JP7314484B2 - Polyester film for surface protection film of flexible display - Google Patents

Description

TECHNICAL FIELD The present invention relates to a polyester film, and more particularly to a polyester film having a high pencil hardness when laminated with a hard coat layer or the like.

Mobile terminal devices such as displays and smartphones are becoming thinner and lighter. At present, since all of them use rigid housings, tempered glass is generally used to protect the displays. On the other hand, in recent years, displays that emphasize design are preferred. For example, displays with special shapes, such as circular, are preferred. Especially in recent years, there has been a demand for a flexible display intended to be installed on a curved surface.

However, since a flexible display cannot use a glass material, an acrylic plate or the like has been used. Polycarbonate, which has high impact resistance and high light resistance, is sometimes used, but it is expensive and has a low pencil hardness. On the other hand, polyester films are inexpensive and have excellent impact resistance, but have the problem of insufficient pencil hardness.

In order to achieve these, Patent Document 1 proposes a material having high hardness and flexibility as a hard coat material. However, the material is expensive and there is a problem in versatility, and there is also a problem that the scratch resistance of the surface is low.

In addition, as in Patent Document 2, a transparent polyimide material that can achieve high pencil hardness without depending on hard coat materials has also been proposed. However, this material is very expensive and has problems in mass production.

JP 2017-008148 A JP 2016-222797 A

An object of the present invention is to solve the problems associated with transparent films as described above, and to provide a polyester film at a low cost that exhibits a high pencil hardness when a hard coat layer is laminated thereon.

That is, the present invention consists of the following configurations.
1. A polyester film for a surface protective film of a flexible display having a thickness of 25 to 100 μm and a depth of indentation of 1.4 μm or less after unloading the test force.
2. 1. The polyester film for a surface protective film of a flexible display as described in 1 above, which has a plane orientation coefficient ΔP of 0.160 or more.
3. 3. The polyester film for a surface protection film of a flexible display according to claim 1, which is a biaxially stretched polyester film and has an easy-adhesion layer on at least one surface of said biaxially stretched polyester film.
4. A hard coat film for a surface protective film of a flexible display, comprising a hard coat layer on at least one surface of the polyester film for a surface protective film of a flexible display according to any one of the first to third above.
5. A display body using the hard coat film for a surface protective film of a flexible display according to 4 above as a surface protective film.

The polyester film of the present invention has a higher surface hardness than a general polyester film when a hard coat layer or the like is laminated, and the hard coat film is used as a surface protective film of a display body, so that the surface of the display body is resistant to scratches, impacts, deformation, etc., and it is possible to provide a display body with excellent design. The display can provide beautiful images.

(Display body)
The display body referred to in the present invention generally refers to display devices, and the types of displays include LCD, organic EL display, inorganic EL display, LED, FED, etc., but LCD, organic EL, and inorganic EL, which have a foldable structure, are preferable. In particular, an organic EL with a reduced number of layers and a wide color gamut is more preferable.

The form of the display includes a large screen, a display, a digital signage, a bendable mobile terminal, a PC, and the like, and can be used without limitation. In particular, it is preferable to use a display with a touch panel, a digital signage, a bendable mobile terminal, a PC, etc., which have a touch panel function to be operated by direct touch. Further, it is particularly preferable that the display body is a flexible display as described below.

(flexible display)
A flexible display has a structure in which one continuous display can be bent according to the application, and at the same time, it is desirable that the display be thin and light. In particular, a foldable display that is a foldable display, a rollable display that is a rollable display, and a paper-like display can be used.

(polyester film)
Transparent films include films with high light transmittance and low haze, such as PI films, polyester films, polycarbonate films, acrylic films, TAC films, and cycloolefin polymer films.

In the present invention, the polyester film may be a single-layer film made of one or more polyester resins, or when two or more types of polyester are used, may be a multi-layer structure film or a super-multi-layer laminated film having a repeating structure.

Polyester resins include, for example, polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, and polyester films composed of copolymers containing these resin components as main components. Among them, a biaxially stretched polyethylene terephthalate film is particularly preferable in terms of mechanical properties, heat resistance, transparency, price, and the like.

When a polyester copolymer is used for the substrate film, examples of the dicarboxylic acid component of the polyester include aliphatic dicarboxylic acids such as adipic acid and sebacic acid; terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; aromatic dicarboxylic acids; and polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid. Examples of glycol components 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 glycols having an average molecular weight of 150 to 20,000. The preferred copolymer has a copolymer content of less than 20% by weight. When the amount is less than 20% by mass, the film strength, transparency and heat resistance are maintained, which is preferable.

Further, in the production of the polyester film, the limiting viscosity of at least one kind of resin pellets is preferably in the range of 0.50 to 1.0 dl/g. When the intrinsic viscosity is 0.55 dl/g or more, the pencil hardness after hard coating is improved, which is preferable. More preferably 0.58 dl/g or more, 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 of the melted fluid does not increase too much, and the film production can be stably operated, which is preferable.

The intrinsic viscosity of the film is preferably 0.50 dl/g or more, regardless of whether the film has a single layer structure or a laminated structure. An increase in the molecular weight of the polyester resin is preferable because the elastic behavior of the film is improved, and deformation against the load of a pencil during pencil hardness measurement can be effectively suppressed. More preferably 0.55 dl/g or more, particularly preferably 0.63 dl/g or more. On the other hand, if the intrinsic viscosity is increased, the pencil hardness is also improved, but there is a concern that the productivity will be lowered.

The thickness of the polyester film is preferably 25-100 μm, more preferably 30-75 μm. When the thickness is 25 μm or more, the handleability and impact resistance are improved, and when the thickness is 100 μm or less, it is advantageous for weight reduction and excellent in flexibility and workability.

The surface of the polyester film of the present invention may be smooth or uneven, but since it is used as a protective film for the surface of a display, deterioration in optical properties due to unevenness is not preferable. The haze is preferably 3% or less, more preferably 2% or less, most preferably 1% or less. If the haze is 3% or less, the visibility of the image can be improved. Although the lower limit of haze is better, it may be 0.1% or more, or 0.3% or more.

For the purpose of reducing haze as described above, it is preferable that the unevenness of the film surface is not so large, but in order to provide a certain degree of slipperiness from the viewpoint of handling, as a method of forming unevenness, it is possible to form by blending a filler in the polyester resin layer of the surface layer or by coating a coat layer containing a filler during film formation.

A known method can be employed as a method for blending the particles into the base film. For example, it can be added at any stage of polyester production, but preferably at the stage of esterification, or after the transesterification reaction is completed and before the start of the polycondensation reaction, it may be added as a slurry dispersed in ethylene glycol or the like to proceed with the polycondensation reaction. Alternatively, a kneading extruder with a vent may be used to blend a slurry of particles dispersed in ethylene glycol or water with a polyester raw material, or a kneading extruder may be used to blend dried particles and a polyester raw material.

Among them, a method of homogenously dispersing the aggregate inorganic particles in a monomer liquid that is part of the polyester raw material and then filtering it is added to the remainder of the polyester raw material before, during, or after the esterification reaction. According to this method, since the monomer liquid has a low viscosity, homogeneous dispersion of particles and highly accurate filtration of the slurry can be easily performed, and when added to the rest of the raw material, the particles have good dispersibility and new aggregates are less likely to occur. From this point of view, it is particularly preferable to add it to the rest of the raw materials in a low temperature state before the esterification reaction.

Further, the number of protrusions on the film surface can be further reduced by a method of kneading and extruding the pellets and pellets not containing particles after obtaining a polyester containing particles in advance (masterbatch method).

Moreover, the polyester film may contain various additives as long as the desired range of total light transmittance is maintained. Additives include, for example, antistatic agents, UV absorbers, and stabilizers.

The total light transmittance of the polyester film is preferably 85% or more, more preferably 87% or more. A transmittance of 85% or more can ensure sufficient visibility. It can be said that the higher the total light transmittance of the polyester film is, the better it is.

The surface of the polyester film of the present invention may be subjected to a treatment for improving adhesion with the resin forming the hard coat layer.

Examples of the surface treatment method include sandblasting, roughening treatment by solvent treatment, etc., corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone/ultraviolet irradiation treatment, flame treatment, chromic acid treatment, oxidation treatment such as hot air treatment, etc., and can be used without particular limitation.

Adhesion can also be improved by an adhesion improving layer such as an easy-adhesion layer. As the easy-adhesion layer, an acrylic resin, a polyester resin, a polyurethane resin, a polyether resin, or the like can be used without particular limitation, and can be formed by a general coating technique, preferably a so-called in-line coating formulation.

The polyester film described above can be produced, for example, through a film forming step of, for example, uniformly dispersing inorganic particles in a monomer liquid that is a part of the polyester raw material, filtering the mixture, and then adding it to the remainder of the polyester raw material to polymerize the polyester.

Next, the method for producing a biaxially stretched polyester film will be described in detail with an example in which pellets of polyethylene terephthalate (hereinafter sometimes referred to as PET) are used as a raw material for the base film, but the method is not limited to these. Also, the number of layers is not limited, such as a single layer structure or a multilayer structure.

PET pellets are mixed at a predetermined ratio and dried, then supplied to a known melt lamination extruder, extruded into a sheet from a slit-shaped die, and cooled and solidified on a casting roll to form an unstretched film. In the case of a single layer, one extruder may be used, but when producing a multi-layer film, two or more extruders, two or more layers of manifolds or confluence blocks (for example, a confluence block having a square confluence) can be used to laminate a plurality of film layers constituting each outermost layer, extrude a sheet of two or more layers from a die, and cool with a casting roll to form an unstretched film.

In this case, during melt extrusion, it is preferable to perform high-precision filtration at an arbitrary location where the molten resin is kept at about 280° C. in order to remove foreign substances contained in the resin. The filter medium used for high-precision filtration of the molten resin is not particularly limited, but the filter medium of a stainless steel sintered body is preferable because it has excellent performance in removing aggregates and high-melting-point organic substances mainly composed of Si, Ti, Sb, Ge, and Cu.

Furthermore, the filtration particle size (initial filtration efficiency of 95%) of the filter medium is preferably 20 μm or less, particularly preferably 15 μm or less. If the filtration particle size of the filter medium (initial filtration efficiency of 95%) exceeds 20 μm, foreign matter with a size of 20 μm or more cannot be sufficiently removed. By performing high-precision filtration of the molten resin using a filter medium having a filter particle size (initial filtration efficiency of 95%) of 20 μm or less, productivity may decrease, but it is preferable for obtaining a film with few protrusions due to coarse particles.

Specifically, for example, PET pellets are sufficiently vacuum-dried, supplied to an extruder, melt-extruded into a sheet at about 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 with rolls heated to 80 to 120° C. to obtain a uniaxially oriented PET film. Further, the ends of the film are gripped with clips, guided to a hot air zone heated to 80 to 180° C., dried, and stretched 2.5 to 5.0 times in the width direction. Subsequently, it is led to a heat treatment zone of 160 to 250° C. and heat treated for 1 to 60 seconds to complete crystal orientation. During this heat treatment step, a relaxation treatment of 1 to 12% may be applied in the width direction or the longitudinal direction, if necessary.

(crystallinity)
Crystallinity is determined primarily by the amount of heat applied to the film in the heat treatment zone. Since the amount of heat is determined by various conditions such as film forming equipment conditions, film forming speed, and film thickness, the degree of crystallinity is not determined only by the heat treatment temperature, but the heat treatment temperature is preferably 160 to 250 ° C., and more preferably 200 to 240 ° C.. A temperature of 160° C. or higher is preferable because crystallization can be promoted. Heat treatment at 250° C. or less is preferable because it is possible to suppress deterioration in crystallization due to remelting of the polyester.

In order to ensure sufficient pencil hardness after hard coating, the crystallinity of the film by density method is preferably 48% or more, and the crystallinity of the film surface by ATR method is preferably 1.20 or more.

The crystallinity of the polyester film measured by the density method is preferably 48% or more, more preferably 50% or more, and even more preferably 52% or more. The crystallinity is preferably high, but the upper limit of the film that can be produced is about 65%.

The crystallinity of the film surface measured by the ATR method is preferably 1.20 or higher, more preferably 1.25 or higher, and even more preferably 1.27 or higher. The crystallinity of the film surface by the ATR method is also preferably high, but the upper limit of the film that can be produced is about 3.0.

The indentation depth after unloading the test force in the film thickness direction by a dynamic ultra-micro 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 reducing the depth of indentation after unloading the test force (final amount of deformation under load), deformation after evaluation of pencil hardness can be suppressed, and pencil hardness can be improved.

The plane orientation ΔP is preferably 0.160 or more, more preferably 0.163 or more, and even more preferably 0.165 or more. By increasing the plane orientation, it becomes strong against the load in the thickness direction, and the deformation amount at the time of unloading can be suppressed. A higher value is preferable, but the upper limit of the film that can be formed is about 0.170.

The area draw ratio is preferably 12.5 times or more, more preferably 13 times or more, and even more preferably 14 times or more. By increasing the area magnification, the orientation in the plane direction of the film can be strengthened, and ΔP can be increased.

The stretching method can be simultaneous biaxial stretching, sequential biaxial stretching, or the like without particular limitation.

(Easy adhesion layer)
The easy-adhesion layer can be obtained by applying the above-described coating liquid to one or both sides of an unstretched or longitudinally uniaxially stretched film, drying at 75 to 150° C., and stretching in one or two directions. It is preferable to control the final coating amount of the easy-adhesion layer to 0.05 to 0.20 g/m ² . A coating amount of 0.05 g/m ² or more is preferable because adhesiveness is satisfied. On the other hand, when the coating amount is 0.20 g/m ² or less, blocking resistance is obtained, which is preferable.

Examples of the resin used for the easy-adhesion layer include polyester-based resins, polyurethane-based resins, polyester-polyurethane resins, polycarbonate-polyurethane resins, acrylic resins, and the like, and can be used without particular limitation. Examples of the cross-linking agent for the easy-adhesion layer include melamine-based, isocyanate-based, oxazoline-based, and epoxy-based cross-linking agents. Two or more kinds can be mixed and used. Due to the nature of in-line coating, these are preferably applied with a water-based coating liquid, and the above resins and cross-linking agents are preferably water-soluble or water-dispersible resins or compounds.

Particles are preferably added to the easy-adhesion layer to impart lubricity. It is preferable that the average particle size of the fine particles is 2 μm or less. When the average particle size of the particles exceeds 2 μm, the particles tend to fall off from the easy-adhesion layer. Examples of particles to be contained in the easy-adhesion layer include inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, calcium fluoride, and organic polymer particles such as styrene, acrylic, melamine, benzoguanamine, and silicone particles. These may be added to the easy-adhesion layer singly, or two or more of them may be added in combination.

Moreover, as a method of applying the coating liquid, a known method can be used as in the case of the coating layer described above. For example, reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, etc., can be used alone or in combination.

(Hard coat layer)
As the resin for forming the hard coat layer, (meth)acrylate, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, epoxy, and the like can be used without particular limitation. Moreover, two or more kinds of materials can be mixed and used, and particles such as inorganic fillers and organic fillers can be added. Among them, (meth)acrylic acid esters and acrylate resins are preferable, and resins having 3 or more reactive groups in the molecule as an essential component are preferable.

(Resin having 3 or more reactive groups)
Examples of resins having 3 or more reactive groups include tris(2-acryloyloxyethyl) isocyanurate, tris(3-acryloyloxypropyl) isocyanurate, tris(2-methacryloyloxyethyl) isocyanurate, tris(3-methacryloyloxypropyl) isocyanurate, tris[(meth)acryloyloxyalkyl]isocyanurate, trimethylolpropane tri(meth)acrylate, dipentaerythritol tris (Meth)acrylates, propionic 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, caprolactone-modified dipentaerythritol hexa(meth)acrylate, and other (meth)acrylates. These may be used individually by 1 type, and may be used in combination of 2 or more type.

In addition, commercially available hard coating agents can also be used. For example, Beamset (registered trademark) series manufactured by Arakawa Chemical Industry Co., Ltd., Aika Itron (registered trademark) series manufactured by Aica Kogyo Co., Ltd., Lioduras (registered trademark) series manufactured by Toyo Ink Co., Ltd., etc. can be used without particular limitation, and can also be used in combination with other (meth)acrylic acid ester compounds.

(Photoinitiator)
When the hard coat layer of the present invention is cured using ultraviolet rays, 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, methyl benzoin benzoate, benzoin dimethyl ketal, 2,4-diethylthioxanthone, 1-hydroxycyclohexylphenyl ketone, benzyldiphenylsulfide, tetramethylthiurammonosulfide, azobisisobutyronitrile, benzyl, dibenzyl, diacetyl, β-Chloranthraquinone, (2,4,6-trimethylbenzyldiphenyl)phosphine oxide, 2-benzothiazole-N,N-diethyldithiocarbamate and the like. In particular, 2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-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, which are said to have excellent surface curability, are preferred, and 2-hydroxy- 2-Methyl-1-phenyl-propan-1-one, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one are particularly preferred. These may be used alone or in combination of two or more.

The amount of the photopolymerization initiator added is not particularly limited. For example, it is preferable to use about 1 to 10% by mass with respect to the resin used. By adding 1% by mass or more, sufficient curability can be obtained, and by adding 10% by mass or less, the content ratio of the resin having 3 or more reactive groups is high, and a hard coat film with a high crosslinking density and high hardness can be obtained.

(film thickness)
The thickness of the hard coat layer is preferably 1 to 55 μm. If it is thicker than 1 μm, it will be sufficiently hardened and good pencil hardness will be obtained. Further, by setting the thickness to 55 μm or less, it is possible to suppress curling due to curing shrinkage of the hard coat and improve the handleability of the film.

(Application method)
The hard coat layer may be applied by a Meyer bar, a gravure coater, a die coater, a knife coater, or the like without particular limitation, and can be appropriately selected depending on the viscosity and film thickness. The drying temperature is preferably 60 to 100°C, more preferably 60 to 90°C. If it is 60° C. or higher, it can be sufficiently dried in a short time, and if it is 100° C. or lower, deformation of the polyester film does not occur, which is preferable.

(Curing conditions)
As a method for curing the hard coat layer, energy beams such as ultraviolet rays, electron beams, and heat can be used, but ultraviolet rays and electron beams are preferred in order to reduce damage to the film.

(ultraviolet rays)
As an ultraviolet lamp, there are a high-pressure mercury lamp, an electrodeless lamp, and the like, and they can be appropriately selected and used. The integrated amount of UV light is preferably 50 to 500 mJ/cm ² , more preferably 100 to 200 mJ/cm ² . Sufficient pencil hardness can be obtained by irradiating an integrated light amount of 50 mJ/cm ² or more, and an integrated light amount of 500 mJ/cm ² or less can increase the processing speed and improve economic efficiency, which is preferable. Curability can also be improved by irradiating ultraviolet rays in an inert gas such as nitrogen, argon, or carbon dioxide, and can be selected as appropriate.

(Electron beam)
The electron beam irradiation apparatus includes an area type, a scanning type, and the like, and can be appropriately selected and used. The cumulative dose of the electron beam is preferably 25-500 kGy, more preferably 50-300 kGy. Sufficient pencil hardness can be obtained by irradiating with an accumulated irradiation dose of 25 kGy or more, and if it is 500 kGy or less, the processing speed can be increased, and economic efficiency is improved, which is preferable. Irradiation is preferably performed in an inert gas such as nitrogen or argon at an oxygen concentration of 500 ppm or less. By setting the oxygen concentration to 500 ppm or less, it is possible to suppress the generation of ozone and improve safety during work.

(Pencil hardness)
The hard coat layer preferably has a pencil hardness of 2H or higher, more preferably 3H or higher, and particularly preferably 4H or higher. If it has a pencil hardness of 2H or more, it will not be easily deformed and visibility will not be lowered. In general, the hard coat layer preferably has a higher pencil hardness, but it may be 10H or less, 8H or less, and even 6H or less can be practically used without any problem.

(Type of hard coat layer)
The hard coat layer in the present invention may have other functions 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 to which functionality such as an antiglare layer having a certain pencil hardness, an antiglare antireflection layer, an antireflection layer, a low reflection layer and an antistatic layer as described above is added is preferably applied in the present invention.

Next, the effects of the present invention will be described using examples and comparative examples. First, the evaluation method of characteristic values used in the present invention is shown below.

(1) Intrinsic Viscosity A film or polyester resin was pulverized and dried, and then dissolved in a mixed solvent of phenol/tetrachloroethane=60/40 (mass ratio). After removing the inorganic particles by centrifuging this solution, using an Ubbelohde viscometer, the flow time of the solution with a concentration of 0.4 (g / dl) at 30 ° C. and the flow time of the solvent alone were measured, and from the time ratio, the intrinsic viscosity was calculated using the Huggins formula, assuming that the Huggins constant was 0.38. In the case of the laminated film, the intrinsic viscosity of each layer alone was evaluated by scraping off the corresponding polyester layer of the film according to the laminated thickness.

(2) Depth of indentation after unloading the test force A sample was cut into a square of about 2 cm, and the opposite side of the measurement surface was fixed on a micro cover glass 18 x 18 mm (manufactured by Matsunami Glass Co., Ltd.) with an adhesive (Cemedine Hi-Super 30). After fixation, it was left at room temperature for 12 hours or more, and then using a dynamic ultra-micro hardness tester "DUH-211" (manufactured by Shimadzu Corporation) under the following conditions, the indentation depth (μm) after unloading the test force was measured.
≪Measurement conditions≫
Test mode: load-unload test
Indenter used: Triangular pyramid indenter with an edge-to-edge angle of 115 degrees
Indenter elastic modulus: 1.140×10 ⁶ N/mm ²
Indenter Poisson's ratio: 0.07
Test force: 50mN
Load speed: 4.44mN/sec
Load holding time: 2 sec
Unloading holding time: 0 sec

(3) Plane orientation coefficient ΔP
Five samples were collected, and according to JIS K7142: 1996, 5.1 (method A), the refractive index in the film longitudinal direction (Nx), the refractive index in the width direction (Ny), and the refractive index in the thickness direction (Nz) were measured with an Abbe refractometer using a sodium D line as a light source, and the plane orientation coefficient (ΔP) was calculated by the following formula. In addition, the average value of the obtained plane orientation coefficient was made into the plane orientation coefficient.
ΔP = (Nx + Ny)/2-Nz
(4) Pencil hardness According to JIS K 5600-5-4:1999, measurement was performed at a load of 750 g and a speed of 1.0 mm/s, and the presence or absence of deformation was visually confirmed. Those with no deformation were evaluated as OK. It was measured 5 times, and the pencil hardness at which the evaluation of OK was 4 times or more was taken as the measured value.

(Polymerization of urethane resin)
72.96 parts by weight of 1,3-bis(isocyanatomethyl)cyclohexane, 12.60 parts by weight of dimethylolpropionic acid, 11.74 parts by weight of neopentyl glycol, 112.70 parts by weight of a polycarbonate diol having a number average molecular weight of 2000, and 85.00 parts by weight of acetonitrile as a solvent and 5.0 parts by weight of N-methylpyrrolidone in a four-necked flask equipped with a stirrer, a Dimroth condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer. 0 part by mass was added and stirred at 75° C. for 3 hours under a nitrogen atmosphere, and it was confirmed that the reaction liquid reached a predetermined amine equivalent weight. Next, after the temperature of this reaction liquid was lowered to 40° C., 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer D solution. Next, 450 g of water was added to a reaction vessel equipped with a homodisper capable of high-speed stirring, adjusted to 25° C., and while stirring and mixing, the isocyanate group-terminated prepolymer was added and dispersed in water. After that, acetonitrile and part of the water were removed under reduced pressure to prepare a water-soluble polyurethane resin (A) having a solid content of 35% by mass.

(Polymerization of water-soluble carbodiimide compound)
200 parts by mass of isophorone diisocyanate and 4 parts by mass of 3-methyl-1-phenyl-2-phosphorene-1-oxide as a carbodiimidization catalyst were charged into a flask equipped with a thermometer, a nitrogen gas inlet tube, a reflux condenser, a dropping funnel, and a stirrer, and stirred at 180° C. for 10 hours under a nitrogen atmosphere to obtain an isocyanate-terminated isophorone carbodiimide (polymerization degree=5). Then, 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 to this at 50° C. to obtain a yellow transparent water-soluble carbodiimide compound (B) having a solid content of 40% by mass.
(Preparation of easy-adhesion layer coating liquid)

A coating liquid was prepared by mixing the following coating agents.
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 an average particle size of 40 nm, solid content concentration of 40% by mass)
Surfactant 0.05 part by mass (silicone type, solid content concentration 100% by mass)

(Preparation of Hard Coating Liquid 10)
Pentaerythritol triacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., A-TMM-3, solid content concentration 100%) 95 parts by weight, photopolymerization initiator (manufactured by BASF Japan, Irgacure (registered trademark) 907, solid content concentration 100%) 5 parts by weight, leveling agent (BYK Chemie Japan Co., Ltd., BYK307, solid content concentration 100%) 0.1 parts by weight, diluted with a solvent of toluene / MEK = 1/1. , a coating solution 10 having a concentration of 40% was prepared.

(Preparation of Hard Coating Liquid 11)
Urethane acrylate (manufactured by Kyoeisha Chemical Co., Ltd., UA-306H, solid content concentration 100%) 50 parts by weight, pentaerythritol triacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., A-TMM-3, solid content concentration 100%) 45 parts by weight, photoinitiator (BASF Japan Co., Ltd., Irgacure (registered trademark) 907, solid content concentration 100%) 5 parts by weight, leveling agent (BYK Chemie Japan Co., Ltd., BYK307 , solid concentration 100%) were mixed and diluted with a solvent of toluene/MEK=1/1 to prepare a coating liquid 11 having a concentration of 40%.

(Preparation of polyethylene terephthalate pellets)
As the esterification reactor, a continuous esterification reactor consisting of three stages of complete mixing tanks having a stirrer, a dephlegmator, a raw material inlet and a product outlet was used. TPA was 2 tons/hr, EG was 2 mol per 1 mol of TPA, and antimony trioxide was adjusted in an amount so that the Sb atom content was 160 ppm relative to the produced PET. . Next, the reaction product in the first esterification reaction can is continuously taken out of the system and supplied to the second esterification reaction can, and 8% by mass of EG distilled from the first esterification reaction can is supplied into the second esterification reaction can with respect to the produced polymer (produced PET). The reaction was allowed to proceed at 260° C. for 0.5 hours. Next, the reaction product in the second esterification reactor was continuously taken out of the system and supplied to the third esterification reactor, and an EG solution containing TMPA in an amount such that the P atom was 20 ppm with respect to the PET produced was added, and the reaction was carried out at normal pressure at 260 ° C. with an average residence time of 0.5 hours. The esterification reaction product produced in the third esterification reaction vessel was continuously supplied to a three-stage continuous polycondensation reaction apparatus for polycondensation, and further filtered with a stainless steel sintered filter material (nominal filtration accuracy 5 μm, 90% cut of particles) to obtain polyethylene terephthalate pellets with an intrinsic viscosity of 0.58 dl / g.

(Example 1)
After the polyethylene terephthalate pellets were dried under reduced pressure (3 Torr) at 180°C for 8 hours, the polyethylene terephthalate pellets were supplied to the extruder and melted at 285°C. This polymer was filtered with a stainless steel sintered filter material (nominal filtration accuracy: 10 μm, 95% cut of particles), extruded from a nozzle in the form of a sheet, and then brought into contact with a casting drum having a surface temperature of 30° C. using an electrostatic casting method to cool and solidify, thereby producing an unstretched film. This unstretched film was stretched 3.4 times in the longitudinal direction at 85°C. The easily adhesive layer coating solution was applied to one side of a PET film by a roll coating method, and then dried at 80° C. for 20 seconds. The final (after biaxial stretching) coating amount after drying was adjusted to 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.

Using a Meyer bar, the hard coat coating liquid 10 was applied onto the easily adhesive layer of the polyethylene terephthalate film so that the film thickness after drying was 5.0 μm, dried at 80° C. for 1 minute, and then irradiated with ultraviolet rays (accumulated light amount: 200 mJ/cm ² ) to obtain a hard coat film.

(Examples 2-4, Comparative Examples 1-3)
The film was drawn in the same manner as in Example 1 except that the draw ratio was changed as shown in Table 1 to obtain polyethylene terephthalate films and hard coat films of Examples 2 to 4 and Comparative Examples 1 to 3.

(Examples 5 and 6)
The film was stretched in the same manner as in Example 1, except that the thickness was changed as shown in Table 1, to obtain polyethylene terephthalate films and hard coat films of Examples 5 and 6.

(Example 7)
Polyethylene-2,6-naphthalate (PEN) having an intrinsic viscosity of 0.62 dl/g was used and stretched in the same manner as in Example 1 except that the stretch ratio was changed to obtain a polyethylene naphthalate film and a hard coat film.

(Examples 8 and 9)
Using the same polyethylene terephthalate film as in Example 1 and adopting the hard coating conditions described in Table 1, a hard coat film was produced.

The prepared hard coat film was attached to an organic EL module via an adhesive layer having a thickness of 25 μm to prepare a flexible display. In each example, the hard coat film was able to protect the organic EL module from external impact, and good visibility was obtained.

According to the present invention, it is possible to provide a polyester film whose surface hardness is increased when a hard coat layer or the like is laminated, and the hard coat film obtained by laminating a hard coat layer on the surface of the polyester film of the present invention is used as a surface protective film for a display such as a flexible display.

Claims (5)

A polyester film for a surface protective film of a flexible display, which has a thickness of 25 to 100 μm, an intrinsic viscosity of 0.50 dl/g or more and 1.00 dl/g or less, a plane orientation coefficient ΔP of 0.160 or more, a film crystallinity by a density method of 48% or more and 65% or less, and a film surface crystallinity of 1.20 or more and 3.0 or less,
A polyester film for a surface protective film of a flexible display, having an indentation depth of 1.4 μm or less after unloading a test force in the thickness direction of the film, measured using a dynamic ultra-micro hardness tester under the following measurement conditions.
≪Measurement conditions≫
Test mode: Load-unload test Indenter used: Ridge angle 115 degrees, triangular pyramid indenter Indenter modulus of elasticity: 1.140 × 106 N/mm ²
Indenter Poisson's ratio: 0.07
Test force: 50mN
Load speed: 4.44mN/sec
Load holding time: 2 sec
Unloading retention time: 0 sec The polyester film for a surface protective film of a flexible display according to claim 1, which has a total light transmittance of 85% or more and a haze of 3% or less. A biaxially stretched polyester film having an easy-adhesion layer on at least one surface of the biaxially stretched polyester film,
3. The polyester film for a surface protective film of a flexible display according to claim 1 or 2, wherein the easy-adhesion layer contains a water-soluble polyurethane resin. Having a hard coat layer on at least one surface of the polyester film for a surface protective film of a flexible display according to any one of claims 1 to 3,
The hard coat layer contains an acrylate resin having 3 or more reactive groups in the molecule,
A hard coat film for a surface protective film of a flexible display, wherein the hard coat layer has a pencil hardness of 2H or more. A display body using the hard coat film for a surface protective film of a flexible display according to claim 4 as a surface protective film.