Polyester film for surface protection of flexible displays

Abstract

To provide an inexpensive polyester film that achieves high pencil hardness when a hard coat layer is laminated onto it. [Solution] A polyester film for surface protection of a flexible display, having a thickness of 25-100 μm and an indentation depth of 1.4 μm or less after unloading the test force. A hard coat film for surface protection of a flexible display using the polyester film for surface protection of a flexible display, and a display body thereof.

Description

[Technical Field]

[0001] The present invention relates to a polyester film, and more particularly to a polyester film that can obtain high pencil hardness when a hard coat layer or the like is laminated onto it. [Background technology]

[0002] Display devices and mobile terminals such as smartphones are becoming thinner and lighter. Currently, all of these devices use rigid housings, so tempered glass is generally used to protect the display. On the other hand, in recent years, displays that prioritize design have become popular. For example, displays with special shapes such as circles are popular. In particular, in recent years, there has been a demand for flexible displays that can be installed on curved surfaces.

[0003] However, since glass cannot be used in flexible displays, acrylic sheets have been used, but thin acrylic sheets have problems with being susceptible to scratches and impacts. Polycarbonate, which is highly impact-resistant and light-resistant, is sometimes used, but it is expensive and has the problem of low pencil hardness. On the other hand, polyester film is inexpensive and has excellent impact resistance, but it has the problem of insufficient pencil hardness.

[0004] To achieve these goals, Patent Document 1 proposes a hard coat material that is both hard and flexible. However, this material is expensive and lacks versatility, and also suffers from poor surface scratch resistance.

[0005] Furthermore, as described in Patent Document 2, a transparent polyimide material has been proposed that can achieve high pencil hardness without relying on a hard coat material. However, this material is very expensive and presents problems in terms of mass production. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2017-008148 [Patent Document 2] Japanese Patent Publication No. 2016-222797 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The present invention aims to solve the problems of transparent films described above, and to provide an inexpensive polyester film that has high pencil hardness when a hard coat layer is laminated onto it. [Means for solving the problem]

[0008] In other words, the present invention consists of the following configuration. 1. A polyester film for surface protection of flexible displays, having a thickness of 25-100 μm and an indentation depth of 1.4 μm or less after unloading the test force. 2. A polyester film for surface protection of a flexible display according to the first description above, wherein the surface orientation coefficient ΔP is 0.160 or greater. 3. A polyester film for a surface protective film of a flexible display according to claim 1 or 2, wherein the biaxially oriented polyester film has an easy-adhesion layer on at least one surface of the biaxially oriented polyester film. 4. A hard coat film for a surface protective film of a flexible display, having a hard coat layer on at least one surface of a polyester film for a surface protective film of a flexible display as described in any of the first to third above. 5. A display unit using the hard coat film for surface protection of the flexible display described in item 4 above as the surface protection film. [Effects of the Invention]

[0009] The polyester film of the present invention, when laminated with a hard coat layer or the like, exhibits higher surface hardness than general polyester films. By using the hard coat film as a surface protective film for the display, it becomes possible to provide a display that is highly resistant to scratches, impacts, and deformation on the display surface, while also offering excellent design. This display can provide beautiful images. [Modes for carrying out the invention]

[0010] (Display body) In this invention, the term "display body" refers to display devices in general. Examples of display types include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs, but LCDs, organic EL displays, and inorganic EL displays, which have a flexible structure, are preferred. Organic EL displays, which have fewer layers and a wider color gamut, are even more preferred.

[0011] The display body can take the form of large screens, displays, digital signage, flexible mobile terminals, PCs, etc., and can be used without limitation. In particular, it is preferable to use it in touch panel displays, digital signage, flexible mobile terminals, PCs, etc., which have a touch panel function that allows for direct touch operation. Furthermore, it is especially preferable that the display body be a flexible display as described below.

[0012] (Flexible display) Flexible displays are those in which a single, continuous display can be bent according to its intended use, and ideally, they should also be thin and lightweight. Examples include foldable displays, rollable displays, and displays that can be used like paper.

[0013] (Polyester film) The transparent film includes films with high light transmittance and low haze, such as PI film, polyester film, polycarbonate film, acrylic film, TAC film, cycloolefin polymer film, etc. Among them, a polyester film with high impact resistance and appropriate pencil hardness is preferred, and a polyethylene terephthalate film that can be manufactured at a low cost is particularly preferred.

[0014] In the present invention, the polyester film may be a single-layer film composed of one or more polyester resins. When using two or more types of polyesters, it may also be a multilayer structure film or a super-multilayer laminated film with a repeating structure.

[0015] Examples of the polyester resin include polyester films composed of polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or copolymers mainly composed of the constituent components of these resins. Among them, a biaxially stretched polyethylene terephthalate film is particularly preferred in terms of mechanical properties, heat resistance, transparency, price, etc.

[0016] When using a polyester copolymer for the base film, examples of the dicarboxylic acid component of the polyester include aliphatic dicarboxylic acids such as adipic acid and sebacic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; and polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid. Examples of the glycol component include fatty acid glycols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, and neopentyl glycol; aromatic glycols such as p-xylene glycol; alicyclic glycols such as 1,4-cyclohexanedimethanol; and polyethylene glycol with an average molecular weight of 150 to 20,000. The mass ratio of the copolymer components of the preferred copolymer is less than 20% by mass. When it is less than 20% by mass, it is preferable because the film strength, transparency, and heat resistance are maintained.

[0017] In the production of the polyester film, the intrinsic viscosity of at least one type of resin pellet is preferably in the range of 0.50 to 1.0 dl / g. When the intrinsic viscosity is 0.55 dl / g or more, it is preferable because the pencil hardness after hard coat processing is improved. More preferably, it is 0.58 dl / g or more, and particularly preferably 0.63 dl / g or more. On the other hand, when the intrinsic viscosity is 1.00 dl / g or less, the filtration pressure increase of the molten fluid does not become too large, and it is preferable for stable film production operation.

[0018] Regardless of whether the film is a single-layer structure or a laminated structure, the intrinsic viscosity of the film is preferably 0.50 dl / g or more. As the molecular weight of the polyester resin increases, the elastic behavior of the film is improved, and deformation can be effectively suppressed against the load of the pencil during pencil hardness measurement, which is preferable. More preferably, it is 0.55 dl / g or more, and particularly preferably 0.63 dl / g or more. On the other hand, although the pencil hardness also improves as the intrinsic viscosity increases, there is a concern that productivity may decrease. Therefore, when the intrinsic viscosity is 1.00 dl / g or less, it is preferable because it can be manufactured with good operability.

[0019] The thickness of the polyester film is preferably 25 to 100 μm, and more preferably 30 to 75 μm. When the thickness is 25 μm or more, handling properties and impact resistance are improved, which is preferable. When the thickness is 100 μm or less, it is advantageous for weight reduction, and it is also preferable because of excellent flexibility and processability.

[0020] The surface of the polyester film of the present invention may be smooth or may have irregularities. However, since it is used for the purpose of a protective film on the surface of the display body, a decrease in optical properties due to irregularities is not preferable. As haze, 3% or less is preferable, 2% or less is more preferable, and 1% or less is most preferable. If the haze is 3% or less, the visibility of the image can be improved. The lower limit of haze is preferably as small as possible, but it may be 0.1% or more, or 0.3% or more.

[0021] As mentioned above, for the purpose of reducing haze, it is preferable to have less surface irregularity on the film. However, in order to provide a certain degree of slipperiness from a handling perspective, irregularities can be formed by compounding fillers into the surface polyester resin layer or by coating with a filler-containing coating layer during the film formation process.

[0022] Known methods can be used to incorporate the particles into the base film. For example, they can be added at any stage in the production of polyester, but preferably, they can be added as a slurry dispersed in ethylene glycol or the like at the esterification stage, or after the transesterification reaction is complete but before the polycondensation reaction begins, to allow the polycondensation reaction to proceed. Alternatively, this can be done by using a vented kneading extruder to blend a slurry of particles dispersed in ethylene glycol or water with the polyester raw material, or by using a kneading extruder to blend dried particles with the polyester raw material.

[0023] In particular, a method is preferred in which aggregated inorganic particles are homogeneously dispersed in a monomer liquid that will become part of the polyester raw material, and then the filtered solution is added to the remaining polyester raw material before, during, or after the esterification reaction. With this method, since the monomer liquid has low viscosity, homogeneous dispersion of particles and high-precision filtration of the slurry can be easily performed, and when added to the remaining raw material, the particles dispersibility is good and new aggregates are less likely to be formed. From this viewpoint, it is especially preferable to add it to the remaining raw material in a low-temperature state before the esterification reaction.

[0024] Furthermore, by obtaining a polyester containing particles beforehand and then kneading and extruding the resulting pellets with pellets that do not contain particles (masterbatch method), the number of protrusions on the film surface can be further reduced.

[0025] Furthermore, the polyester film may contain various additives, within a range that maintains a desirable range of total light transmittance. Examples of additives include antistatic agents, UV absorbers, and stabilizers.

[0026] The total light transmittance of the polyester film is preferably 85% or higher, and more preferably 87% or higher. A transmittance of 85% or higher is sufficient to ensure adequate visibility. While a higher total light transmittance of the polyester film is desirable, it is acceptable even if it is 99% or lower, or even 97% or lower.

[0027] The surface of the polyester film of the present invention can be treated to improve adhesion with a resin that forms a hard coat layer or the like.

[0028] Surface treatment methods include, for example, sandblasting, solvent treatment to create uneven surfaces, and oxidation treatments such as corona discharge, electron beam irradiation, plasma treatment, ozone / ultraviolet irradiation, flame treatment, chromic acid treatment, and hot air treatment, and can be used without any particular limitations.

[0029] Furthermore, adhesion can be improved by using adhesion-enhancing layers such as an easy-adhesion layer. The easy-adhesion layer can be made of any material, including acrylic resin, polyester resin, polyurethane resin, and polyether resin, and can be formed using a general coating method, preferably a so-called in-line coating formulation.

[0030] The polyester film described above can be manufactured, for example, through a polymerization step in which inorganic particles are homogeneously dispersed in a monomer liquid that will be part of the polyester raw material, filtered, and then added to the remainder of the polyester raw material to polymerize the polyester; and a film forming step in which the polyester is melt-extruded into a sheet through a filter, cooled, and then stretched to form a base film.

[0031] Next, we will explain in detail a method for manufacturing biaxially oriented polyester film, using polyethylene terephthalate (PET) pellets as the raw material for the base film, but this is not the only method. Furthermore, the number of layers (single-layer, multi-layer, etc.) is not limited.

[0032] PET pellets are mixed in a predetermined ratio, dried, and then supplied to a known molten lamination extruder. The pellets are extruded through a slit-shaped die into a sheet, and then cooled and solidified on a casting roll to form an unstretched film. For single-layer films, one extruder is sufficient. However, when manufacturing multi-layer films, two or more extruders, two or more manifolds or confluence blocks (for example, confluence blocks with a rectangular confluence section) are used to laminate multiple film layers constituting each outermost layer. Two or more sheets are then extruded from a die and cooled on a casting roll to form an unstretched film.

[0033] In this case, during melt extrusion, it is preferable to perform high-precision filtration to remove foreign matter contained in the resin at any location where the molten resin is maintained at approximately 280°C. The filter material used for high-precision filtration of the molten resin is not particularly limited, but a stainless steel sintered body filter material is preferred because it has excellent performance in removing aggregates mainly composed of Si, Ti, Sb, Ge, and Cu, as well as high-melting-point organic matter.

[0034] Furthermore, the filtration particle size of the filter media (initial filtration efficiency 95%) is preferably 20 μm or less, and particularly preferably 15 μm or less. If the filtration particle size of the filter media (initial filtration efficiency 95%) exceeds 20 μm, foreign matter larger than 20 μm cannot be sufficiently removed. Although high-precision filtration of molten resin using a filter media with a filtration particle size (initial filtration efficiency 95%) of 20 μm or less may reduce productivity, it is preferable for obtaining a film with fewer protrusions caused by coarse particles.

[0035] Specifically, for example, PET pellets are thoroughly vacuum-dried, then fed into an extruder, melted and extruded into a sheet at approximately 280°C, and cooled and solidified to form an unstretched PET sheet. The obtained unstretched sheet is stretched 3.1 to 5.0 times in the longitudinal direction on a roll heated to 80 to 120°C to obtain a uniaxially oriented PET film. Furthermore, the ends of the film are gripped with clips and guided into a hot air zone heated to 80 to 180°C, and after drying, stretched 2.5 to 5.0 times in the width direction. Subsequently, it is guided into a heat treatment zone at 160 to 250°C and heat-treated for 1 to 60 seconds to complete crystal orientation. During this heat treatment process, a relaxation treatment of 1 to 12% in the width direction or longitudinal direction may be applied as needed.

[0036] (Degree of crystallinity) The degree of crystallinity is mainly determined by the amount of heat applied to the film in the heat treatment zone. The amount of heat is determined by various conditions such as the film-making equipment conditions, film-making speed, and film thickness, so the degree of crystallinity is not determined solely by the heat treatment temperature. However, a heat treatment temperature of 160 to 250°C is preferable, and 200 to 240°C is even more preferable. A temperature of 160°C or higher is preferable because it can promote crystallinity. Heat treatment at 250°C or lower is preferable because it can suppress the decrease in crystallinity due to the remelting of polyester.

[0037] To ensure sufficient pencil hardness after hard coating, it is preferable that the crystallinity of the film by density method be 48% or higher, and that the crystallinity of the film surface by ATR method be 1.20 or higher.

[0038] The degree of crystallinity of the polyester film obtained by the density method is preferably 48% or higher, more preferably 50% or higher, and even more preferably 52% or higher. While a high degree of crystallinity is preferable, the upper limit for the film that can be manufactured is approximately 65%.

[0039] The crystallinity of the film surface produced by the ATR method is preferably 1.20 or higher, more preferably 1.25 or higher, and even more preferably 1.27 or higher. While a high crystallinity of the film surface produced by the ATR method is also preferable, the upper limit of the film that can be manufactured is approximately 3.0.

[0040] The indentation depth after unloading the test force in the film thickness direction using a dynamic ultramicro hardness tester is preferably 1.4 μm or less, more preferably 1.35 μm or less, and even more preferably 1.30 μm or less. By lowering the indentation depth after unloading the test force (the final amount of deformation under load), deformation after pencil hardness evaluation can be suppressed, and the pencil hardness can be improved.

[0041] The surface orientation ΔP is preferably 0.160 or higher, more preferably 0.163 or higher, and even more preferably 0.165 or higher. Increasing the surface orientation makes the film stronger against loads in the thickness direction and suppresses the amount of deformation during unloading. While a higher value is preferable, the upper limit of the film that can be manufactured is approximately 0.170.

[0042] The area stretching ratio is preferably 12.5 times or more, more preferably 13 times or more, and even more preferably 14 times or more. Increasing the area stretching ratio can strengthen the orientation of the film in the planar direction, thereby increasing ΔP.

[0043] The stretching method is not particularly limited and the film can be formed using simultaneous biaxial stretching, sequential biaxial stretching, or any other method.

[0044] (Easy adhesion layer) The easy-adhesion layer can be obtained by applying the coating liquid to one or both sides of an unstretched or uniaxially oriented film in the longitudinal direction, drying it at 75-150°C, and then stretching it in one or two directions. The final amount of the easy-adhesion layer applied is 0.05-0.20 g / m². 2 It is preferable to manage the amount to 0.05 g / m². 2 The above is preferable as it satisfies the adhesive requirements. On the other hand, the application amount is 0.20 g / m². 2The following conditions are preferable as they provide resistance to blocking.

[0045] The resin used in the easy-adhesion layer can be any resin without particular limitation, such as polyester resins, polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, or acrylic resins. Examples of crosslinking agents for the easy-adhesion layer include melamine-based, isocyanate-based, oxazoline-based, and epoxy-based crosslinking agents. Two or more types can also be mixed and used. Due to the nature of inline coatings, these are preferably applied with a water-based coating solution, and the aforementioned resins and crosslinking agents are preferably water-soluble or water-dispersible resins or compounds.

[0046] It is preferable to add particles to the easy-adhesion layer to provide slipperiness. The average particle size of the fine particles is preferably 2 μm or less. If the average particle size exceeds 2 μm, the particles tend to fall off the easy-adhesion layer. Examples of particles to be included in the easy-adhesion layer include inorganic particles such as titanium dioxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, and calcium fluoride, as well as organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles. These may be added to the easy-adhesion layer individually, or two or more may be added in combination.

[0047] Furthermore, known methods similar to those used for the coating layer can be used for applying the coating solution. Examples include the reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife coating method, wire bar coating method, and pipe doctor method, and these methods can be used individually or in combination.

[0048] (Hard coat layer) The resin used to form the hard coat layer is not particularly limited and can be any resin such as (meth)acrylic acid ester, siloxane-based, inorganic hybrid, urethane acrylate-based, polyester acrylate-based, or epoxy-based resin. Furthermore, two or more materials can be mixed and used, and particles such as inorganic fillers or organic fillers can be added. Among these, (meth)acrylic acid ester and acrylate-based resins are preferred, and resins having three or more reactive groups in the molecule are preferred as essential components.

[0049] (Resin having 3 or more reactive groups) Examples of resins having three or more reactive groups include tris[(meth)acryloyloxyalkyl]isocyanurates such as tris(2-acryloyloxyethyl) isocyanurate, tris(3-acryloyloxypropyl) isocyanurate, tris(2-methacryloyloxyethyl) isocyanurate, and tris(3-methacryloyloxypropyl) isocyanurate, as well as trimethylolpropane tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, and pr Examples of (meth)acrylates include hopionic acid-modified dipentaerythritol tri(meth)acrylate, pentaerythritol tri(meth)acrylate, propylene oxide-modified trimethylolpropane tri(meth)acrylate, dipentaerythritol tetra(meth)acrylate, propionic acid-modified dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and caprolactone-modified dipentaerythritol hexa(meth)acrylate. These may be used individually or in combination of two or more.

[0050] In addition, commercially available hard coating agents can also be used. For example, Arakawa Chemical Industries' Beamset® series, Aica Industrial Co., Ltd.'s Aica Aitron® series, and Toyo Ink Co., Ltd.'s Lioduras® series can be used without any particular limitations, and they can also be used in mixture with other (meth)acrylic acid ester compounds.

[0051] (Photopolymerization initiator) When curing the hard coat layer of the present invention using ultraviolet light, it is necessary to add a photopolymerization initiator. Specific examples of photopolymerization initiators include benzophenone, acetophenone, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, benzoin benzoic acid, benzoin methyl benzoate, benzoin dimethyl ketal, 2,4-diethylthioxanthone, 1-hydroxycyclohexylphenyl ketone, benzyl diphenyl sulfide, tetramethylthiuram monosulfide, azobisisobutyronitrile, benzyl, dibenzyl, diacetyl, β-chloranthraquinone, (2,4,6-trimethylbenzyldiphenyl)phosphine oxide, and 2-benzothiazole-N,N-diethyldithiocarbamate. In particular, 2-hydroxy-1-{4-[4-(2-hydroxy-2-methylpropionyl)-benzyl]-phenyl}-2-methylpropan-1-one, 1-hydroxy-cyclohexyl-phenyl-ketone, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, and 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one are preferred, and among these, 2-hydroxy-2-methyl-1-phenyl-propan-1-one and 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one are especially preferred. These may be used individually or in combination of two or more.

[0052] The amount of photopolymerization initiator added is not particularly limited. For example, it is preferable to use about 1 to 10% by mass relative to the resin used. Adding 1% by mass or more will provide sufficient curability, while adding 10% by mass or less will result in a high content of resin having 3 or more reactive groups, making it possible to produce a hard coat film with high crosslink density and high hardness.

[0053] (film thickness) The thickness of the hard coat layer is preferably 1 to 55 μm. A thickness greater than 1 μm allows for sufficient curing and good pencil hardness. Furthermore, by limiting the thickness to 55 μm or less, curling due to hard coat curing shrinkage can be suppressed, improving the handling properties of the film.

[0054] (Application method) The hard coat layer can be applied using various methods without particular limitations, such as Meyer bar coating, gravure coating, die coating, or knife coating, and can be appropriately selected depending on viscosity and film thickness. The drying temperature is preferably 60 to 100°C, and more preferably 60 to 90°C. A temperature of 60°C or higher allows for sufficient drying in a short time, while a temperature of 100°C or lower is preferable as it prevents deformation of the polyester film.

[0055] (Curing conditions) While methods such as curing with energy rays like ultraviolet light and electron beams, or curing with heat, can be used to cure the hard coat layer, ultraviolet light or electron beams are preferred to minimize damage to the film.

[0056] (Ultraviolet rays) UV lamps include high-pressure mercury lamps and electrodeless lamps, which can be selected and used as appropriate. The cumulative UV light intensity is 50-500 mJ / cm². 2 Preferably, 100-200 mJ / cm² 2 This is even more preferable. 50 mJ / cm 2 By irradiating with the above cumulative light intensity, sufficient pencil hardness can be obtained, 500 mJ / cm². 2 The following cumulative light intensity levels allow for increased processing speed, improving cost-effectiveness and making them preferable. Curing properties can also be improved by irradiating with ultraviolet light in an inert gas such as nitrogen, argon, or carbon dioxide, and these can be selected as appropriate.

[0057] (Electron beam) Electron beam irradiation devices come in various types, such as area type and scanning type, and can be selected and used as appropriate. The cumulative irradiation dose of the electron beam is preferably 25 to 500 kGy, and more preferably 50 to 300 kGy. An integrated irradiation dose of 25 kGy or more is sufficient to obtain pencil hardness, and if it is 500 kGy or less, the processing speed can be increased, improving economic efficiency, which is preferable. Irradiation is preferably carried out in an inert gas such as nitrogen or argon, with an oxygen concentration of 500 ppm or less. By keeping the oxygen concentration at 500 ppm or less, ozone generation can be suppressed, improving safety during operation.

[0058] (Pencil hardness) The pencil hardness of the hard coat layer is preferably 2H or higher, more preferably 3H or higher, and particularly preferably 4H or higher. A pencil hardness of 2H or higher prevents easy deformation and does not reduce visibility. Generally, a higher pencil hardness of the hard coat layer is preferable, but it can be 10H or lower, 8H or lower, or even 6H or lower without practical problems.

[0059] (Types of hard coat layers) The hard coat layer in the present invention may have other functions added, as long as it can be used for the purpose of protecting the display by increasing the pencil hardness of the surface as described above. For example, a hard coat layer with added functionality such as an anti-glare layer, anti-glare anti-reflective layer, anti-reflective layer, low-reflection layer, and anti-static layer having a certain pencil hardness as described above is also preferably applied in the present invention. [Examples]

[0060] Next, the effects of the present invention will be explained using examples and comparative examples. First, the evaluation method for characteristic values ​​used in the present invention is shown below.

[0061] (1) Intrinsic viscosity The film or polyester resin was crushed and dried, then dissolved in a phenol / tetrachloroethane mixed solvent of 60 / 40 (mass ratio). After removing inorganic particles from this solution by centrifugation, the flow time of the solution at a concentration of 0.4 g / dl and the flow time of the solvent alone were measured using an Ubbelohde viscometer at 30°C. The intrinsic viscosity was calculated from the ratio of these times using Huggins' equation, assuming that Huggins' constant is 0.38. In the case of laminated films, the intrinsic viscosity of each individual layer was evaluated by scraping off the corresponding polyester layer of the film according to the laminate thickness.

[0062] (2) Depth of indentation after unloading of test force The sample was cut into approximately 2 cm squares and fixed to an 18 × 18 mm microcover glass (manufactured by Matsunami Glass Co., Ltd.) with the opposite side of the measurement surface using adhesive (Cemedine High Super 30). After fixing, it was left at room temperature for more than 12 hours, and then the indentation depth (μm) after unloading the test force was measured using a dynamic ultramicrohardness tester "DUH-211" (manufactured by Shimadzu Corporation) under the following conditions. <Measurement Conditions> Test mode: Load-unload test Indenter used: 115 degree ridge angle, triangular pyramid indenter Indenter modulus: 1.140 × 10⁶ N / mm 2 Indenter Poisson's ratio: 0.07 Test force: 50mN Load speed: 4.44mN / sec Load holding time: 2sec Unloading holding time: 0sec

[0063] (3) Surface orientation coefficient ΔP Five samples were taken, and the refractive index in the longitudinal direction (Nx), the refractive index in the width direction (Ny), and the refractive index in the thickness direction (Nz) were measured using an Abbe refractometer with sodium D line as the light source, according to JIS K7142:1996 5.1 (Method A). The surface orientation coefficient (ΔP) was calculated using the following formula. The average value of the obtained surface orientation coefficients was used as the surface orientation coefficient. ΔP = (Nx + Ny) / 2 - Nz (4) Pencil hardness In accordance with JIS K 5600-5-4:1999, measurements were taken with a load of 750g and a speed of 1.0mm / s, and the presence or absence of deformation was visually confirmed. Pencils without deformation were considered OK. Five measurements were taken, and pencil hardness with four or more OK ratings was used as the measured value.

[0064] (Polymerization of urethane resin) In a four-necked flask equipped with a stirrer, a Liebig condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer, 72.96 parts by mass of 1,3-bis(isocyanatemethyl)cyclohexane, 12.60 parts by mass of dimethylolpropionic acid, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of polycarbonate diol with a number average molecular weight of 2000, and 85.00 parts by mass of acetonitrile and 5.00 parts by mass of N-methylpyrrolidone as solvents were added. The mixture was stirred at 75°C for 3 hours under a nitrogen atmosphere, and it was confirmed that the reaction solution reached the predetermined amine equivalent. Next, the reaction solution was cooled to 40°C, and then 9.03 parts by mass of triethylamine was added to obtain polyurethane prepolymer D solution. Next, in a reaction vessel equipped with a homodisperser capable of high-speed stirring, 450 g of water was added, and the temperature was adjusted to 25°C. The isocyanate-terminated prepolymer was added and dispersed in water while stirring. Subsequently, a water-soluble polyurethane resin (A) with a solid content of 35% by mass was prepared by removing some of the acetonitrile and water under reduced pressure.

[0065] (Polymerization of water-soluble carbodiimide compounds) In a flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer, 200 parts by mass of isophorone diisocyanate and 4 parts by mass of the carbodiimide catalyst 3-methyl-1-phenyl-2-phosphorene-1-oxide were added and stirred at 180°C for 10 hours under a nitrogen atmosphere to obtain isocyanate-terminated isophorone carbodiimide (degree of polymerization = 5). Next, 111.2 g of the obtained carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100°C for 24 hours. Water was gradually added at 50°C to obtain a yellow, transparent, water-soluble carbodiimide compound (B) with a solid content of 40% by mass. (Preparation of easy-adhesion layer coating solution)

[0066] The following coating agents were mixed to prepare the coating solution. Water 16.97 parts by mass Isopropanol 21.96 parts by mass Water-soluble polyurethane resin (A) 3.27 parts by mass Water-soluble carbodiimide compound (B) 1.22 parts by mass Particles 0.51 parts by mass (Silica sol with average particle size of 40 nm, solid content concentration of 40% by mass) Surfactant 0.05 parts by mass (Silicone-based, solid content concentration 100% by mass)

[0067] (Preparation of hard coat coating solution 10) 95 parts by weight of pentaerythritol triacrylate (manufactured by Shin-Nakamura Chemical Industry Co., Ltd., A-TMM-3, 100% solids content), 5 parts by weight of photopolymerization initiator (manufactured by BASF Japan, Irgacure® 907, 100% solids content), and 0.1 parts by weight of leveling agent (manufactured by BIC Chemie Japan, BYK307, 100% solids content) were mixed and diluted with a toluene / MEK = 1 / 1 solvent to prepare a 40% concentration coating solution 10.

[0068] (Preparation of hard coat coating solution 11) 50 parts by weight of urethane acrylate (manufactured by Kyoeisha Chemical Co., Ltd., UA-306H, 100% solids content), 45 parts by weight of pentaerythritol triacrylate (manufactured by Shin Nakamura Chemical Industry Co., Ltd., A-TMM-3, 100% solids content), 5 parts by weight of photopolymerization initiator (manufactured by BASF Japan, Irgacure® 907, 100% solids content), and 0.1 parts by weight of leveling agent (manufactured by BIC Chemie Japan, BYK307, 100% solids content) were mixed and diluted with a toluene / MEK = 1 / 1 solvent to prepare a 40% concentration coating solution 11.

[0069] (Preparation of polyethylene terephthalate pellets) As the esterification reactor, a continuous esterification reactor consisting of a three-stage complete mixing tank equipped with a stirrer, a partial condenser, a raw material inlet, and a product outlet was used. TPA was supplied at 2 tons / hr, EG at 2 moles per mole of TPA, and antimony trioxide was added in an amount that resulted in 160 ppm of Sb atoms relative to the generated PET. This slurry was continuously supplied to the first esterification reactor of the esterification reactor and reacted at atmospheric pressure at 255°C with an average residence time of 4 hours. Next, the reaction product in the first esterification reactor was continuously removed from the system and supplied to the second esterification reactor. EG distilled off from the first esterification reactor was supplied to the second esterification reactor at 8% by mass relative to the generated polymer (generated PET). Furthermore, an EG solution containing magnesium acetate in an amount that resulted in 65 ppm of Mg atoms relative to the generated PET, and an EG solution containing TMPA in an amount that resulted in 20 ppm of P atoms relative to the generated PET were added, and the reaction was carried out at atmospheric pressure at 260°C with an average residence time of 1.5 hours. Next, the reaction product from the second esterification reaction vessel was continuously removed from the system and supplied to the third esterification reaction vessel. Further, an EG solution containing TMPA in an amount that resulted in 20 ppm of P atoms relative to the generated PET was added, and the reaction was carried out at atmospheric pressure at 260°C with an average residence time of 0.5 hours. The esterification reaction product generated in the third esterification reaction vessel was continuously supplied to a three-stage continuous polycondensation reactor for polycondensation, and then filtered through a stainless steel sintered filter material (nominal filtration accuracy, 90% cut of 5 μm particles) to obtain polyethylene terephthalate pellets with an intrinsic viscosity of 0.58 dl / g.

[0070] (Example 1) After drying the above polyethylene terephthalate pellets under reduced pressure (3 Torr) at 180 °C for 8 hours, the polyethylene terephthalate pellets were respectively supplied to an extruder and melted at 285 °C. This polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% cut for 10 μm particles), extruded into a sheet form from a die, and then cooled and solidified by contacting a casting drum with a surface temperature of 30 °C using an electrostatic application casting method to produce an unstretched film. This unstretched film was stretched 3.4 times in the longitudinal direction at 85 °C. After applying the easy adhesion layer coating solution to one side of the PET film by a roll coating method, it was dried at 80 °C for 20 seconds. The coating amount after drying in the final (after biaxial stretching) was adjusted to be 0.06 g / m 2 This uniaxially stretched film was stretched 4.3 times in the width direction at 95 °C using a tenter and heat-treated at 235 °C for 5 seconds to obtain a polyethylene terephthalate film.

[0071] Using a Meyer bar, a hard coat coating solution 10 was applied onto the easy adhesion layer of the above polyethylene terephthalate film so that the film thickness after drying was 5.0 μm. After drying at 80 °C for 1 minute, ultraviolet rays were irradiated (integrated light quantity: 200 mJ / cm 2 ) to obtain a hard coat film.

[0072] (Examples 2 - 4, Comparative Examples 1 - 3) Stretching was carried out in the same manner as in Example 1 except that the stretching ratio was changed as shown in Table 1, and polyethylene terephthalate films and hard coat films of Examples 2 - 4 and Comparative Examples 1 - 3 were obtained.

[0073] (Examples 5, 6) Stretching was carried out in the same manner as in Example 1 except that the thickness was changed as shown in Table 1, and polyethylene terephthalate films and hard coat films of Examples 5 and 6 were obtained.

[0074] (Example 7) Using polyethylene-2,6-naphthalate (PEN) with an intrinsic viscosity of 0.62 dl / g, polyethylene naphthalate film and hard coat film were obtained by stretching in the same manner as in Example 1, except that the stretching ratio was changed.

[0075] (Examples 8 and 9) Using the same polyethylene terephthalate film as in Example 1, a hard-coated film was prepared using the hard-coat conditions described in Table 1.

[0076] The fabricated hard coat film was laminated to an organic EL module via a 25 μm thick adhesive layer to create a flexible display. In each example, the hard coat film protected the organic EL module from external impacts, and good visibility was achieved.

[0077] [Table 1] [Industrial applicability]

[0078] According to the present invention, a polyester film with high surface hardness when a hard coat layer is laminated is provided. The hard coat film obtained by laminating a hard coat layer on the surface of the polyester film of the present invention can be used as a surface protective film for display elements such as flexible displays, thereby enabling the provision of display elements that are resistant to scratches, impacts, and deformation on the surface of the display element and also have excellent design.

Claims

1. A laminated polyester film comprising a polyester film having an easy-adhesion layer on at least one surface and an easy-adhesion layer, A laminated polyester film for flexible displays, having a film crystallinity of 48% to 65% by density method, and a film surface crystallinity of 1.20 to 3.0 by ATR method.

2. A laminated polyester film for a protective film of a flexible display according to claim 1, wherein the intrinsic viscosity is 0.50 dl / g or more and 1.00 dl / g or less.

3. A laminated polyester film for protective films of flexible displays according to claim 1, wherein the indentation depth after unloading the test force in the film thickness direction, measured using a dynamic ultramicrohardness tester under the following measurement conditions, is 1.4 μm or less. <Measurement Conditions> Test mode: Load-unload test Indenter used: ridge angle 115 degrees, triangular pyramid indenter Indenter modulus: 1.140 × 10⁶ N / mm ２ Indenter Poisson's ratio: 0.07 Test force: 50 mN Load speed: 4.44mN / sec Load holding time: 2sec Unloading holding time: 0sec

4. A laminated polyester film for a protective film of a flexible display according to claim 1, wherein the total light transmittance is 85% or more and the haze is 3% or less.

5. A laminated polyester film for a protective film of a flexible display according to claim 1, having a thickness of 25 to 100 μm.

6. It is a biaxially oriented polyester film, The laminated polyester film for a protective film of a flexible display according to claim 1, comprising a water-soluble polyurethane resin in the easy-adhesion layer.

7. A hard coat film for a protective film of a flexible display, having a hard coat layer on at least one surface of a laminated polyester film for a protective film of a flexible display according to any one of claims 1 to 6.

8. The hard coat layer contains an acrylate resin having three or more reactive groups in its molecule. The hard coat film for a protective film of a flexible display according to claim 7, wherein the pencil hardness of the hard coat layer is 2H or higher.

9. A display unit using the hard coat film for protective film of a flexible display described in claim 7 as a protective film.

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