JP7334837B2 - 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 flexible display comprising a polyester film having an intrinsic viscosity of 0.60 to 1.0 dL/g, a crystallinity of 48% or more by a density method, and a crystallinity of 1.20 or more on the polyester film surface by an ATR method. polyester film for surface protection film.
2. 1. The polyester film for a surface protective film of a flexible display as described in 1 above, which has a thickness of 25 to 188 μm.
3. 3. The polyester film for a surface protective film of a flexible display according to the above 1 or 2, wherein the polyester film is a biaxially stretched polyester film, and an easily adhesive layer is provided on at least one surface of the 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 according to any one of the first to third above.
5. A display using the hard coat film according to the fourth aspect as a surface protective film.

When the polyester film of the present invention is laminated with a hard coat layer or the like, the surface hardness becomes higher than that of a general polyester film, and the hard coat film is used as a surface protective film of the display body, thereby preventing scratches on the surface of the display body. It is possible to provide a display body that is resistant to impact, deformation, etc., and has 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., and have a bendable structure. LCD, organic EL, and inorganic EL are preferable. In particular, an organic EL with a reduced number of layers and a wide color gamut is more preferable.

The forms of the display include large screens, displays, digital signage, bendable portable terminals, personal computers, and the like, and can be used without limitation. In particular, it is preferably used for a display with a touch panel, a digital signage, a bendable mobile terminal, a personal computer, and the like, 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 polyimide film, polyester film, polycarbonate film, acrylic film, triacetyl cellulose film, cycloolefin polymer film, and other films with high light transmittance and low haze. A polyester film having a pencil hardness is preferred, and a polyethylene terephthalate film, which is inexpensive to produce, is particularly preferred.

In the present invention, the polyester film may be a single-layer film composed of one or more polyester resins, or when two or more types of polyester are used, may be a multi-layer structure film, or may be 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 polyester film is preferable, and a biaxially stretched polyethylene terephthalate film is particularly preferable, from the viewpoints 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-naphthalene dicarboxylic acid. aromatic dicarboxylic acids such as 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; 1,4-cyclohexanedimethanol and the like. 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 intrinsic viscosity of at least one resin pellet is preferably in the range of 0.60 to 1.0 dl/g. When the intrinsic viscosity is 0.60 dl/g or more, the pencil hardness after hard coating is improved, which is preferable. More preferably 0.65 dl/g or more, particularly preferably 0.70 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 0.60 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 0.65 dl/g or more, particularly preferably 0.70 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 25-188 μm, more preferably 25-125 μm. When the thickness is 25 μm or more, the handleability and impact resistance are improved, and it is preferable.

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 better not to make the unevenness of the film surface too large. It can be formed by blending a filler in the 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 is preferably added as a slurry dispersed in ethylene glycol or the like at the stage of esterification, or after the completion of the transesterification reaction and before the start of the polycondensation reaction. Then, the polycondensation reaction may proceed. Alternatively, a method of blending a slurry of particles dispersed in ethylene glycol or water with a polyester raw material using a kneading extruder with a vent, or a method of blending dried particles and a polyester raw material using a kneading extruder. etc.

Among them, after uniformly dispersing the aggregate inorganic particles in the monomer liquid that is part of the polyester raw material, the filtered material is added to the remainder of the polyester raw material before, during, or after the esterification reaction. A method of adding is preferred. According to this method, since the monomer liquid has a low viscosity, homogeneous dispersion of the particles and high-precision filtration of the slurry can be easily performed. 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.

Surface treatment methods include, for example, sandblasting, unevenness treatment by solvent treatment, corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone/ultraviolet irradiation treatment, flame treatment, chromic acid treatment, hot air treatment, and the like. Examples include oxidation treatment and the like, which 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 is produced, for example, by homogenously dispersing inorganic particles in a monomer liquid that is part of the polyester raw material, filtering it, and then adding it to the remainder of the polyester raw material to polymerize the polyester. It can be manufactured through a film forming step of melt-extrusion through a filter into a sheet, cooling and stretching to form a base film.

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

PET pellets are mixed in a predetermined ratio, dried, fed to a known melt lamination extruder, extruded into a sheet from a slit-shaped die, cooled and solidified on a casting roll to form an unstretched film. . In the case of a single layer, one extruder is sufficient, but in the case of producing a film with a multilayer structure, two or more extruders, two or more layers of manifolds or confluence blocks (for example, a confluence having a square confluence A block) can be used to laminate a plurality of film layers constituting each outermost layer, two or more sheets can be extruded from a spinneret, and cooled with casting rolls 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 the sintered stainless steel has excellent performance in removing agglomerates and high-melting-point organic substances mainly composed of Si, Ti, Sb, Ge, and Cu. Therefore, it is preferable.

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 molten resin using a filter medium having a filtration particle size (initial filtration efficiency of 95%) of 20 μm or less, productivity may decrease, but a film with few protrusions due to coarse particles can be obtained. preferred above.

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 2.5 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. It can be said that 240°C is more preferable. 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, it is preferable that the crystallinity of the film by the density method is 48% or more and the crystallinity of the film surface by the ATR method is 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.

(Easy adhesion layer)
The easy-adhesion layer can be obtained by coating the coating liquid on one or both sides of an unstretched or longitudinally uniaxially stretched film, drying at 100 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. Particles contained in the easy-adhesion layer include, for example, titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, Examples include inorganic particles such as 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., and these methods can be used alone. Or it can be performed 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 three or more reactive groups include tris(2-acryloyloxyethyl) isocyanurate, tris(3-acryloyloxypropyl) isocyanurate, tris(2-methacryloyloxyethyl) isocyanurate, tris(3- Tris[(meth)acryloyloxyalkyl]isocyanurate such as methacryloyloxypropyl)isocyanurate, trimethylolpropane tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, propionic acid-modified dipentaerythritol tri(meth) Acrylates, 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 Examples include (meth)acrylates such as acrylates and caprolactone-modified dipentaerythritol hexa(meth)acrylate. 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 Industries, 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. ) It can also be used by mixing with 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, benzoin methyl benzoate, benzoin dimethyl ketal, 2,4 -diethylthioxanthone, 1-hydroxycyclohexylphenyl ketone, benzyldiphenylsulfide, tetramethylthiuram monosulfide, azobisisobutyronitrile, benzyl, dibenzyl, diacetyl, β-chloranthraquinone, (2,4,6-trimethyl benzyldiphenyl)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, which is said to have excellent surface curability, 1-hydroxy-cyclohexyl-phenyl-ketone, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1 -one is preferred, especially 2-hydroxy-2-methyl-1-phenyl-propan-1-one, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one preferable. 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 the hard coat film has a high crosslinking density and high hardness. can be

(film thickness)
The thickness of the hard coat layer is preferably 1 to 40 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 40 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, in the present invention, a hard coat layer having added 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 is also included in the present invention. is preferably applied.

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 centrifuging the solution to remove the inorganic particles, the flow-down time of the solution with a concentration of 0.4 (g/dl) at 30° C. and the flow-down time of the solvent alone were measured using an Ubbelohde viscometer. , and the time ratio thereof, the intrinsic viscosity was calculated using the Huggins equation, assuming that the Huggins constant is 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) Crystallinity by Density Method The density of the film sample was measured at three points, and the average value was defined as the crystallinity. The resin component is a mixture of completely amorphous and completely crystalline, and its density is as described later. The density of the sample is the sum of the masses of the components that make up the sample divided by the sum of the volumes of the components. , the crystallinity (weight ratio) of each resin was estimated. The sample density was determined according to the method (density gradient tube method) based on JISK-7112-1980. The formula for calculating the degree of crystallinity is as follows, and the values below were used for the densities of individual components (unit: g/cm3).

Crystallinity (%) = {dc (d-da) / d (dc-da)} × 100

Here, dc: density of completely crystalline portion, da: density of completely amorphous portion, d: density of sample polyethylene terephthalate resin: density of completely amorphous portion (da) = 1.34,
Density of perfectly crystalline portion (dc) = 1.46
Polyethylene naphthalate resin: Density of completely amorphous portion (da) = 1.32,
Density of perfectly crystalline portion (dc) = 1.41

(3) Surface crystallinity (ATR method)
One side of the stretched film (if one side of the film is corona-treated, the non-corona-treated side) is subjected to total reflection infrared absorption measurement (FT-IRATR measurement) under the following conditions, The crystallinity was calculated from the intensity ratio (1340 cm-1/1410 cm-1) of the absorption appearing near 1340 cm-1 and the absorption appearing near 1410 cm-1. Here, 1340 cm-1 is absorption due to bending vibration of CH2 (trans structure) of ethylene glycol, and 1410 cm-1 is absorption irrespective of crystal and orientation.
(measurement equipment, conditions)
FT-IR device: Bio Rad DIGILAB "FTS-60A/896" single reflection ATR attachment: SPECAC "golden gate MKII" internal reflection element: diamond
Incident angle: 45°
Resolution: 4cm-1
Accumulated times: 128 times

(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 mass of 1,3-bis(isocyanatomethyl)cyclohexane and 12.60 parts of dimethylolpropionic acid were added to a four-necked flask equipped with a stirrer, a Dimroth condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer. Part by mass, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of polycarbonate diol having 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, and a nitrogen atmosphere was added. After stirring at 75° C. for 3 hours, it was confirmed that the reaction solution 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 placed in a flask equipped with a thermometer, a nitrogen gas inlet tube, a reflux condenser, a dropping funnel, and a stirrer. and stirred for 10 hours at 180° C. under a nitrogen atmosphere to obtain an isocyanate-terminated isophorone carbodiimide (degree of polymerization=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 solution)

A coating liquid was prepared by mixing the following coating agents.
Water 16.97 parts by mass Isopropanol 21.96 parts by mass Polyurethane resin (A) 3.27 parts by mass Water-soluble carbodiimide compound (B) 1.22 parts by mass Particles 0.51 parts by mass (silica sol having an average particle size of 40 nm, solid content Concentration 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 (BASF Japan Co., Ltd., Irgacure (registered trademark) 907, solid content concentration 100% ) 5 parts by weight and 0.1 part by weight of a leveling agent (BYK307 manufactured by BYK Chemie Japan, solid content concentration 100%), diluted with a solvent of toluene/MEK = 1/1, and applied at a concentration of 40% Liquid 10 was prepared.

(Preparation of Hard Coating Liquid 11)
Urethane acrylate (Kyoeisha Chemical Co., Ltd., UA-306H, solid content concentration 100%) 50 parts by weight, pentaerythritol triacrylate (Shin Nakamura Chemical Co., Ltd., A-TMM-3, solid content concentration 100%) 45 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 They 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 (a))
As the esterification reactor, a continuous esterification reactor comprising a three-stage complete mixing tank having a stirrer, a partial condenser, a raw material inlet and a product outlet was used, TPA was 2 tons/hr, and EG was TPA1. The amount of antimony trioxide is adjusted to 2 mol per mol, and the amount of antimony trioxide is adjusted so that the Sb atom is 160 ppm with respect to the produced PET. The reaction was carried out at 255° C. with an average residence time of 4 hours. Next, the reaction product in the first esterification reaction can is continuously taken out of the system, supplied to the second esterification reaction can, and distilled from the first esterification reaction can into the second esterification reaction can. 8% by mass of EG is supplied to the produced polymer (produced PET), and an EG solution containing magnesium acetate in an amount such that the Mg atom is 65 ppm relative to the produced PET, and the P atom is 20 ppm relative to the produced PET. EG solution containing the amount of TMPA was added and reacted at normal pressure at 260° C. with an average residence time of 1.5 hours. Next, the reaction product in the second esterification reactor is continuously taken out of the system and supplied to the third esterification reactor, and further contains TMPA in an amount such that the P atom is 20 ppm with respect to the produced PET. The EG solution was added and reacted 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 can is continuously supplied to a three-stage continuous polycondensation reaction apparatus for polycondensation, and a stainless steel sintered filter material (nominal filtration accuracy of 5 μm 90% particle cut) to obtain polyethylene terephthalate pellets (a) with a limiting viscosity of 0.58 dl/g.

(Preparation of polyethylene terephthalate pellets (b))
Regarding the production process of polyethylene terephthalate pellets (a), the intrinsic viscosity was adjusted to 0.62 dl/g by the same method except that the residence time of the third esterification reaction was adjusted, and polyethylene terephthalate pellets (b) were obtained. .

(Preparation of polyethylene terephthalate pellets (c))
Polyethylene terephthalate pellets (a) were solid-phase polymerized at 220° C. under a reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus to prepare polyethylene terephthalate pellets (c) having an intrinsic viscosity of 0.67 dL/g.

(Preparation of polyethylene terephthalate pellets (d))
Polyethylene terephthalate pellets (a) are solid-phase polymerized at 220° C. under a reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus, changing the time from (c) above, to obtain an intrinsic viscosity of 0.75 dL / g. A polyethylene terephthalate pellet (d) was produced.

(Preparation of polyethylene terephthalate pellets (e))
Polyethylene terephthalate pellets (a) were solid-phase polymerized at 220° C. under a reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus with different time periods from (c) and (d), resulting in an intrinsic viscosity of 0.0. 83 dL/g polyethylene terephthalate pellets (e) were produced.

(Examples 1 to 5, Comparative Examples 1 to 3)
After drying the polyethylene terephthalate pellets (c) at 180°C for 8 hours under reduced pressure (3 Torr), the polyethylene terephthalate pellets (a) were supplied to the extruder and melted at 285°C. This polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 10 μm, 95% cut of particles), extruded from a spinneret in the form of a sheet, and cast onto a casting drum with a surface temperature of 30° C. using an electrostatic casting method. They were brought into contact and cooled to solidify to form 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 4.2 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 of Example 1 in Table 1.

In addition, using polyethylene terephthalate pellets (b) to (e), Examples 2 to 5 and Comparative Examples 1 to 3 in Table 1 were performed in substantially the same manner as in Example 1 except that the heat treatment temperature and heat treatment time were changed. of polyethylene terephthalate film was obtained.

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

(Examples 1-1 to 5-1, 1-2, 2-2 and Comparative Examples 1-1 to 3-1)
Using the conditions shown in Table 2, a hard coat film was produced in the same manner as in Example 1-1.

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. 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 having a high surface hardness when laminated with a hard coat layer or the like. By being used as a body surface protective film, it is possible to provide a display body that is resistant to scratches, impacts, deformation, etc. on the surface of the display body and has excellent design.

Claims (4)

A polyethylene terephthalate film having an intrinsic viscosity of 0.60 to 1.0 dL/g, a crystallinity of 48% or more and 65% or less as determined by a density method, and a surface crystallinity of 1.0% as determined by an ATR method. a polyethylene terephthalate film of 20 or more and 3.0 or less;
and a hard coat layer having a thickness of 1 to 40 μm provided on at least one surface of a polyethylene terephthalate film. 2. The hard coat film for a display surface protective film according to claim 1, which has a total light transmittance of 85% or more, a haze of 3% or less, and a thickness of 25 to 188 μm. 2. The hard coat film for a display surface protection film according to claim 1, wherein the polyethylene terephthalate film has an easy-adhesion layer on at least one surface thereof. A display using the hard coat film according to claim 1 as a surface protection film.