizers prevent deterioration of image quality due to reflection of external light by components inside the display. Circular polarizers have a linear polarizer and a retardation plate. The linear polarizer has a protective film on at least the viewing side of the polarizer. A protective film may also be provided on the side opposite the viewing side of the polarizer, or a retardation plate may be directly laminated on the polarizer. The retardation plate may be a resin film having a retardation, such as a polycarbonate or cyclic olefin, or a resin film provided with a retardation layer made of a liquid crystal compound. The highly adhesive polyester film for foldable displays of the present invention can be used as a polarizer protective film. In these cases, the slow axis direction of the polyester film is preferably parallel or perpendicular to the absorption axis direction of the polarizer. A deviation of up to 10 degrees, preferably 5 degrees, from this parallel or perpendicular orientation is permitted.

[0023] When an impact is applied to a display from above, there is a risk that the circuits of an organic EL module or a touch panel module may be disconnected, so a surface protective film is often provided. The highly adhesive polyester film for a foldable display of the present invention is used as this surface protective film. Surface protective films include those called cover windows that are incorporated into the outermost surface of the display, and those called outers that can be attached, peeled off, and replaced by the user themselves. In either case, the highly adhesive polyester film for a foldable display of the present invention is used. The functional layer is provided on the surface of the foldable display with the functional layer on the viewing side. The functional layer may be provided on both sides.

[0024] It is also preferable that a protective film is provided on the back side of the display, and the highly adhesive polyester film for a foldable display of the present invention can be used as this protective film on the back side.

[0025] The highly adhesive polyester film for a foldable display of the present invention may be any other film than those described above, as long as it is used in a folded portion of a constituent member of a foldable display. Among these, the highly adhesive polyester film for a foldable display of the present invention is preferably used as a cover window surface protective film, an after-surface protective film, a base film for a touch panel module, or a back surface protective film, and more preferably as a cover window surface protective film or an after-surface protective film.

[0026] In addition, the polyester film of the present invention does not necessarily have to be used for all of the above-mentioned applications in the foldable display. In the foldable display, a film having a functional layer based on a polyester film, a polyimide film, a polyamide film, a polyamideimide film, a polycarbonate film, an acrylic film, a triacetyl cellulose film, a cycloolefin polymer film, a polyphenylene sulfide film, a polymethylpentene film, etc. can be used appropriately according to suitability.

[0027] Polyester film The polyester film used as the substrate in the highly adhesive polyester film of the present invention is a film mainly composed of polyester resin. Here, "a film mainly composed of polyester resin" means a film formed from a resin composition containing 50% by mass or more of polyester resin. When blended with other polymers (e.g., polycarbonate resin, polyimide resin, etc.), this means that the polyester resin is contained in an amount of 50% by mass or more, and when copolymerized with other monomers, this means that the polyester structural unit is contained in an amount of 50 mol% or more. Preferably, the polyester film contains 90% by mass or more of polyester resin, more preferably 95% by mass or more, and even more preferably 100% by mass.

[0028] The polyester resin material is not particularly limited, but a copolymer formed by polycondensation of a dicarboxylic acid component and a diol component, or a blend resin thereof can be used. Examples of dicarboxylic acid components constituting the polyester resin include terephthalic acid, isophthalic acid, orthophthalic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, diphenylcarboxylic acid, diphenoxyethanedicarboxylic acid, diphenylsulfonecarboxylic acid, anthracenedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2,2-dimethylglutaric acid, adipic acid, 2-methyladipic acid, trimethyladipic acid, pimelic acid, azelaic acid, dimer acid, sebacic acid, suberic acid, and dodecadicarboxylic acid.

[0029] Examples of diol components constituting the polyester resin include ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, decamethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexadiol, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone.

[0030] The dicarboxylic acid component and the diol component may each be used alone or in combination of two or more kinds. In addition, other polycarboxylic acid components such as trimellitic acid and other polyol components such as trimethylolpropane may also be added as appropriate.

[0031] Specific examples of polyester resins include polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. Among these, polyethylene terephthalate is preferred in terms of the balance between physical properties and cost. In addition, it is also a preferred embodiment to contain other copolymerization components or other polymers in order to control optical properties such as polarization. From the viewpoint of controlling the optical properties of the polyester film, preferred copolymerization components include diethylene glycol and copolymerization components having norbornene in the side chain.

[0032] The intrinsic viscosity of the polyester resin (solvent: phenol / tetrachloroethane=60:40) is, for example, 0.50 to 1.0 dl / g.

[0033] In order to improve the handling properties of polyester films, such as slipperiness and windability, inert particles can be incorporated into the film. Examples of inert particles include inorganic particles such as silica, kaolinite, talc, light calcium carbonate, heavy calcium carbonate, zeolite, alumina, barium sulfate, carbon black, zinc oxide, zinc sulfate, zinc carbonate, titanium dioxide, satin white, aluminum silicate, diatomaceous earth, calcium silicate, aluminum hydroxide, hydrated halloysite, magnesium carbonate, and magnesium hydroxide. The average particle diameter of the inert particles is, for example, 200 to 5000 nm, and preferably 250 to 4500 nm. This average particle diameter is measured using the method described in the Examples (number-based average particle diameter by SEM). In order to maintain high transparency, it is preferable that the content of inert particles in the film is as small as possible. Therefore, it is preferable to have a multi-layer structure in which particles are contained only in the surface layer of the film, or to have the film essentially free of particles and to contain fine particles only in a coating layer laminated on at least one side of the polyester film.

[0034] Here, "substantially free of particles" means, for example, in the case of inorganic particles, that the content of elements derived from the particles is 50 ppm or less, preferably 10 ppm or less, and most preferably below the detection limit when quantitatively analyzed by fluorescent X-ray analysis. This is because even if particles are not intentionally added to the base film, contaminants derived from foreign matter or dirt adhering to the raw material resin or the lines or equipment in the film manufacturing process may peel off and be inevitably mixed into the film.

[0035] Furthermore, when the polyester film has a multi-layer structure, it may have a two-type three-layer structure in which the inner layer does not substantially contain inert particles and only the outermost layer (second layer) contains inert particles, which is preferable because it can achieve both transparency and processability.

[0036] The polyester film serving as the substrate may be a single layer or a laminate of two or more layers. Furthermore, various additives may be incorporated into the film as needed, as long as the effects of the present invention are achieved. Examples of additives include antioxidants, light stabilizers, antigelling agents, organic wetting agents, antistatic agents, UV absorbers, surfactants, and the like. When the film has a laminated structure, it is also preferable to incorporate additives into each layer as needed, depending on the function of each layer. For example, adding a UV absorber or the like to an inner layer is a preferred embodiment to prevent photodegradation of the polarizer.

[0037] The polyester film can be produced by a conventional method. For example, it can be obtained by melt-extruding a material containing the above-mentioned polyester resin into a film shape, and then cooling and solidifying it on a casting drum to form a film. As the polyester film in the present invention, either a non-stretched film or a stretched film can be used, but a stretched film is preferred from the viewpoint of durability such as mechanical strength and chemical resistance.

[0038] When the polyester film is a stretched film, the stretching method is not particularly limited, and may be a longitudinal uniaxial stretching method, a transverse uniaxial stretching method, a longitudinal and transverse sequential biaxial stretching method, a longitudinal and transverse simultaneous biaxial stretching method, or the like. When stretching a polyester film, the stretching may be carried out before laminating an easy-adhesion coating layer described below, or may be carried out after laminating the easy-adhesion coating layer. It is also possible to uniaxially stretch the polyester film in the longitudinal or transverse direction before laminating the easy-adhesion coating layer, and then stretch the polyester film in the other direction after laminating the coating layer.

[0039] The polyester film has optical transparency. In this specification, "optical transparency" means the property of transmitting light, and includes, for example, a total light transmittance of 50% or more, preferably 70% or more, more preferably 80% or more, and particularly preferably 90% or more. Optical transparency does not necessarily mean transparency, and may also be translucent.

[0040] The thickness of the polyester film is preferably 10 μm or more and 100 μm or less. A polyester film thickness of 10 μm or more can suppress curling of the optical film, provide sufficient hardness, and furthermore, even when the optical film is produced by roll-to-roll processing, wrinkles are less likely to occur, resulting in no deterioration of the appearance. On the other hand, a polyester film thickness of 100 μm or less provides good folding performance of the optical film, satisfies the requirements of a continuous folding test, and is also preferable in terms of reducing the weight of the optical film. The thickness of the polyester film is determined by photographing a cross section of the polyester film using a scanning electron microscope (SEM), measuring the film thickness of the polyester film at 10 points on the image of the cross section, and calculating the arithmetic average of the film thicknesses at the 10 points. The lower limit of the polyester film 11 is more preferably 25 μm or more, and the upper limit of the polyester film is more preferably 80 μm or less.

[0041] The substrate of the highly adhesive polyester film is made of a polyester film, and has a resin layer A on a first surface side of the substrate, and a resin layer B on a second surface side of the substrate opposite to the first surface. The highly adhesive polyester film for a foldable display of the present invention may have a resin layer A having high adhesiveness on one surface of the polyester film as the substrate, and a resin layer B on the other surface thereof.

[0042] (Resin layer A) The highly adhesive polyester film for a foldable display of the present invention has a resin layer A on the first surface side of the substrate. For example, the resin layer A is a layer that improves adhesion between the substrate and the functional layer. The binder resin constituting the resin layer A is not particularly limited as long as it is a resin that has adhesiveness with the functional layer described below. Because the resin layer A is located between the polyester film (substrate) and the functional layer, it is necessary to consider adhesiveness with the polyester film, and therefore it preferably contains a polyester resin. By employing such a resin layer A, the adhesion between the polyester film substrate and the functional layer is dramatically improved.

[0043] The functional layer is obtained by UV curing through UV irradiation and has a cross-linked network structure. The polyester resins in the coating layer interact with each other, making them more compatible and entangled. Furthermore, it is believed that these entangled resins become entangled in the network structure of the functional layer formed on top of them, resulting in a coating layer with better adhesion than conventional coating layers. Furthermore, since the coating layer contains polyester resin, it is believed that adhesion to the polyester film substrate is also improved, resulting in a coating layer with better adhesion.

[0044] The thickness of the resin layer A is preferably 10 nm or more and 200 nm or less, for example, 30 nm or more and 180 nm or less. If the thickness of the resin layer A is 10 nm or more, sufficient adhesion with the functional layer applied thereon can be ensured, and if it is 200 nm or less, interference fringes can be further suppressed and foldability can be improved. The thickness of the resin layer A is more preferably 50 nm or more, for example, more preferably 150 nm or less.

[0045] The film thickness of resin layer A is determined by photographing a cross section of resin layer A using a scanning transmission electron microscope (STEM) or transmission electron microscope (TEM). The film thickness of resin layer A is measured at 20 locations on the cross section image, and the arithmetic mean value of the film thicknesses at those 20 locations is used. The specific method for photographing the cross section is described below. First, a 1 mm x 10 mm piece of optical film is embedded in an embedding resin to create a block. From this block, uniform, hole-free slices with a thickness of 70 nm to 100 nm are cut using a standard sectioning method. An "Ultramicrotome EM UC7" (Leica Microsystems) or similar instrument can be used to prepare the slices. These uniform, hole-free slices are used as the measurement sample. Then, a cross-sectional photograph of the measurement sample is taken using a scanning transmission electron microscope (STEM) (product name "S-4800" manufactured by Hitachi High-Technologies Corporation). When taking cross-sectional photographs using the S-4800, the detector is set to "TE," the acceleration voltage to "30 kV," and the emission current to "10 μA" for cross-sectional observation. The magnification is adjusted appropriately between 5,000x and 200,000x while adjusting the focus and observing whether the contrast and brightness of each layer can be distinguished. The preferred magnification is 10,000x to 100,000x, more preferably 10,000x to 50,000x, and most preferably 25,000x to 50,000x. When taking cross-sectional photographs using the S-4800, the aperture may be set to "beam monitor aperture 3," the objective lens aperture to "3," and the working distance to "8 mm." When observing the cross-section, it is important to be able to observe the interfacial contrast between the functional layer and other layers (e.g., the polyester film substrate) as clearly as possible. If the interface is difficult to see due to insufficient contrast, dyeing with osmium tetroxide, ruthenium tetroxide, phosphotungstic acid, or the like can be performed, as this makes the interface between the organic layers easier to see. Furthermore, the contrast of the interface may be more difficult to see at higher magnifications. In such cases, observations at lower magnifications are also performed. For example, observations are performed at two magnifications, high and low, such as 25,000x and 50,000x, or 50,000x and 100,000x, and the arithmetic mean value described above is calculated at both magnifications. This mean value is then used as the film thickness value of the resin layer.

[0046] The highly adhesive polyester film for a foldable display of the present invention has a resin layer A on one surface of a polyester film substrate. The resin layer A may be, for example, a layer for suppressing the occurrence of interference fringes. In this embodiment, the resin layer A may have both the above-mentioned improved adhesion and the suppression of the occurrence of interference fringes. From the viewpoint of suppressing the occurrence of interference fringes, the refractive index of the resin layer A is preferably lower than the refractive index of the polyester film used as the substrate and higher than the refractive index of the functional layer.

[0047] The difference in refractive index between the resin layer A and the functional layer (for example, the refractive index of the resin layer A minus the refractive index of the functional layer) is The refractive index difference is preferably 0.005 or more and 0.100 or less. If this refractive index difference is 0.005 or more, interfacial reflection occurs between the resin layer A and the functional layer, but interference fringes can be made invisible, and if it is 0.100 or less, interference fringes can be slightly observed, but can be made invisible enough to cause no problems in practical use. The lower limit of this refractive index difference is more preferably 0.007 or more, and the upper limit is more preferably 0.090 or less.

[0048] The resin layer A may be composed solely of resin, but preferably contains a binder resin and particles for adjusting the refractive index. The binder resin of the resin layer A is preferably at least one resin selected from the group consisting of (meth)acrylic resins, cellulose resins, urethane resins, vinyl chloride resins, polyester resins, polyolefin resins, polycarbonates, nylons, polystyrenes, and ABS resins. The particles contained in the resin layer A include inorganic particles and organic polymer particles. Examples of inorganic particles include titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, and mixtures thereof. Furthermore, other common inorganic particles such as calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, calcium fluoride, and the like can also be used in combination. Examples of organic polymer particles include styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based polymer particles. In one embodiment, it is preferably at least one selected from the group consisting of low refractive index particles such as silica and magnesium fluoride, metal oxide particles such as titanium oxide and zirconium oxide, inorganic pigments such as cobalt blue, etc. Among these, from the viewpoints of adhesion and adjusting the refractive index difference, a combination of a polyester resin and metal oxide particles such as titanium oxide and zirconium oxide may be used.

[0049] Furthermore, in order to form a crosslinked structure between resins in the resin layer A, the composition used to form the coating layer may contain a crosslinking agent. By incorporating a crosslinking agent, it becomes possible to further improve adhesion under high temperature and high humidity conditions. Specific crosslinking agents include, for example, urea-based, epoxy-based, melamine-based, isocyanate-based, oxazoline-based, and carbodiimide-based crosslinking agents. Among these, isocyanate-based crosslinking agents are preferred from the viewpoints of the stability of the coating liquid over time and the effect of improving adhesion under high temperature and high humidity treatment. Furthermore, a catalyst or the like can be used as needed to promote the crosslinking reaction.

[0050] When the coating layer-forming composition contains a binder resin and a crosslinking agent, the binder resin content is preferably 50 to 95% by weight, more preferably 55 to 90% by weight, even more preferably 60 to 90% by weight, and most preferably 80 to 90% by weight, based on 100% by weight of the total weight of the binder resin and crosslinking agent, from the viewpoint of adhesion. A binder resin content of 95% by weight or less maintains the strength of the coating layer and provides good adhesion under high temperature and high humidity conditions. A binder resin content of 50% by weight or more maintains the flexibility of the coating layer and provides good adhesion under normal temperature and high temperature and high humidity conditions, which is preferable. Furthermore, when the total weight of the binder resin and crosslinking agent is taken as 100% by weight, the crosslinking agent content is preferably 5 to 50% by weight, more preferably 10 to 45% by weight, even more preferably 10 to 40% by weight, and most preferably 10 to 20% by weight.

[0051] (additives) The resin layer in the present invention may contain known additives, such as surfactants, antioxidants, heat stabilizers, weather stabilizers, ultraviolet absorbers, organic lubricants, pigments, dyes, organic or inorganic particles, antistatic agents, nucleating agents, etc., within the range that does not impair the effects of the present invention. However, it is preferable not to use substances that are undesirable from an environmental perspective, etc.

[0052] In order to further improve the blocking resistance of the resin layer A, it is also a preferred embodiment to add inert particles to the coating layer. Examples of particles to be contained in the resin layer A include inorganic particles and organic polymer particles. Examples of inorganic particles include titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, and mixtures thereof. Furthermore, other common inorganic particles such as calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, calcium fluoride, and the like can also be used in combination. Examples of organic polymer particles include styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based polymer particles.

[0053] The average particle size of the inactive particles in the resin layer A (average particle size based on the number of particles measured by SEM; the same applies below) is preferably 0.04 to 2.0 μm, more preferably 0.1 to 1.0 μm. When the average particle size of the inactive particles is 0.04 μm or more, it is easy to form irregularities on the surface of the coating layer, which improves the handling properties, such as the slipperiness and winding properties, of the highly adhesive polyester film for foldable displays, and favors processability during lamination, which is preferable. On the other hand, when the average particle size of the inactive particles is 2.0 μm or less, particle detachment is less likely to occur, which is preferable. The particle concentration in the coating layer is preferably 1 to 20 mass % relative to the resin content.

[0054] (functional layer) In one aspect, the present invention provides an optical film. The optical film has an easily adhesive polyester film for a foldable display and a functional layer provided on the side of resin layer A opposite to the substrate side. That is, the optical film of the present invention has resin layer A and the functional layer in this order on a polyester film substrate. The functional layer is preferably a layer that functions as a hard coat layer, for example. The functional layer may have a function other than hard coat properties in addition to hard coat properties. In this specification, the term "hard coat layer" refers to a layer having a Martens hardness of 375 MPa or more at the center of the cross section of the hard coat layer. In this specification, "Martens hardness" refers to the hardness measured by nanoindentation when an indenter is pressed 500 nm into the hardness measurement. The Martens hardness measurement by nanoindentation is performed on the measurement sample using a HYSITRON TI950 TriboIndenter. Specifically, first, a 1 mm x 10 mm optical film is embedded in an embedding resin to prepare a block. Then, a uniform, hole-free section with a thickness of 70 nm to 100 nm is cut from this block using a conventional section preparation method. An ultramicrotome EM UC7 (Leica Microsystems) or the like can be used to prepare the section. The remaining block from which the uniform slice without holes etc. has been cut out is used as a measurement sample. Next, in the cross section obtained by cutting out the slice from such a measurement sample, a Berkovich indenter (triangular pyramid) is pressed 500 nm into the center of the cross section of the functional layer under the following measurement conditions, and the residual stress is relaxed by holding the indenter constant. Then, the load is released and the maximum load after relaxation is measured. max (μN) and the area of ​​the 500 nm deep depression A (nm 2 ) and P max The Martens hardness is calculated from the ratio of the hardness of the specimen to the hardness of the specimen. The Martens hardness is the arithmetic mean value of the values ​​measured at 10 locations.

[0055] (Measurement conditions) ·Loading speed: 10nm / sec ·Holding time: 5 seconds ·Loading and unloading speed: 10nm / sec ·Measurement temperature: 25℃

[0056] The functional layer preferably has a Martens hardness of 500 MPa or more and 2000 MPa or less at the center of its cross section. If the Martens hardness of the functional layer is 500 MPa or more, sufficient hardness as a hard coat layer can be obtained, and if it is 2000 MPa or less, good folding performance of the optical film can be obtained. The lower limit of the Martens hardness at the center of the cross section of the functional layer is preferably 600 MPa or more, and the upper limit is preferably 1500 MPa or less.

[0057] The thickness of the functional layer is preferably 2 μm or more and 40 μm or less. If the thickness of the functional layer is 2 μm or more, sufficient hardness as a hard coat layer can be obtained, and if it is 40 μm or less, deterioration of processability can be suppressed. In this specification, the "thickness of the functional layer" means the thickness (total thickness) of the functional layer when the functional layer has a multi-layer structure. The upper limit of the thickness of the functional layer is more preferably 30 μm or less, and even more preferably 20 μm or less. The thickness of the functional layer can be determined by the same method as that of the resin layer A described above.

[0058] The functional layer 12 may be an antistatic hard coat layer, and may contain a binder resin and an antistatic agent present in the binder resin. In addition to the binder resin, the functional layer 12 may contain additives such as inorganic particles, organic particles, ultraviolet absorbers, adhesion improvers, leveling agents, thixotropy-imparting agents, coupling agents, plasticizers, antifoaming agents, bulking agents, colorants, and fillers, as needed, within the scope of the present invention.

[0059] (binder resin) The binder resin contains a polymer (cured product) of a polymerizable compound (curable compound). The polymerizable compound has at least one polymerizable functional group in the molecule. Examples of the polymerizable functional group include ethylenically unsaturated groups such as a (meth)acryloyl group, a vinyl group, and an allyl group. The term "(meth)acryloyl group" includes both an "acryloyl group" and a "methacryloyl group."

[0060] The polymerizable compound is preferably a polyfunctional (meth)acrylate. Examples of the polyfunctional (meth)acrylate include trimethylolpropane tri(meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, and dipentaerythritol penta(meth)acrylate. acrylate, tripentaerythritol octa(meth)acrylate, tetrapentaerythritol deca(meth)acrylate, isocyanuric acid tri(meth)acrylate, isocyanuric acid di(meth)acrylate, polyester tri(meth)acrylate, polyester di(meth)acrylate, bisphenol di(meth)acrylate, diglycerin tetra(meth)acrylate, adamantyl di(meth)acrylate, isoboronyl di(meth)acrylate, dicyclopentane di(meth)acrylate, tricyclodecane di(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, and those modified with PO, EO, caprolactone, etc.

[0061] Among these, tri- to hexa-functional compounds are preferred because they can suitably satisfy the above-mentioned Martens hardness, and examples thereof include pentaerythritol triacrylate (PETA), dipentaerythritol hexaacrylate (DPHA), pentaerythritol tetraacrylate (PETTA), dipentaerythritol pentaacrylate (DPPA), trimethylolpropane tri(meth)acrylate, tripentaerythritol octa(meth)acrylate, and tetrapentaerythritol deca(meth)acrylate. In this specification, "(meth)acrylate" refers to both "acrylate" and "methacrylate."

[0062] The composition may further contain a monofunctional (meth)acrylate monomer in order to adjust the hardness or viscosity of the composition, improve adhesion, etc. Examples of the monofunctional (meth)acrylate monomer include hydroxyethyl acrylate (HEA), glycidyl methacrylate, methoxypolyethylene glycol (meth)acrylate, isostearyl (meth)acrylate, 2-acryloyloxyethyl succinate, acryloylmorpholine, N-acryloyloxyethyl hexahydrophthalimide, cyclohexyl acrylate, tetrahydrofuryl acrylate, isobornyl acrylate, phenoxyethyl acrylate, and adamantyl acrylate.

[0063] From the viewpoint of improving the hardness of the functional layer, the weight-average molecular weight of the monomer is preferably less than 1,000, and more preferably from 200 to 800. The weight-average molecular weight of the polymerizable oligomer is preferably from 1,000 to 20,000, more preferably from 1,000 to 10,000, and even more preferably from 2,000 to 7,000.

[0064] (antistatic agent) The antistatic agent used in the functional layer is not particularly limited as long as it has good compatibility with the binder resin. Antistatic agents include ion-conductive antistatic agents and electron-conductive antistatic agents, and ion-conductive antistatic agents are preferred from the viewpoint of compatibility with the binder resin.

[0065] The functional layer may further include an ultraviolet absorber, a spectral transmittance adjuster, and / or an antifouling agent.

[0066] (ultraviolet absorber) Optical films are particularly suitable for use in mobile devices such as foldable smartphones and tablet devices. However, these mobile devices are often used outdoors, which poses a problem of the polarizer located on the display element side of the optical film being easily exposed to ultraviolet light and degraded. However, since the functional layer 12 is located on the viewer side of the polarizer, if the functional layer contains an ultraviolet absorber, it can effectively prevent the polarizer from degrading due to exposure to ultraviolet light. The ultraviolet absorber (UVA) may be contained in the polyester film serving as the substrate, rather than in the functional layer.

[0067] Examples of the ultraviolet absorber include triazine-based ultraviolet absorbers, benzophenone-based ultraviolet absorbers, and benzotriazole-based ultraviolet absorbers.

[0068] Examples of the triazine-based ultraviolet absorber include 2-(2-hydroxy-4-[1-octyloxycarbonylethoxy]phenyl)-4,6-bis(4-phenylphenyl)-1,3,5-triazine, 2-[4-[(2-hydroxy-3-dodecyloxypropyl)oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine, and 2,4-bis[2-hydroxy-4-butoxyphenyl]-6 -(2,4-dibutoxyphenyl)-1,3,5-triazine, 2-[4-[(2-hydroxy-3-tridecyloxypropyl)oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine, and 2-[4-[(2-hydroxy-3-(2'-ethyl)hexyl)oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine, etc. Commercially available triazine-based ultraviolet absorbers include, for example, TINUVIN 460 and TINUVIN 477 (both manufactured by BASF), and LA-46 (manufactured by ADEKA).

[0069] Examples of the benzophenone-based ultraviolet absorbers include 2-hydroxybenzophenone, 2,4-dihydroxybenzophenone, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, 2,2',4,4'-tetrahydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, hydroxymethoxybenzophenone sulfonic acid and its trihydrate, sodium hydroxymethoxybenzophenone sulfonate, etc. Examples of commercially available benzophenone-based ultraviolet absorbers include CHMASSORB81 / FL (manufactured by BASF).

[0070] Examples of the benzotriazole-based ultraviolet absorber include 2-ethylhexyl-3-[3-tert-butyl-4-hydroxy-5-(5-chloro-2H-benzotriazol-2-yl)phenyl]propionate, 2-(2H-benzotriazol-2-yl)-6-(straight-chain and branched-chain dodecyl)-4-methylphenol, 2-[5-chloro(2H)-benzotriazol-2-yl]-4-methyl-6-(tert-butyl)phenol, 2-(2H-benzotriazol-2-yl)-4,6-di-tert-pentylphenol, 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-3',5'-di-tert-butyl)benzotriazole, and 2-ethylhexyl-3-[3-tert-butyl-4-hydroxy-5-(5-chloro-2H-benzotriazol-2-yl)phenyl]propionate. 2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-3',5'-di-tert-butylphenyl)-5-chlorobenzotriazole, 2-(2'-hydroxy-3'-(3'',4'',5'',6''-tetrahydrophthalimidomethyl)-5'-methylphenyl)benzotriazole, 2,2-methylenebis(4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol), and 2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole. Examples of commercially available benzotriazole-based ultraviolet absorbers include KEMISORB71D and KEMISORB79 (both manufactured by Chemipro Chemical Co., Ltd.), JF-80 and JAST-500 (both manufactured by Johoku Chemical Co., Ltd.), ULS-1933D (manufactured by Ippo), and RUVA-93 (manufactured by Otsuka Chemical Co., Ltd.).

[0071] Among the UV absorbers, triazine-based UV absorbers and benzotriazole-based UV absorbers are particularly preferred. The UV absorber preferably has high solubility in the resin components constituting the functional layer, and preferably exhibits minimal bleed-out after the above-mentioned continuous folding test. The UV absorber is preferably polymerized or oligomerized. The UV absorber is preferably a polymer or oligomer having a benzotriazole, triazine, or benzophenone skeleton, and more preferably a thermal copolymer of a (meth)acrylate having a benzotriazole or benzophenone skeleton and methyl methacrylate (MMA) in any ratio. When the optical film is applied to an organic light-emitting diode (OLED) display device, the UV absorber can also protect the OLED from UV rays.

[0072] The content of the ultraviolet absorber is not particularly limited, but is preferably 1 part by mass or more and 6 parts by mass or less per 100 parts by mass of the solid content of the functional layer composition. If it is 1 part by mass or more, the effect of adding the ultraviolet absorber to the functional layer can be fully obtained, and if it is 6 parts by mass or less, the functional layer will not suffer from significant coloration or a decrease in strength. The lower limit of the content of the ultraviolet absorber is more preferably 2 parts by mass or more, and the upper limit is more preferably 5 parts by mass or less.

[0073] The sesamol-type benzotriazole monomer is not particularly limited, but specific substance names include 2-[2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yl]ethyl methacrylate, 2-[2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yl]ethyl acrylate, 3-[2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yl]propyl methacrylate, 3-[2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yl]propyl acrylate, 4-[2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yl]butyl methacrylate, 4-[2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yl]butyl methacrylate, 2-[2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yloxy]ethyl acrylate, 2-[2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yloxy]ethyl acrylate, 2-[3-{2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yloxy]ethyl acrylate )-2H-benzotriazol-5-yl}propanoyloxy]ethyl methacrylate, 2-[3-{2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yl}propanoyloxy]ethyl acrylate, 4-[3-{2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yl}propanoyloxy]butyl methacrylate, 4-[3-{ 2 -(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yl}propanoyloxy]butyl acrylate, 2-[3-{2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazol-5-yl}propanoyloxy]ethyl methacrylate, 2-[3-{2-(6-hydroxybenzo[1,Examples of suitable sesamol-type benzotriazole monomers include 2-(methacryloyloxy)ethyl 2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazole-5-yl}propanoyloxy)ethyl acrylate, 2-(methacryloyloxy)ethyl 2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazole-5-carboxylate, 2-(acryloyloxy)ethyl 2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazole-5-carboxylate, 4-(methacryloyloxy)butyl 2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazole-5-carboxylate, and 4-(acryloyloxy)butyl 2-(6-hydroxybenzo[1,3]dioxol-5-yl)-2H-benzotriazole-5-carboxylate. These sesamol-type benzotriazole monomers may be used alone or in combination of two or more.

[0074] (Anti-fouling agent) The antifouling agent is not particularly limited, and examples thereof include silicone-based antifouling agents, fluorine-based antifouling agents, and silicone-based and fluorine-based antifouling agents, which may be used alone or in combination. The antifouling agent may also be an acrylic-based antifouling agent.

[0075] The content of the antifouling agent is preferably 0.01 to 3.0 parts by mass relative to 100 parts by mass of the polymerizable compound. If the content is 0.01 part by mass or more, sufficient antifouling performance can be imparted to the functional layer, and if the content is 3.0 parts by mass or less, there is no risk of the hardness of the functional layer decreasing.

[0076] The antifouling agent preferably has a weight-average molecular weight of 5000 or less, and is a compound having preferably one or more, more preferably two or more reactive functional groups to improve the durability of the antifouling performance. In particular, the use of an antifouling agent having two or more reactive functional groups can impart excellent scratch resistance.

[0077] If the antifouling agent does not have a reactive functional group, the antifouling agent will be transferred to the back surface of the optical film when the optical film is stacked, whether the optical film is in roll form or sheet form, and when an attempt is made to attach or apply another layer to the back surface of the optical film, the other layer may peel off. Furthermore, the other layer may easily peel off when a multiple consecutive folding test is performed.

[0078] Furthermore, the antifouling agent having the reactive functional group has good antifouling performance durability (durability), and in particular, the functional layer containing the above-mentioned fluorine-based antifouling agent is less susceptible to fingerprints (less noticeable) and has good wiping properties. Furthermore, since the surface tension of the functional layer composition can be reduced during application, the leveling properties are good and the appearance of the formed functional layer is good.

[0079] Functional layers containing silicone-based antifouling agents have good slip properties and good steel wool resistance. Touch sensors incorporating optical films containing such silicone-based antifouling agents in their functional layers have good slip properties when touched with a finger or pen, resulting in a good tactile feel. Fingerprints are also less likely to be left on the functional layer (less noticeable), and they are easy to wipe off. Furthermore, the surface tension of the functional layer composition can be reduced during application, resulting in good leveling properties and a good appearance for the functional layer formed.

[0080] Commercially available silicone antifouling agents include, for example, SUA1900L10 (manufactured by Shin-Nakamura Chemical Co., Ltd.), SUA1900L6 (manufactured by Shin-Nakamura Chemical Co., Ltd.), Ebecryl 1360 (manufactured by Daicel-Cytec Co., Ltd.), UT3971 (manufactured by Nippon Synthetic Co., Ltd.), BYKUV3500 (manufactured by BYK-Chemie), BYKUV3510 (manufactured by BYK-Chemie), BYKUV3570 (manufactured by BYK-Chemie), X22-164E, X Examples include 22-174BX, X22-2426, KBM503, KBM5103 (manufactured by Shin-Etsu Chemical Co., Ltd.), TEGO-RAD2250, TEGO-RAD2300, TEGO-RAD2200N, TEGO-RAD2010, TEGO-RAD2500, TEGO-RAD2600, TEGO-RAD2700 (manufactured by Evonik Japan), and Megafac RS854 (manufactured by DIC Corporation).

[0081] Commercially available fluorine-based antifouling agents include, for example, Optool DAC and Optool DSX (manufactured by Daikin Industries, Ltd.), Megafac RS71 and Megafac RS74 (manufactured by DIC Corporation), LINC152EPA, LINC151EPA, and LINC182UA (manufactured by Kyoeisha Chemical Co., Ltd.), Ftergent 650A, Ftergent 601AD, and Ftergent 602.

[0082] Commercially available fluorine-based and silicone-based antifouling agents having reactive functional groups include, for example, Megafac RS851, Megafac RS852, Megafac RS853, Megafac RS854 (manufactured by DIC Corporation), Opstar TU2225, Opstar TU2224 (manufactured by JSR Corporation), and X71-1203M (manufactured by Shin-Etsu Chemical Co., Ltd.).

[0083] (Resin layer B) The highly adhesive polyester film for a foldable display of the present invention has a resin layer B on the second surface side of the substrate opposite to the first surface. The binder resin constituting the resin layer B is a resin with high adhesive properties, characterized by being formed from a composition containing a polyester resin having a condensed polycyclic aromatic group and an isocyanate crosslinking agent. By using a resin layer B made from these combinations, the adhesion between the polyester film substrate and the adhesive layer is dramatically improved. Furthermore, an optical film can be obtained that does not crack or break even when subjected to a 180° folding test repeated 10,000 times. When an impact is applied to the surface of the optical film, not only is the optical film itself deformed, but plastic deformation of the adhesive layer can also be suppressed, even if the adhesive layer is located further inside the image display device than the optical film. Furthermore, cracking of the optical film during folding can be suppressed. Furthermore, when the container is folded and held, and then opened again, it can have good restoring properties.

[0084] The resin layer B of the present invention is compatible with the composition of the pressure-sensitive adhesive layer described below. The polyester resin having a condensed polycyclic aromatic group and the isocyanate crosslinking agent in the resin layer B interact with each other, making them more compatible and entangled. Furthermore, it is believed that this entangled resin becomes entangled in the network structure of the pressure-sensitive adhesive layer formed thereon, resulting in a resin layer with superior adhesion compared to conventional resins. Furthermore, since the resin layer contains a polyester resin, it is believed that adhesion to the polyester film substrate is also improved, resulting in a resin layer with superior adhesion.

[0085] The resin layer B is formed from a composition containing a polyester resin having a condensed polycyclic aromatic group and an isocyanate crosslinking agent.

[0086] The polyester resin is a copolymer polyester obtained by polycondensation of a dicarboxylic acid component and a diol component.

[0087] The dicarboxylic acid component has a structure containing a condensed polycyclic aromatic group, and in the present invention, it is particularly preferable that the dicarboxylic acid component has a structure containing a naphthalene ring, so that the polyester resin can have a naphthalene skeleton in the molecule.

[0088] Examples of dicarboxylic acid components include 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, anthracenedicarboxylic acid, etc. Among these, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, etc. are particularly preferred because they have the above-mentioned structure containing a naphthalene ring.

[0089] Examples of diol components include ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, 1,4-cyclohexanediethanol, decamethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexadiol, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone.

[0090] The ratio of the dicarboxylic acid component and the diol component when combined is not particularly limited, and can be adjusted so as to obtain a polyester having the required properties.

[0091] The polyester resin having a condensed polycyclic aromatic structure improves the interaction with the adhesive layer and dramatically improves the adhesion to the adhesive layer, which results in the adhesive layer remaining tightly adhered without peeling after a bending test or the like.

[0092] (Crosslinking agent) In the present invention, in order to form a crosslinked structure between resins in the coating layer, the composition used to form the coating layer must contain a crosslinking agent. By including a crosslinking agent, it becomes possible to further improve adhesion under high temperature and high humidity conditions, and it is preferable to use an isocyanate-based crosslinking agent. Furthermore, in order to promote the crosslinking reaction, a catalyst or the like can be used as needed.

[0093] When the coating layer-forming composition contains a binder resin and a crosslinking agent, the binder resin content is preferably 50 to 95% by weight, more preferably 55 to 90% by weight, even more preferably 60 to 90% by weight, and most preferably 80 to 90% by weight, based on 100% by weight of the total weight of the binder resin and crosslinking agent, from the viewpoint of adhesion. A binder resin content of 95% by weight or less maintains the strength of the coating layer and provides good adhesion under high temperature and high humidity conditions. A binder resin content of 50% by weight or more maintains the flexibility of the coating layer and provides good adhesion under normal temperature and high temperature and high humidity conditions, which is preferable. Furthermore, when the total weight of the binder resin and crosslinking agent is taken as 100% by weight, the crosslinking agent content is preferably 5 to 50% by weight, more preferably 10 to 45% by weight, even more preferably 10 to 40% by weight, and most preferably 10 to 20% by weight.

[0094] When using an isocyanate-based crosslinking agent as a crosslinking agent, it is preferable to use a blocking agent to control the reactivity of isocyanate. Examples of blocking agents include bisulfite compounds such as sodium bisulfite; pyrazole compounds such as 3,5-dimethylpyrazole, 3-methylpyrazole, 4-bromo-3,5-dimethylpyrazole, and 4-nitro-3,5-dimethylpyrazole; phenols such as phenol and cresol; aliphatic alcohols such as methanol and ethanol; active methylene compounds such as dimethyl malonate and acetylacetone; mercaptans such as butyl mercaptan and dodecyl mercaptan; acid amides such as acetanilide and acetic acid amide; lactams such as ε-caprolactam and δ-valerolactam; acid imides such as succinimide and maleimide; oximes such as acetaldoxime, acetoneoxime, and methylethylketoxime; and amines such as diphenylaniline, aniline, and ethyleneimine. From the viewpoint of reactivity, a blocking agent having a pyrazole skeleton, also called a pyrazole-type blocked isocyanate, is suitable for this system.

[0095] It is preferable to introduce a hydrophilic group into the blocked isocyanate crosslinking agent from the viewpoint of imparting water dispersibility in aqueous solvents. Furthermore, the hydrophilic group is preferably an anionic group such as a carboxyl group or a sulfonic acid group, or a nonionic group such as an oxyalkyl group. Crosslinking agents having these hydrophilic groups can be prepared by previously reacting a polyisocyanate, which serves as the base of the blocked isocyanate, with a compound having a hydrophilic group and a reactive group such as a hydroxyl group.

[0096] The thickness of the resin layer B is greater than 1 nm and smaller than 30 nm, for example, 3 nm or more and 29 nm or less, and may be 4 nm or more and 28 nm or less. If the film thickness of resin layer B is less than 30 nm, not only can the adhesion between the pressure-sensitive adhesive layer and resin layer B be ensured, but also the film haze of the highly adhesive polyester film for a foldable display and the film haze of the optical film to which the functional layer and pressure-sensitive adhesive layer are attached can be reduced, making it suitable as an optical film. Furthermore, it can contribute to achieving a 90° peel adhesive strength of 45 N / dm or more. Since the present invention can adjust the 90° peel adhesive strength within the above range, for example, in an optical film, no cracks or breaks occur when a test in which the optical film is folded 180° with the functional layer on the inside and the distance between opposing sides of the optical film is 10 mm is repeated 10,000 times. Furthermore, in an optical film, no cracks or breaks occur when a test in which the optical film is folded 180° with the functional layer on the outside and the distance between opposing sides of the optical film is 30 mm is repeated 10,000 times. For example, if the thickness of resin layer B according to the present invention is 30 nm or more, the optical film may crack or break after 10,000 repeated folding tests. Although the detailed mechanism has not been analyzed, it is presumed that the stress when folding the optical film can be alleviated by achieving a 90° peel adhesive strength of 45 N / dm or more. Furthermore, it is presumed that resin layer B containing the components according to the present invention and having a thickness of less than 30 nm can effectively achieve a 90° peel adhesive strength of 45 N / dm or more, and furthermore, can exhibit a favorable failure mode after 10,000 repeated folding tests. The absence of cracks or breaks is evaluated by checking for any gaps between the substrate and the functional layer, and checking for any cracks or breaks in the bent portion. Cracks or breaks can be evaluated visually. The thickness of the resin layer B can be determined by the same method as that for the resin layer A described above.

[0097] The 90° peel adhesive strength is 45 N / dm or more, and may be 45 N / dm or more and 70 N / dm or less. For example, the 90° peel adhesive strength may be 45 N / dm or more and 65 N / dm or less. When the 90° peel adhesive strength is in this range, a good failure mode can be exhibited after repeating a folding test 10,000 times.

[0098] While the resin layer B may be composed solely of a polyester resin having a condensed polycyclic aromatic group and an isocyanate crosslinking agent, adding inactive particles to further improve blocking resistance is also a preferred embodiment. Examples of particles to be contained in the resin layer B include inorganic particles and organic polymer particles. Examples of inorganic particles include titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, and mixtures thereof. Furthermore, other common inorganic particles, such as calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, calcium fluoride, and the like, can also be used in combination. Examples of organic polymer particles include styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based polymer particles.

[0099] The average particle size of the inactive particles in the resin layer B (average particle size based on the number of particles measured by SEM; the same applies below) is preferably 0.04 to 2.0 μm, more preferably 0.1 to 1.0 μm. When the average particle size of the inactive particles is 0.04 μm or more, it is easy to form irregularities on the surface of the coating layer, which improves the handling properties, such as the slipperiness and winding properties, of the highly adhesive polyester film for foldable displays, and favors processability during lamination, which is preferable. On the other hand, when the average particle size of the inactive particles is 2.0 μm or less, particle detachment is less likely to occur, which is preferable. The particle concentration in the coating layer is preferably 1 to 20 mass % relative to the resin content.

[0100] The resin layer B may contain an antistatic agent to obtain antistatic properties. When the resin layer B contains an antistatic agent, the resin layer B also functions as an antistatic layer. When the resin layer B contains an antistatic agent, the surface resistance value on the surface of the highly adhesive polyester film for a foldable display can be further stabilized.

[0101] (additives) In the present invention, known additives such as surfactants, antioxidants, heat stabilizers, weather stabilizers, ultraviolet absorbers, organic lubricants, pigments, dyes, organic or inorganic particles, antistatic agents, nucleating agents, etc. may be added to the resin layer B within the range that does not impair the effects of the present invention. However, it is preferable not to use substances that are undesirable from an environmental perspective, etc.

[0102] The highly adhesive polyester film for foldable displays preferably has a spectral transmittance of 8% or less at a wavelength of 380 nm. If the spectral transmittance of the optical film exceeds 8%, when the optical film is used in a mobile terminal, the polarizer may be exposed to ultraviolet light and easily deteriorate. The transmittance can be measured using a spectrophotometer (product name "UV-2450", manufactured by Shimadzu Corporation). The measurement conditions for the spectral transmittance are the same as those for the spectral transmittance at wavelengths of 300 nm to 780 nm. The transmittance is measured three times on a piece of highly adhesive polyester film for foldable displays cut into a size of 50 mm x 100 mm, and the arithmetic mean value of the values ​​obtained from the three measurements is used. The upper limit of the transmittance of the highly adhesive polyester film for foldable displays is more preferably 5%. The transmittance of the optical film can be achieved by, for example, adjusting the amount of ultraviolet absorber added, as described below.

[0103] The total light transmittance of the easy-adhesion polyester film for foldable displays is preferably 85% or more. If the total light transmittance of the easy-adhesion polyester film for foldable displays is 85% or more, sufficient image visibility can be obtained when the easy-adhesion polyester film for foldable displays is used in a mobile terminal. The total light transmittance of the easy-adhesion polyester film for foldable displays is more preferably 87% or more, and most preferably 90% or more.

[0104] The total light transmittance can be measured using a haze meter (product name "HM-150", manufactured by Murakami Color Research Laboratory Co., Ltd.) according to a method conforming to JIS K7361-1:1997. The total light transmittance is measured three times for each easy-to-adhere polyester film for foldable displays after cutting the optical film into a size of 50 mm x 100 mm. The optical film is then placed in a state free of curls, wrinkles, fingerprints, dust, etc., and the arithmetic mean value of the values ​​obtained from the three measurements is used. In this specification, "measured three times" does not mean measuring the same location three times, but rather measuring three different locations. In the easy-to-adhere polyester film for foldable displays, the film thickness variation is also within a range of ±10%. Therefore, by measuring the total light transmittance at three different locations on the cut-out easy-to-adhere polyester film for foldable displays, it is thought that an approximate average value of the total light transmittance throughout the entire in-plane of the easy-to-adhere polyester film for foldable displays can be obtained. The variation in total light transmittance is within ±10%, even when the measurement target is as long as 1 m x 3000 m, or as small as a 5-inch smartphone. If the highly adhesive polyester film for foldable displays cannot be cut to the above size, for example, the HM-150 has an entrance opening of 20 mmφ for measurement, so a sample size of 21 mm or more is required. Therefore, the highly adhesive polyester film for foldable displays may be appropriately cut to a size of 22 mm x 22 mm or more. If the highly adhesive polyester film for foldable displays is small, measurement points are set to three by gradually shifting the light source spot or changing the angle within the range where it does not shift.

[0105] The haze value (total haze value) of the easy-adhesion polyester film for foldable displays is preferably 1.2% or less. If the haze value of the easy-adhesion polyester film for foldable displays is 1.2% or less, when the easy-adhesion polyester film for foldable displays is used in a mobile terminal, whitening of the image display surface can be suppressed. The haze value is more preferably 1.0% or less, and even more preferably 0.8% or less.

[0106] The haze value can be measured using a haze meter (product name "HM-150" manufactured by Murakami Color Research Laboratory Co., Ltd.) according to JIS K7136:2000. The haze value is measured three times for each piece of adhesive polyester film for foldable displays, cut into pieces measuring 50 mm x 100 mm, and then placed in a state free of curls, wrinkles, fingerprints, dust, etc. The arithmetic mean value of the three measurements is used. The variation in film thickness of adhesive polyester films for foldable displays is also within a range of ±10%. Therefore, by measuring the haze value at three different locations on a cut-out adhesive polyester film for foldable displays, it is believed that the average haze value for the entire in-plane area of ​​the optical film can be obtained. The variation in haze value is within ±10%, even when the measurement target is as long as 1 m x 3000 m or as small as a 5-inch smartphone. If the adhesive polyester film for a foldable display cannot be cut to the above size, for example, the HM-150 has an entrance opening of 20 mmφ for measurement, so a sample size of 21 mm or more is required. Therefore, the adhesive polyester film for a foldable display may be appropriately cut to a size of 22 mm x 22 mm or more. If the adhesive polyester film for a foldable display is small, the measurement points are set to three by shifting the light source spot slightly or changing the angle within the range where it does not miss the light source spot.

[0107] The optical film of the present invention can be obtained by providing a functional layer on resin layer A and a pressure-sensitive adhesive layer on resin layer B of an easily adhesive polyester film for a foldable display having resin layer A and resin layer B formed thereon.

[0108] (Adhesive layer) The optical film according to the present invention is composed of a pressure-sensitive adhesive layer provided on the resin layer B on the side opposite to the substrate. The pressure-sensitive adhesive layer is a layer made of a resin having optical transparency. The pressure-sensitive adhesive layer is a layer having impact absorption properties. The pressure-sensitive adhesive layer may have a multi-layer structure made of two or more pressure-sensitive adhesive layers.

[0109] The thickness of the adhesive layer is 30 μm or more and 300 μm or less. If the thickness of the adhesive layer is 30 μm or more, a decrease in hardness of the adhesive layer can be suppressed, and if it is 300 μm or less, the adhesive layer can be made thinner without deteriorating processability. The thickness of the adhesive layer is determined by photographing a cross section of the adhesive layer using a scanning electron microscope (SEM), measuring the thickness of the adhesive layer at 20 points on the image of the cross section, and calculating the arithmetic mean value of the thicknesses at the 20 points. The lower limit of the adhesive layer is more preferably 50 μm or more, and the upper limit of the adhesive layer is more preferably 150 μm or less, and even more preferably 100 μm or less.

[0110] The resin constituting the pressure-sensitive adhesive layer is not particularly limited. Examples of such resins include acrylic gels, urethane gels, silicone gels, urethane resins, and epoxy resins. Among these, urethane gels are preferred. "Gel" generally refers to a dispersion system that is highly viscous and has lost fluidity. The pressure-sensitive adhesive layer may contain rubber or a thermoplastic elastomer in addition to the acrylic gel or urethane resin.

[0111] The urethane resin is a resin having a urethane bond. Examples of the urethane resin include a cured product of an ionizing radiation-curable urethane resin composition and a cured product of a thermosetting urethane resin composition. Among these, a cured product of an ionizing radiation-curable urethane resin composition is preferred from the viewpoints of obtaining high hardness, a fast curing rate, and excellent mass productivity.

[0112] The ionizing radiation-curable urethane resin composition contains a urethane (meth)acrylate, and the thermosetting urethane resin contains a polyol compound and an isocyanate compound. The urethane (meth)acrylate, the polyol compound, and the isocyanate compound may be any of a monomer, an oligomer, and a prepolymer.

[0113] The number of (meth)acryloyl groups (number of functional groups) in the urethane (meth)acrylate is preferably 2 or more and 4 or less. If the number of (meth)acryloyl groups in the urethane (meth)acrylate is less than 2, the pencil hardness may be reduced, and if it exceeds 4, the cure shrinkage may be large, causing the optical film to curl and the resin layer to crack when bent. The upper limit of the number of (meth)acryloyl groups in the urethane (meth)acrylate is more preferably 3 or less. The term "(meth)acryloyl group" includes both "acryloyl group" and "methacryloyl group."

[0114] The weight-average molecular weight of the urethane (meth)acrylate is preferably 1,500 or more and 20,000 or less. If the weight-average molecular weight of the urethane (meth)acrylate is less than 1,500, the impact resistance may decrease, and if it exceeds 20,000, the viscosity of the ionizing radiation-curable urethane resin composition may increase, resulting in poor coatability. The lower limit of the weight-average molecular weight of the urethane (meth)acrylate is more preferably 2,000 or more, and the upper limit is more preferably 15,000 or less.

[0115] The structure of the polymer chain (repeating unit) of the resin constituting the pressure-sensitive adhesive layer can be determined by analyzing the pressure-sensitive adhesive layer using, for example, pyrolysis GC-MS and FT-IR. Pyrolysis GC-MS is particularly useful because it can detect the monomer units contained in the pressure-sensitive adhesive layer as monomer components.

[0116] The pressure-sensitive adhesive layer may contain an ultraviolet absorber, a spectral transmittance adjuster, an antifouling agent, inorganic particles and / or organic particles, etc. As the ultraviolet absorber, etc., the same ultraviolet absorbers, etc. as those explained in the section on the functional layer can be used, and therefore explanation thereof will be omitted here.

[0117] The adhesion between the resin layer B provided on the highly adhesive polyester film for foldable displays and the adhesive layer laminated thereon can be confirmed by conducting a 90° peel strength (adhesion) test, which is one of constant load peel tests, to evaluate the adhesive strength of the adhesive layer to an adherend.

[0118] In various evaluations, the resin composition used for the adhesive layer was applied to a 12.5 μm thick polyimide film (Apical (registered trademark), manufactured by Kaneka Corporation) using an applicator so that the thickness after drying would be 50 μm, and then dried for 10 minutes at 130 ° C. The adhesive film thus obtained was attached to a pre-prepared easy-adhesion polyester film for a foldable display, and evaluations could be performed.

[0119] For example, the adhesive resin composition was applied to a 12.5 μm thick polyimide film (Apical (registered trademark), manufactured by Kaneka Corporation) so that the thickness after drying would be 25 μm, and then dried at 130° C. for 10 minutes. The adhesive film (B-stage product) thus obtained was laminated to a 12.5 μm thick polyimide film (PI) or an 18 μm thick rolled copper foil (BHY series, manufactured by JX Nippon Mining & Metals Corporation) (Cu). The lamination was performed with the shiny side of the rolled copper foil in contact with the adhesive layer, at 160° C. and 40 kgf / cm. 2The adhesive was then pressed under pressure for 30 seconds to bond the film. The film was then cured by heat treatment at 200°C for 1 hour to obtain a sample for peel strength evaluation. The peel strength was measured by a 90° peel test at 25°C, with the film pulled at a tensile speed of 50 mm / min. This test indicates the adhesive strength at room temperature.

[0120] The optical film is foldable. Specifically, when a test in which the optical film is folded 180° with the functional layer facing inward and the distance between opposing sides of the optical film being 10 mm is repeated 10,000 times, no cracks or breaks occur. It is preferable that the optical film 10 does not crack or break even when the folding test is repeated 20,000 times, and it is even more preferable that the optical film does not crack or break even when the folding test is repeated 100,000 times. When an optical film is repeatedly folded 10,000 times, if cracks or the like occur in the optical film, the foldability of the optical film is insufficient.

[0121] On the other hand, it is preferable that the optical film does not crack or break even when a test (also called a continuous folding test) in which the optical film is continuously folded 10,000 times with the functional layer on the outside and the distance between opposing sides of the optical film being 30 mm is repeated, it is more preferable that the optical film does not crack or break even when the continuous folding test is repeated 20,000 times, and it is even more preferable that the optical film does not crack or break even when the test is repeated 100,000 times. Here, when bending the optical film so that the functional layer faces outward, stress that stretches the functional layer in the longitudinal direction is applied compared to a folding test conducted so that the functional layer faces inward, so it is desirable to conduct a test in which the optical film is continuously folded so that the distance between opposing sides of the optical film is 30 mm. This is because it is known that the functional layer is weaker against the elongation stress applied by tension than the contraction stress applied by compression, and for example, bending the optical film so that the functional layer faces outward increases the elongation stress applied to the functional layer, so measurement conditions are applied in which the folding distance is changed. That is, the evaluation method may be different depending on whether the substrate is bent so that the functional layer faces inward or outward. The present invention, in which the resin layer B has the composition described in this specification, can lead to good results in an evaluation in which the substrate is bent so that the functional layer faces outward.

[0122] A continuous folding test in which an optical film is continuously folded so that the functional layer faces inward is performed as follows. In the continuous folding test, the optical film is first fixed to fixing parts arranged in parallel so that the sides of the optical film face each other. Note that the optical film may have any shape, but it is preferable that the optical film used in the continuous folding test be rectangular (for example, a rectangle of 30 mm x 100 mm).

[0123] Next, the fixing parts are moved closer to each other, deforming the optical film so that the functional layer is on the inside and folding it, and then the fixing parts are moved to a position where the distance between the two opposing sides of the optical film fixed by the fixing parts is 10 mm, and then the fixing parts are moved in the opposite direction to eliminate the deformation of the optical film. After these steps, the optical film is folded 180° 10,000 times so that the functional layer faces inward and the distance between the opposing sides of the optical film is 10 mm. The optical film of the present invention does not crack or break even when the folding test is repeated 10,000 times.

[0124] In the present invention, the absence of cracks or breaks in the optical film is evaluated by checking whether there is a gap between the substrate and the functional layer, and whether there is a crack or break at the bent portion. The crack or break can be evaluated visually.

[0125] By moving the fixing part, the optical film can be folded 180°. Furthermore, by conducting a continuous folding test so that the bent part of the optical film does not protrude from the end of the fixing part and controlling the distance between the fixing parts to 10 mm when they are closest to each other, the distance between the two opposing sides of the optical film can be set to 10 mm. In this case, the outer diameter of the bent part is replaced with 10 mm. Since the thickness of the optical film is sufficiently small compared to the distance between the fixing parts (10 mm), it is considered that the results of the continuous folding test of the optical film are not affected by differences in the thickness of the optical film. In the present invention, the optical film does not crack or break when the continuous folding test is repeated 10,000 times in which the optical film is folded 180° with the functional layer on the inside and the distance between the opposing sides of the optical film is 10 mm, and also does not crack or break when the continuous folding test is repeated 10,000 times in which the optical film is folded 180° with the functional layer on the inside and the distance between the opposing sides of the optical film is 2 mm. Preferably, resin layer B has the composition described in this specification, and can therefore show good results in these successive tests.

[0126] As another evaluation of folding, the optical film was fixed to a fixing part so that the distance between one end of the optical film and the other opposite end was 10 mm, and the optical film was kept in a folded state at 70°C for 12 hours (a folding retention test was performed). After the folding retention test, the optical film was released from the folded state, and after 30 minutes at room temperature, the opening angle θ of the optical film, which is the opening angle of the optical film, was measured, as shown in Figure 3. In this evaluation test, the opening angle θ of the optical film is preferably 100° or more. A larger opening angle θ indicates better restorability, and the maximum opening angle θ is 180°. The folding retention test may be performed by folding the optical film so that the functional layer is on the inside, or so that the functional layer is on the outside. In either case, the opening angle θ is preferably 100° or more.

[0127] The optical film surface (the surface of the functional layer) preferably has a hardness (pencil hardness) of 3H or more, more preferably 4H or more, as measured by the pencil hardness test specified in JIS K5600-5-4:1999. The pencil hardness test is performed by fixing a 30 mm × 100 mm piece of optical film cut out onto a glass plate with Nichiban Co., Ltd.'s Cellotape (registered trademark) to prevent folds or wrinkles, and then using a pencil hardness tester (product name "Pencil Scratch Coating Hardness Tester (Electric)" manufactured by Toyo Seiki Seisakusho Co., Ltd.) to move a pencil (product name "Uni" manufactured by Mitsubishi Pencil Co., Ltd.) against the surface of the optical film at a speed of 1 mm / sec while applying a 750 g load to the pencil. The pencil hardness is defined as the highest hardness that does not scratch the surface of the optical film in the pencil hardness test. When measuring the pencil hardness, multiple pencils with different hardnesses are used, and the pencil hardness test is performed five times for each pencil. If the surface of the optical film is not scratched four or more times out of the five times, it is determined that the pencil of that hardness did not scratch the surface of the optical film. The scratches refer to those visible when the surface of the optical film that has been subjected to the pencil hardness test is observed through transmission under fluorescent light.

[0128] The optical film preferably has a yellow index (YI) of 15 or less. When the YI of the optical film is 15 or less, the yellowness of the optical film can be suppressed, making it suitable for applications requiring transparency. The upper limit of the yellow index (YI) of the optical film is more preferably 10 or less. The yellow index (YI) is determined by calculating the chromaticity tristimulus values ​​X, Y, and Z according to the formula described in JIS Z8722:2009 from the transmittance of the optical film at wavelengths of 300 nm to 780 nm measured in a spectrophotometer (product name "UV-2450" manufactured by Shimadzu Corporation, light source: tungsten lamp and deuterium lamp) with the back side facing the light source, and then calculating the chromaticity tristimulus values ​​X, Y, and Z according to the formula described in ASTM D1925:1962. The upper limit of the yellow index (YI) of the optical film is more preferably 10 or less. The yellow index (YI) is calculated by measuring one optical film three times and calculating the arithmetic mean of the three measurements. With the UV-2450, the yellow index is calculated by loading the transmittance measurement data on a monitor connected to the UV-2450 and checking the "YI" box in the calculations. The transmittance at wavelengths of 300 nm to 780 nm is measured at least five points within 1 nm of each wavelength from 300 nm to 780 nm under the following conditions, and then calculating the average value. If the spectral transmittance spectrum exhibits undulations, smoothing with a delta of 5.0 nm may be performed.

[0129] (Measurement conditions) ·Wavelength range: 300nm~780nm Scan speed: Fast Slit width: 2.0 Sampling interval: Auto (0.5 nm interval) ·Lighting:C Light source: D2 and WI ·Field of view: 2° Light source switching wavelength: 360nm S / R switching: Standard Detector: PM Autozero: Performed at 550 nm after baseline scan

[0130] In order to adjust the yellow index (YI) of the optical film, for example, a blue dye, which is the complementary color of yellow, may be contained in at least one of the polyester film as the base material, the functional layer, the resin layer A, and the resin layer B.

[0131] The blue pigment may be either a pigment or a dye. However, for example, when the optical film is used in an organic light-emitting diode display device, a pigment that combines light resistance and heat resistance is preferred. Polycyclic organic pigments and metal complex organic pigments, etc., are preferred as blue pigments for applications requiring light resistance, because they are less susceptible to molecular cleavage by ultraviolet rays and have significantly better light resistance than molecularly dispersed dyes. More specifically, phthalocyanine organic pigments are preferred. However, because pigments are dispersed in a solvent, transparency is hindered by particle scattering. Therefore, it is preferable to set the particle size of the pigment dispersion within the Rayleigh scattering region. On the other hand, when the transparency of the optical film is important, it is preferable to use a dye that molecularly disperses in a solvent as the blue pigment. Alternatively, an inorganic pigment such as cobalt blue may be used as the blue pigment.

[0132] The optical film is irradiated with light having a continuous spectrum in the wavelength range of 300 nm to 780 nm from the functional layer side at an incident angle of 0°, and the L of the light that passes through the optical film (transmitted light) is measured. * a * b * Color coordinate a of the color system * , b * When we asked for a * is between -3.0 and 2.0, and b * is preferably -2.0 or more and 8.0 or less. * and b * When each of a and b is within the above range, the yellow index can be set to 15 or less. * and b *The measurement can be carried out using a spectrophotometer (product name "UV-2450", manufactured by Shimadzu Corporation). The light source can be a tungsten halogen (WI) lamp alone, or a combination of a deuterium (D2) lamp and a tungsten halogen (WI) lamp. In this specification, "light at an incident angle of 0°" means light in the normal direction when the normal direction to the first surface of the optical film is set to 0°. "L * a * b * "Color system", "a * " and "b * " conforms to JIS Z8729:2004.

[0133] The optical film preferably has a spectral transmittance of 8% or less at a wavelength of 380 nm. If the spectral transmittance of the optical film exceeds 8%, when the optical film is used in a mobile terminal, the polarizer may be exposed to ultraviolet light and easily deteriorate. The transmittance can be measured in the same manner as in the case of the aforementioned highly adhesive polyester film for foldable displays.

[0134] The total light transmittance of the optical film is preferably 85% or more. If the total light transmittance of the optical film is 85% or more, sufficient image visibility can be obtained when the optical film is used in a mobile terminal. The total light transmittance of the optical film is more preferably 87% or more, and most preferably 90% or more.

[0135] The total light transmittance of the optical film can be measured in the same manner as in the case of the above-mentioned highly adhesive polyester film for a foldable display.

[0136] The haze value (total haze value) of the optical film is preferably 1.0% or less. If the haze value of the optical film is 1.0% or less, whitening of the image display surface can be suppressed when the optical film is used in a mobile terminal. The haze value is more preferably 0.7% or less, and even more preferably 0.5% or less.

[0137] The haze value can be measured in the same manner as in the case of the highly adhesive polyester film for a foldable display described above.

[0138] If another film, such as a polarizing plate, is attached to the first surface of the optical film via a pressure-sensitive adhesive layer or adhesive layer, the other film must be peeled off together with the pressure-sensitive adhesive layer before performing the folding test, folding retention test, yellow index measurement, total light transmittance measurement, and haze measurement. The peeling of the other film can be performed, for example, as follows: First, a laminate in which the other film is attached to the optical film via a pressure-sensitive adhesive layer or adhesive layer is heated with a dryer, and the blade of a cutter is inserted into the area believed to be the interface between the optical film and the other film, and the film is slowly peeled off. By repeating this heating and peeling process, the pressure-sensitive adhesive layer, adhesive layer, and other film can be peeled off. Note that the presence of such a peeling step does not significantly affect these tests or measurements. The haze measurement should be performed after the pressure-sensitive adhesive layer or adhesive layer has been peeled off and any dirt on the pressure-sensitive adhesive layer or adhesive layer has been thoroughly wiped off with alcohol.

[0139] In one aspect, the present invention provides a method for producing an optical film, comprising: A functional layer is produced on a first surface side of a polyester film as a substrate by a coating process. A method for producing an optical film is provided.

[0140] In one aspect, the present invention provides a foldable image display device comprising a display element and an optical film according to the present invention, the display element being an organic light-emitting diode element. [Example]

[0141] To explain the present invention in detail, the following examples are provided, but the present invention is not limited to these examples. The "values ​​calculated based on 100% solids" below refer to values ​​when the solids content in the solvent-diluted product is taken as 100%. The composition ratios are expressed as percentages of the solid mass of each of the resin and crosslinking agent relative to the sum of the solid masses of the resin and crosslinking agent.

[0142] The refractive index of the functional layer and various resin layers was determined by applying a functional layer composition and a resin layer composition to a 50 μm thick PET sheet without any adhesive treatment, respectively, to form a cured film having a thickness of 1 to 10 μm. To prevent backside reflection, black vinyl tape (e.g., Yamato Vinyl Tape No. 200-38-21, 38 mm wide) with a width larger than the measurement spot area was applied to the side of the PET sheet that was not coated with the functional layer composition or resin layer composition (back side). The average reflectance at wavelengths of 380 to 780 nm was measured using a spectrophotometer (product name "UV-2450", manufactured by Shimadzu Corporation). The average reflectance thus obtained was used to calculate the refractive index according to the above formula (2).

[0143] The thickness of each layer was measured by photographing the cross section of the optical film using a scanning transmission electron microscope (STEM) (product name "S-4800" manufactured by Hitachi High-Technologies Corporation). The thickness of each layer was measured at 20 locations on the cross-sectional image, and the arithmetic mean value of the thicknesses at those 20 locations was used. Cross-sectional photographs of the optical film were taken as follows. First, a 1 mm × 10 mm piece of optical film was embedded in an embedding resin to prepare a block. From this block, uniform, hole-free sections with a thickness of 70 nm to 100 nm were cut using a standard sectioning method. Sections were prepared using an "Ultramicrotome EM UC7" (Leica Microsystems Inc.) or similar. These uniform, hole-free sections were used as measurement samples. Cross-sectional photographs of the measurement samples were then taken using a scanning transmission electron microscope (STEM). When taking these cross-sectional photographs, STEM observation was performed using the detector set to "TE," the accelerating voltage set to "30 kV," and the emission current set to "10 μA." The magnification was adjusted appropriately between 5,000x and 200,000x while adjusting the focus and observing whether the contrast and brightness could be distinguished between the individual layers. When taking cross-sectional photographs, the aperture was set to "Beam monitor aperture 3," the objective lens aperture to "3," and the WD to "8 mm."

[0144] In the present invention, various physical properties can be evaluated according to the following criteria.

[0145] <Interference fringe evaluation> The optical films according to the examples and comparative examples were evaluated for the presence or absence of interference fringes. Specifically, a black acrylic plate was attached to the back surface of each optical film via a transparent adhesive to prevent back surface reflection, and light was irradiated onto each optical film from the front surface side, and the presence or absence of interference fringes was visually observed. A three-wavelength fluorescent lamp was used as the light source. The occurrence of interference fringes was evaluated according to the following criteria. ○: No interference fringes were observed. △: A small amount of interference fringes was observed, but it was at a level that would not cause any problems in practical use. ×: Interference fringes were clearly observed.

[0146] <Pencil hardness> The pencil hardness of the surface of the optical films according to the examples and comparative examples was measured according to JIS K5600-5-4:1999. The pencil hardness test was carried out by fixing a 30 mm × 100 mm piece of optical film cut onto a glass plate with Cellotape (registered trademark) manufactured by Nichiban Co., Ltd. to prevent folds or wrinkles, and then using a pencil hardness tester (product name "Pencil Scratch Coating Hardness Tester (electric)" manufactured by Toyo Seiki Seisaku-sho Co., Ltd.) to move a pencil (product name "Uni" manufactured by Mitsubishi Pencil Co., Ltd.) against the surface of the optical film at a speed of 1 mm / sec while applying a load of 750 g to the pencil. The pencil hardness was determined as the highest hardness that did not scratch the surface of the optical film in the pencil hardness test. When measuring the pencil hardness, a plurality of pencils with different hardnesses are used, and the pencil hardness test is performed five times for each pencil. If no scratches are visually observed on the surface of the optical film when the surface of the optical film is observed through a fluorescent lamp four or more times out of the five times, it is determined that the pencil of that hardness did not scratch the surface of the optical film. Note that the optical film used was the optical film before the durability test.

[0147] <Yellow Index (YI)> The yellow index was measured for each of the optical films according to the examples and comparative examples. Specifically, an optical film cut to a size of 50 mm x 50 mm was placed in a spectrophotometer (product name "UV-2450," manufactured by Shimadzu Corporation; light source: tungsten lamp and deuterium lamp) with the substrate side of the optical film facing the light source. The optical film was free of defects (contamination of foreign matter), cracks, wrinkles, and stains, and was held in the spectrophotometer in a flat, curled state. In this state, transmittance was measured at least five points within 1 nm of each wavelength from 300 nm to 780 nm under the following measurement conditions, and the average value was calculated. The transmittance measurement data was then read on a monitor connected to the UV-2450, and the YI was obtained by checking "YI" in the calculation section. The optical film used was the optical film before the durability test. (Measurement conditions) ·Wavelength range: 300nm~780nm Scan speed: Fast Slit width: 2.0 Sampling interval: Auto (0.5 nm interval) ·Lighting:C Light source: D2 and WI ·Field of view: 2° Light source switching wavelength: 360nm S / R switching: Standard Detector: PM Autozero: Performed at 550 nm after baseline scan

[0148] <Total light transmittance measurement> The total light transmittance and haze value of the optical films according to the examples and comparative examples were measured using a haze meter (product name "HM-150", manufactured by Murakami Color Research Laboratory Co., Ltd.) in accordance with JIS K7361-1:1997. The total light transmittance was measured three times for each optical film, after which the optical film was cut into a size of 50 mm x 100 mm, and placed in a state free of curls, wrinkles, fingerprints, dust, etc., with the hard coat layer side facing away from the light source. The arithmetic mean value of the values ​​obtained from the three measurements was used.

[0149] <90° peel strength> A pressure-sensitive adhesive composition, which will be described later, was applied to the side of resin layer B opposite to the substrate to form a pressure-sensitive adhesive layer. The adhesive layer of a test piece having a substrate, resin layer B, and adhesive layer in this order was bonded to a 12.5 μm thick polyimide film (Apical (registered trademark), manufactured by Kaneka Corporation), and the resulting adhesive film (B-stage product) was bonded to a 12.5 μm thick polyimide film (PI) or an 18 μm thick rolled copper foil (BHY series, manufactured by JX Nippon Mining Corporation) (Cu). The adhesive layer was bonded to the shiny surface of the rolled copper foil at 160°C and 40 kgf / cm 2The adhesive was then pressed under pressure for 30 seconds to bond the film. The film was then cured by heat treatment at 200°C for 1 hour to obtain a sample for peel strength evaluation. The peel strength was measured by a 90° peel test at 25°C, with the film pulled at a tensile speed of 50 mm / min. This test indicates the adhesive strength at room temperature. <Failure mode evaluation> The following measurements were carried out on a test piece having an easy-adhesion polyester film for a foldable display, which was composed of a functional layer provided on the side of resin layer A opposite to the substrate side, and a pressure-sensitive adhesive layer provided on the side of resin layer B opposite to the substrate side. The pressure-sensitive adhesive layer was protected with a known polyester film and the experiment was carried out. The optical film was repeatedly folded 10,000 times by 180° so that the functional layer was on the inside and the distance between the opposing sides of the optical film was 10 mm. The optical film was repeatedly folded 10,000 times by 180° so that the functional layer was on the outside and the distance between the opposing sides of the optical film was 30 mm. (evaluation) 〇: In both the folding test where the functional layer is on the inside and the folding test where the functional layer is on the outside, No cracks or breaks occurred in the optical film. △: In either the folding test where the functional layer was on the inside or the folding test where the functional layer was on the outside As a result, cracks or breaks were observed in the optical film. ×: In both the folding test in which the functional layer is on the inside and the folding test in which the functional layer is on the outside, Cracks or breaks were observed in the optical film.

[0150] [Polyurethane resin] Synthesis of polyurethane resin (PCPU-1): A four-neck flask equipped with a stirrer, Dimroth condenser, nitrogen inlet tube, silica gel drying tube, and thermometer was charged with 29.4 parts by weight of dicyclohexylmethane-4,4'-diisocyanate, 63.6 parts by weight of a polycarbonate diol with a melting point of 33°C, mainly composed of 1,5-pentanediol / 1,6-hexanediol (45 / 55 molar ratio) with a number average molecular weight of 1000, 7 parts by weight of dimethylolpropionic acid, and 200 parts by weight of ethyl methyl ketone as a solvent, and stirred for 3 hours at 75°C under a nitrogen atmosphere. The infrared spectrum of the reaction solution was measured to confirm the disappearance of isocyanate groups in the reaction solution. Next, the solution was cooled to room temperature, and 8.2 parts by weight of triethylamine was added to obtain a polyurethane resin (PCPU-1) solution with a solids content of 50.0% by weight.

[0151] Preparation of aqueous dispersion (PCPU-1WD) of polyurethane resin (PCPU-1): A predetermined amount 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. The mixture was stirred for 2000 min. -1 While stirring and mixing at 50°C, the polyurethane resin (PCPU-1) solution was gradually added to disperse it in water. The solvent, ethyl methyl ketone, was then removed under reduced pressure. The concentration was adjusted with water to prepare a water dispersion (PCPU-1WD) of polycarbonate polyurethane resin (PCPU-1) with a solids content of 35.0% by mass.

[0152] Synthesis of polyurethane resin (PCPU-2) and preparation of aqueous dispersion (PCPU-2WD): The main raw material of polycarbonate diol was changed to 1,4-cyclohexanedimethanol / 1,6-hexanediol (=75 / 25 (molar ratio)) instead of 1,4-butanediol / 1,6-hexanediol (=75 / 25 (molar ratio)), except that (PCPU-2) was prepared in the same manner as the synthesis of (PCPU-1) above, and an aqueous dispersion (PCPU-2WD) was obtained in the same manner as the preparation of (PCPU-1WD). The proportion (content) of cyclohexane ring structures in the entire polycarbonate polyurethane (PCPU-1) was 29.1% by mass, and the proportion (content) of methylene chain structures having 5 to 10 carbon atoms was 8.5% by mass.

[0153] <Reduced viscosity ηsp / c of ​​polyester resin (unit: dl / g)> 0.10 g of polyester resin was dissolved in 25 ml of a mixed solvent of phenol / tetrachloroethane (mass ratio 6 / 4), and the viscosity was measured at 30°C using an Ubbelohde viscosity tube.

[0154] <Polyester resin composition> The polyester resin was dissolved in deuterated chloroform and analyzed using a Varian Gemini-200 nuclear magnetic resonance analyzer (NMR). 1 H-NMR analysis was carried out, and the molar percentage ratio of each component was determined from the integral ratio.

[0155] <Viscosity of Polyester Resin Dispersion> The polyester resin aqueous dispersion was placed in a 140 cc glass bottle, and the viscosity was measured for 1 minute at a rotation speed of 60 rpm using a No. 1 or No. 2 rotor of a viscometer model BL (TOKIMEC INC.) in a thermostatic bath at 25°C.

[0156] [Polyester resin] Preparation of polyester resin (PEs-1): A polyester resin (PEs-1) was polymerized according to a known polymerization method. The composition of the resulting polymer was: 1H-NMR analysis was performed, and the molar percentage of each component was determined from the integral ratio. The results are shown in Table 4. The reduced viscosity of the resulting polyester resin was 0.493 dL / g. The abbreviations listed in Table 1 are listed below. TPA: Terephthalic acid IPA: Isophthalic acid NDC: Naphthalenedicarboxylic acid SA: Sebacic acid DSS: dimethyl-5-sodium sulfoisophthalate EG: Ethylene glycol HD: hexanediol DEG: Diethylene glycol NPG: Neopentyl glycol

[0157] Preparation of polyester water dispersion (PEs-1WD): A reactor equipped with a stirrer, thermometer, and reflux device was charged with 30 parts by weight of copolymer polyester resin (PEs-1) and 15 parts by weight of ethylene glycol-n-butyl ether, heated to 110°C, and stirred to dissolve the resin. After the resin was completely dissolved, 55 parts by weight of water was gradually added to the polyester solution while stirring. After the addition, the solution was cooled to room temperature while stirring, producing a milky white polyester resin (PEs-1) aqueous dispersion (PEs-1WD) with a solids content of 24.9% by weight. The liquid viscosity of the resulting aqueous dispersion was 32 mPa·s.

[0158] Preparation of polyester resin (PEs-2) and polyester water dispersion (PEs-2WD): Polyester resin (PEs-2) was produced using a known polymerization method similar to that used to produce polyester resin (PEs-1). As with PEs-1, the composition ratio was determined, and the reduced viscosity of the resulting resin was evaluated. The results are shown in Table 1. The proportion (content) of cyclohexane ring structures in the entire polyester (PEs-2) was 12.8% by mass, and the proportion (content) of methylene chain structures having 5 to 10 carbon atoms was 0.0% by mass. A polyester water dispersion (PEs-2WD) was prepared in the same manner as the polyester water dispersion (PEs-1WD) described above. The solid content and liquid viscosity were evaluated in the same manner as for PEs-1WD. The results are shown in Table 2.

[0159] Preparation of polyester water dispersion (PEs-3WD): (PEs-3WD) was prepared containing the components described in (PEs-1) except that it contained about 10% of the above NDC component.

[0160] Preparation of polyester water dispersion (PEs-4WD): An aqueous dispersion of polyester resin (PEs-4WD) was prepared in the same manner as PEs-1WD, except that it did not contain any aromatic compounds.

[0161] [Crosslinking agent] Preparation of water dispersion (C-1WD) of blocked isocyanate crosslinker (C-1): A flask equipped with a stirrer, thermometer, and reflux condenser was charged with 125.2 parts by mass of a polyisocyanate compound having a biuret structure made from hexamethylene diisocyanate (Asahi Kasei Chemicals, Duranate 24A-100, NCO concentration 23.1%), 50.0 parts by mass of dipropylene glycol dimethyl ether, and 68.8 parts by mass of 3,5-dimethylpyrazole, and the mixture was stirred at 70°C under a nitrogen atmosphere for 2 hours. The infrared spectrum of the reaction solution was then measured, and it was confirmed that the absorption of the isocyanate group had disappeared. After cooling to room temperature, 6 parts by mass of polyethylene glycol (n=12) monolaurate was added, and the mixture was stirred for 2000 min. -1 Water was added while stirring and mixing at 50°C. The concentration was adjusted with water to prepare an aqueous dispersion (C-1WD) of the blocked isocyanate crosslinking agent (C-1) with a solids content of 30.0% by mass.

[0162] <Average particle size> [Measurement method using a scanning electron microscope] The average particle size of particles present in the resin layer in the present invention can be measured by the following method: Particles are photographed with a scanning electron microscope (SEM), and the maximum diameters (the distance between the two most distant points) of 300 to 500 particles are measured at a magnification such that the size of the smallest particle is 2 to 5 mm, and the arithmetic mean of these is taken as the average particle size.

[0163] [Dynamic Light Scattering Method] The average particle size can also be determined by dynamic light scattering during particle or film production. The sol is diluted with a dispersion medium, and measurements are taken using a submicron particle analyzer N4 PLUS (Beckman Coulter) using the dispersion medium parameters. The average particle size is then calculated using the cumulant method. Dynamic light scattering observes the average particle size of the particles in the sol, and if there is particle aggregation, the average particle size of the aggregated particles is observed.

[0164] <Refractive index of particles> The refractive index of particles can be measured by the following method. After drying inorganic particles at 150°C and pulverizing them in a mortar, the fine particles were immersed in solvent 1 (which has a lower refractive index than the particles), and then solvent 2 (which has a higher refractive index than the particles) was added in small amounts until the fine particles became almost transparent. The refractive index of this solution was measured using an Abbe refractometer (Abbe refractometer manufactured by Atago Co., Ltd.). Measurements were performed at 23°C using D-line (wavelength 589 nm). Solvents 1 and 2 were selected to be miscible with each other, and examples of solvents that could be used depending on the refractive index include 1,1,1,3,3,3-hexafluoro-2-propanol, 2-propanol, chloroform, carbon tetrachloride, toluene, and glycerin.

[0165] (zirconia particles) A 3-liter glass container was charged with 2283.6 g of purified water and 403.4 g of oxalic acid dihydrate and heated to 40°C to prepare a 10.72 wt% oxalic acid aqueous solution. While stirring, 495.8 g of zirconium oxycarbonate powder (ZrOCO3, manufactured by AMR International Corp., containing 39.76 wt% ZrO2) was gradually added and mixed for 30 minutes, followed by heating at 90°C for 30 minutes. Next, 1747.2 g of a 25.0 wt% tetramethylammonium hydroxide aqueous solution (manufactured by Tama Chemicals Co., Ltd.) was gradually added over 1 hour. At this point, the mixture was in a slurry state and contained 4.0 wt% ZrO2. This slurry was transferred to a stainless steel autoclave and subjected to hydrothermal treatment at 145°C for 5 hours. The product after this hydrothermal treatment was completely solated with no undissolved matter. The resulting sol contained 4.0% ZrO2 by mass, had a pH of 6.8, and had an average particle size of 19 nm as measured by dynamic light scattering. The sol was adjusted to a ZrO2 concentration of 2.0% by mass with pure water, and the transmittance measured was 88%. Observation of the particles using a transmission electron microscope revealed that most of the particles were aggregates of ZrO2 primary particles of approximately 7 nm. 4000 g of zirconia sol with a ZrO2 concentration of 4.0% by mass obtained by the hydrothermal treatment described above was washed and concentrated using an ultrafiltration device while gradually adding pure water. 953 g of zirconia sol with a ZrO2 concentration of 13.1% by mass, pH 4.9, and a transmittance of 76% was obtained at a ZrO2 concentration of 13.1% by mass. The refractive index of the resulting zirconia-based microparticles was 1.75.

[0166] (zirconia sol) To 300 g of the zirconia sol with a ZrO2 concentration of 13.1 wt% obtained after the washing and concentration steps, 3.93 g of a 20 wt% aqueous citric acid solution and 11.0 g of a 25 wt% aqueous tetramethylammonium hydroxide solution were added, followed by further concentration using an ultrafiltration device, yielding 129 g of a high-concentration zirconia sol with a ZrO2 concentration of 30.5 wt%. The resulting high-concentration zirconia sol had a pH of 9.3 and an average particle size of 19 nm as measured by dynamic light scattering. Furthermore, this zirconia sol was free of sediment and stable for more than one month at 50°C.

[0167] <Composition 1 for resin layer A> A coating solution having the following composition was prepared. Water 42.38 parts by mass Isopropyl alcohol 29.79 parts by mass Zirconia sol 4.50 parts by mass (zirconia sol with an average particle size of 20 nm, solid content concentration of 30% by mass) Silica sol 0.87 parts by mass (Silica sol with an average particle size of 450 nm, solid content of 4% by mass) PCPU-1WD 5.62 parts by mass (Solid content concentration 35.0% by mass) PEs-1WD 11.08 parts by mass (Solid content concentration 25.1% by mass) C-1WD 5.20 parts by mass (Solid content concentration 30.0% by mass) Surfactant 0.30 parts by mass (Silicone-based, solid content 10.0% by mass) High boiling point solvent 0.26 parts by mass

[0168] <Composition 2 for resin layer A> A coating solution having the following composition was prepared. Water 40.30 parts by mass Isopropyl alcohol 29.79 parts by mass Zirconia sol 4.50 parts by mass (zirconia sol with an average particle size of 20 nm, solid content concentration of 30% by mass) Silica sol 0.87 parts by mass (Silica sol with an average particle size of 450 nm, solid content of 4% by mass) PCPU-1WD 1.31 parts by mass (Solid content concentration 35.0% by mass) PEs-1WD 17.46 parts by mass (Solid content concentration 25.1% by mass) C-1WD 5.20 parts by mass (Solid content concentration 30.0% by mass) Surfactant 0.30 parts by mass (Silicone-based, solid content 10.0% by mass) High boiling point solvent 0.26 parts by mass

[0169] <Composition 1 for resin layer B> A coating solution having the following composition was prepared. Water 52.75 parts by mass Isopropyl alcohol 30.95 parts by mass Zirconia sol 2.60 parts by mass (zirconia sol with an average particle size of 20 nm, solid content concentration of 30% by mass) Silica sol 0.50 parts by mass (Silica sol with an average particle size of 450 nm, solid content of 4% by mass) PCPU-1WD 3.25 parts by mass (Solid content concentration 35.0% by mass) PEs-1WD 6.40 parts by mass (Solid content concentration 25.1% by mass) C-1WD 3.00 parts by mass (Solid content concentration 30.0% by mass) Surfactant 0.30 parts by mass (Silicone-based, solid content 10.0% by mass) High boiling point solvent 0.26 parts by mass

[0170] <Composition 2 for resin layer B> A coating solution having the following composition was prepared. Water 51.56 parts by mass Isopropyl alcohol 30.95 parts by mass Zirconia sol 2.60 parts by mass (zirconia sol with an average particle size of 20 nm, solid content concentration of 30% by mass) Silica sol 0.50 parts by mass (Silica sol with an average particle size of 450 nm, solid content of 4% by mass) PCPU-1WD 0.76 parts by mass (Solid content concentration 35.0% by mass) PEs-1WD 10.08 parts by mass (Solid content concentration 25.1% by mass) C-1WD 3.00 parts by mass (Solid content concentration 30.0% by mass) Surfactant 0.30 parts by mass (Silicone-based, solid content 10.0% by mass) High boiling point solvent 0.26 parts by mass

[0171] <Functional layer composition 1> A coating solution having the following composition was prepared. Isopropanol 21.00 parts by mass Toluene 49.00 parts by mass Pentaerythritol triacrylate 21.38 parts by mass (Shin Nakamura Chemical A-TMM-3) Urethane acrylate 7.12 parts by mass (Mitsubishi Chemical UV-7600B) Photopolymerization initiator 1.50 parts by mass (Omnirad184 manufactured by IGM Resins BV)

[0172] <Functional layer composition 2> A coating solution having the following composition was prepared. Methyl ethyl ketone 64.40 parts by mass Dipentaerythritol hexaacrylate 27.20 parts by mass (Shin Nakamura Chemical A-DPH) Polyethylene glycol diacrylate 3.40 parts by mass (Kyoeisha Chemical Light Acrylate 9EG-A) Bisphenol A diacrylate 4.00 parts by mass (Kyoeisha Chemical Light Acrylate BP-4PA) Photopolymerization initiator 1.00 parts by mass (Omnirad184 manufactured by IGM Resins BV)

[0173] <Composition for adhesive layer> A coating solution having the following composition was prepared. Methyl isobutyl ketone 10.00 parts by mass Urethane acrylate 72.25 parts by mass (UV3310B manufactured by Nippon Synthetic Chemical Industry) Phenoxyethyl acrylate 12.75 parts by mass (Viscoat #192 manufactured by Osaka Organic Chemical Industry Co., Ltd.) Photopolymerization initiator 5.00 parts by mass (Omnirad184 manufactured by IGM Resins BV)

[0174] Example 1 A polyester film substrate with a refractive index of 1.650 and a thickness of 50 μm was prepared. Resin layer A composition 1 was applied to the first surface of the polyester substrate using a bar coater to form a coating film. The resulting coating film was then heated at 90°C for 1 minute to evaporate the solvent in the coating film, and then heated at 230°C for 1 minute to cure the coating film, forming a resin layer A-1 with a refractive index of 1.57 and a thickness of 100 nm. Functional layer composition 1 was applied to the surface of resin layer A-1 using a bar coater to form a coating film. The resulting coating film was then heated at 70°C for 1 minute to evaporate the solvent in the coating film, and then UV irradiation was performed in air using an ultraviolet irradiation device (Fusion UV Systems Japan, light source H bulb) to irradiate UV light at an integrated dose of 200 mJ / cm. 2 The coating was cured by irradiating the coating with light so that the refractive index was 1.52 and the thickness was 5 μm, forming a functional layer-1. Next, resin layer B composition 1 was applied with a bar coater to the surface opposite to the applied functional layer-1, i.e., the second surface of the polyester substrate, to form a coating. The formed coating was then heated at 90°C for 1 minute to evaporate the solvent in the coating, and then heated at 230°C for 1 minute to cure the coating, forming a resin layer B-1 with a refractive index of 1.57 and a thickness of 20 nm. Furthermore, a composition for adhesive layer was applied to the surface of the resin layer B-1 using a bar coater to form a coating film. The formed coating film was then heated at 70°C for 1 minute to evaporate the solvent in the coating film, and then ultraviolet light was irradiated in air using an ultraviolet irradiation device (manufactured by Fusion UV Systems Japan, light source H bulb) at an integrated light intensity of 1200mJ / cm. 2The coating was cured by irradiating it to a temperature of 1.50, forming a resin layer made of a urethane resin with a refractive index of 1.50 and a thickness of 50 μm. This produced an optical film in which resin layer A was adjacent to the functional layer and resin layer B was adjacent to the adhesive layer, separated by a polyester film as the base. The various evaluation results of the obtained optical film are shown in Table 3.

[0175] <Examples 2 to 9> As shown in Table 3, optical films were produced in the same manner as in Example 1, except that the various components or the thickness of resin layer B were changed. The evaluation results of the obtained optical films are as shown in Table 3.

[0176] <Comparative Example 1> An optical film was produced in the same manner as in Example 1, except that the resin layer B did not contain the polyester resin according to the present invention. The evaluation results of the obtained optical film are shown in Table 3.

[0177] <Comparative Example 2> An optical film was produced in the same manner as in Example 1, except that the thickness of the resin layer B was set to 50 nm. The results of various evaluations of the obtained optical film are shown in Table 3.

[0178] <Comparative Example 3> An optical film was produced in the same manner as in Example 1, except that the resin layer B did not contain a polyester resin having an aromatic ring. The evaluation results of the obtained optical film are as shown in Table 3.

[0179] <Comparative Example 4> An optical film was produced in the same manner as in Example 1, except that the resin layer B did not contain an isocyanate crosslinking agent. Table 3 shows the results of various evaluations of the obtained optical film.

[0180] [Table 1]

[0181] [Table 2]

[0182] [Table 3] [Industrial Applicability]

[0183] The optical film of the present invention is suitable for use as a foldable optical film, has high adhesion reliability between the functional layer and the substrate, and is excellent in optical interference. It also has excellent reliability as an image display device, and can be widely used in optical applications, etc.

Claims

1. A foldable, easily adhesive polyester film for a foldable display used in an image display device, The substrate of the highly adhesive polyester film is made of a polyester film, a resin layer A on the first surface side of the substrate; a resin layer B on a second surface side of the base material opposite to the first surface, The resin layer B has a thickness of less than 30 nm, The resin layer B is formed from a composition containing a polyester resin having a condensed polycyclic aromatic group and an isocyanate crosslinking agent.

2. The highly adhesive polyester film for a foldable display according to claim 1, wherein the polyester resin used in the resin layer B has a naphthalene skeleton in the molecule.

3. The highly adhesive polyester film for a foldable display according to claim 1, wherein the isocyanate crosslinking agent used in the resin layer B is a pyrazole-type blocked isocyanate.

4. The highly adhesive polyester film for a foldable display according to claim 1, wherein the resin layer A has a thickness of 10 nm or more and 200 nm or less.

5. An optical film having the highly adhesive polyester film for a foldable display according to claim 1, a functional layer provided on the resin layer A on the side opposite to the substrate; a pressure-sensitive adhesive layer provided on the resin layer B on the opposite side to the substrate, The optical film is characterized in that no cracks or breaks occur when a test in which the optical film is folded 180° with the functional layer facing inside and the distance between opposing sides of the optical film being 10 mm is repeated 10,000 times. Optical film.

6. The optical film described in [5], wherein the optical film does not crack or break when subjected to a test in which the optical film is folded 180° so that the functional layer is on the outside and the distance between opposing sides of the optical film is 30 mm, repeated 10,000 times.

7. The optical film according to claim 5 , having a yellow index of 15 or less.

8. A method for producing the optical film according to claim 5, and producing a functional layer on the first surface side of the polyester film as the substrate by a coating step. A method for manufacturing an optical film.

9. A foldable image display device comprising: a display element; and the optical film according to claim 5, which is disposed closer to a viewer than the display element.

10. The image display device according to claim 9 , wherein the display element is an organic light-emitting diode element.

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