JP2020012087A - Polyester film for surface protective film of flexible display - Google Patents

Abstract

To inexpensively provide a polyester film showing a high pencil hardness scale when a hard coat layer is laminated.SOLUTION: A polyester film for a surface protective film of a flexible display is provided, which has a thickness of 25 to 100 μm and an indentation depth of 1.4 μm or less after a test load is removed. A hard coat film for a surface protective film of a flexible display and a display body are provided, which use the above polyester film for a surface protective film of a flexible display.SELECTED DRAWING: None

Description

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

  Portable terminal devices such as displays and smartphones are becoming thinner and thinner. At present, since all use rigid housings, tempered glass is generally used to protect the display. On the other hand, in recent years, a display emphasizing design has been preferred. For example, a display having a special shape such as a circle is preferred. Particularly in recent years, a flexible display for the purpose of installing the display on a curved surface has been demanded.

  However, since a glass material cannot be used for a flexible display, an acrylic plate or the like has been used. However, a thin acrylic plate has a problem that it is weak in scratches and impact resistance. Polycarbonate having high impact resistance and high light resistance may be used, but is expensive and has a problem of low pencil hardness. On the other hand, the polyester film is inexpensive and has excellent impact resistance, but has a problem that the pencil hardness is insufficient.

  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 surface has low scratch resistance.

  Further, as disclosed in Patent Document 2, a transparent polyimide material which can realize high pencil hardness without depending on a hard coat material has also been proposed. However, this material is very expensive and has a problem in mass productivity.

JP-A-2017-008148 JP-A-2006-222797

  The present invention is intended to solve the above-mentioned problems of the transparent film, and is to provide a polyester film having a high pencil hardness when a hard coat layer is laminated, at a low cost.

That is, the present invention has 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 pressing depth after unloading a test force of 1.4 μm or less.
2. 2. The polyester film for a surface protective film of a flexible display according to the above item 1, wherein the plane orientation coefficient ΔP is 0.160 or more.
3. The polyester film for a surface protective film of a flexible display according to claim 1 or 2, wherein the polyester film is a biaxially stretched polyester film, and has an easily adhesive layer on at least one surface of the biaxially stretched polyester film.
4. 4. A hard coat film for a surface protection film of a flexible display, comprising a hard coat layer on at least one surface of the polyester film for a surface protection film of the flexible display according to any one of the first to third aspects.
5. A display using the hard coat film for a surface protective film of the flexible display according to the fourth aspect as a surface protective film.

  The polyester film of the present invention has a surface hardness higher than that of 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 the display body, so that the surface of the display body is damaged. This makes it possible to provide a display body that is resistant to shock, deformation, etc., and has excellent design. The display can provide a beautiful image.

(Display body)
The display according to the present invention generally refers to a display device, and as a type of display, there are an LCD, an organic EL display, an inorganic EL display, an LED, an FED, etc., but it has a bendable structure. LCD, organic EL, and inorganic EL are preferable. In particular, an organic EL having a small layer structure and a wide color gamut is more preferable.

  Examples of the form of the display include a large screen, a display, a digital signage, a bent-type portable terminal, a PC, and the like, which can be used without limitation. In particular, it is preferably used for a display with a touch panel having a touch panel function of directly touching and operating, a digital signage, a bending-type portable terminal, a PC, and the like. The display is particularly preferably a flexible display as described below.

(Flexible display)
The 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 lightweight. In particular, a folder-type display that is a folding type display, a rollable display that is a roll-up type display, and the like that can be used in a paper-like manner are included.

(Polyester film)
Transparent films include PI films, polyester films, polycarbonate films, acrylic films, TAC films, cycloolefin polymer films, and other films with high light transmission and low haze, but among them, high impact resistance and moderate pencil hardness Is preferred, and a polyethylene terephthalate film which can be produced at low cost is particularly preferred.

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

  Examples of the polyester resin include a polyester film made of polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or a copolymer containing a main component of these resins. 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 base 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 such as acids; 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; and 1,4-cyclohexanedimethanol. Alicyclic glycols; polyethylene glycols having an average molecular weight of 150 to 20,000. The mass ratio of the copolymer component of the preferred copolymer is less than 20% by mass. When the amount is less than 20% by mass, the film strength, transparency and heat resistance are preferably maintained.

  In the production of the polyester film, the intrinsic viscosity of at least one or more 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. It is more preferably at least 0.58 dl / g, particularly preferably at least 0.63 dl / g. On the other hand, when the intrinsic viscosity is 1.00 dl / g or less, the increase in the filtration pressure of the molten fluid does not become too large, and it is easy to stably operate the film production, which is preferable.

  Regardless of whether the film has a single-layer structure or a laminated structure, the intrinsic viscosity of the film is preferably 0.50 dl / g or more. An increase in the molecular weight of the polyester resin is preferable because the elastic behavior of the film is improved, and deformation can be effectively suppressed with respect to the load of the pencil when measuring the pencil hardness. It is more preferably at least 0.55 dl / g, particularly preferably at least 0.63 dl / g. On the other hand, although the pencil hardness is improved by increasing the intrinsic viscosity, there is a concern that the productivity may decrease. Therefore, when the intrinsic viscosity is 1.00 dl / g or less, it is preferable because the operability can be produced with good operability.

  The thickness of the polyester film is preferably from 25 to 100 μm, more preferably from 30 to 75 μm. When the thickness is 25 μm or more, handling properties and impact resistance are improved, and when the thickness is 100 μm or less, it is advantageous not only for weight reduction but also for flexibility and workability.

  The surface of the polyester film of the present invention may be smooth or may have irregularities. However, since the surface is used for a protective film on the surface of a display, a decrease in optical properties due to irregularities is not preferred. The haze is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. When the haze is 3% or less, the visibility of the image can be improved. The smaller the lower limit of the haze, the better, but it may be 0.1% or more, or 0.3% or more.

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

  A known method can be adopted as a method of blending the particles with the base film. For example, it can be added at any stage of producing the polyester, but is preferably added as a slurry dispersed in ethylene glycol or the like at the stage of esterification or after the end of the transesterification reaction and before the start of the polycondensation reaction. Then, the polycondensation reaction may proceed. Also, a method of blending a polyester raw material with a slurry of particles dispersed in ethylene glycol or water using a kneading extruder with a vent, or a method of blending dried particles with a polyester raw material using a kneading extruder And so on.

  Above all, after homogenously dispersing the aggregated inorganic particles in a monomer liquid that becomes a part of the polyester raw material, the filtered product is used before the esterification reaction, during the esterification reaction or after the esterification reaction in the remaining polyester raw material. The addition method 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, and when added to the rest of the raw materials, the dispersibility of the particles is good, and a new Aggregates are less likely to occur. From this viewpoint, it is particularly preferable to add the remaining raw material in a low temperature state before the esterification reaction.

  Further, after a polyester containing particles is obtained in advance, the number of protrusions on the film surface can be further reduced by a method (master batch method) of kneading and extruding the pellets and the pellets containing no particles.

  Further, the polyester film may contain various additives as long as the preferable range of the total light transmittance is maintained. Examples of the additive include an antistatic agent, a UV absorber, and a stabilizer.

  The total light transmittance of the polyester film is preferably 85% or more, more preferably 87% or more. If the transmittance is 85% or more, the visibility can be sufficiently ensured. The higher the total light transmittance of the polyester film, the better, but it may be 99% or less, or 97% or less.

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

  Examples of the surface treatment method include, for example, sand blast treatment, roughening treatment by solvent treatment, corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone / ultraviolet irradiation treatment, flame treatment, chromic acid treatment, and hot air treatment. Oxidation treatment etc. are mentioned, and it can be used without particular limitation.

  Further, the adhesion can be improved by an adhesion improving layer such as an easy adhesion layer. The easy-adhesion layer can be used without any particular limitation, such as an acrylic resin, a polyester resin, a polyurethane resin, and a polyether resin, and can be formed by a general coating method, preferably a so-called in-line coating formulation.

  The above-mentioned polyester film is, for example, after uniformly dispersing the inorganic particles in a monomer liquid to be a part of the polyester raw material and filtering, and then adding the remaining polyester raw material to the polymerization step of polymerizing the polyester, It can be manufactured by melt-extruding into a sheet through a filter, cooling it, stretching it, and performing a film forming step of forming a base film.

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

  After mixing and drying the PET pellets at a predetermined ratio, the PET pellets are supplied to a known extruder for melt lamination, extruded from a slit die into a sheet, 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 in the case of producing a multilayer film, two or more extruders, two or more layers of a manifold or a merging block (for example, a merging having a square merging section) Using a block), a plurality of film layers constituting each outermost layer are laminated, and two or more sheets are extruded from a die and cooled by a casting roll to form an unstretched film.

  In this case, at the time of melt extrusion, it is preferable to perform high-precision filtration at an arbitrary place where the molten resin is maintained 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. However, the filter medium of the stainless sintered body is excellent in the performance of removing aggregates and high-melting organic substances containing Si, Ti, Sb, Ge, and Cu as main components. Therefore, it is preferable.

  Further, the filtration particle size (initial filtration efficiency: 95%) of the filter medium is preferably 20 μm or less, particularly preferably 15 μm or less. When the filtration particle size (initial filtration efficiency 95%) of the filter medium exceeds 20 μm, foreign substances having a size of 20 μm or more cannot be sufficiently removed. By performing high-precision filtration of the molten resin using a filter material having a filter particle size (initial filtration efficiency of 95%) of 20 μm or less, productivity may be reduced, but a film having few projections due to coarse particles is obtained. Preferred above.

  More specifically, for example, PET pellets are sufficiently vacuum-dried, fed 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 a roll heated to 80 to 120 ° C to obtain a uniaxially oriented PET film. Furthermore, the end of the film is gripped with a clip, 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, the resultant is guided to a heat treatment zone of 160 to 250 ° C., and heat treatment is performed for 1 to 60 seconds to complete the crystal orientation. During this heat treatment step, if necessary, a 1 to 12% relaxation treatment may be performed in the width direction or the longitudinal direction.

(Crystallinity)
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 film forming equipment conditions, film forming speed, and film thickness. Therefore, 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 200 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 a temperature of 250 ° C. or lower is preferable because a decrease in crystallization due to remelting of the polyester can be suppressed.

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

  The crystallinity of the polyester film determined by the density method is preferably at least 48%, more preferably at least 50%, even more preferably at least 52%. 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 more, more preferably 1.25 or more, and even more preferably 1.27 or more. 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 the dynamic ultra-micro hardness tester is preferably 1.4 μm or less, more preferably 1.35 μm or less, and more preferably 1.30 μm or less. More preferred. By lowering the indentation depth after test force unloading (final deformation under load), deformation after pencil hardness evaluation can be suppressed, and pencil hardness can be improved.

  The plane orientation ΔP is preferably at least 0.160, more preferably at least 0.163, even more preferably at least 0.165. By increasing the plane orientation, it becomes strong against a load in the thickness direction, and the amount of deformation during unloading can be suppressed. Although it is preferably high, the upper limit of the film that can be formed is about 0.170.

  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. 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 performed without any particular limitation such as simultaneous biaxial stretching and sequential biaxial stretching.

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

  As the resin used for the easy adhesion layer, for example, a polyester resin, a polyurethane resin, a polyester polyurethane resin, a polycarbonate polyurethane resin, an acrylic resin, and the like can be used without particular limitation. Examples of the crosslinking agent for the easily adhesive layer include melamine-based, isocyanate-based, oxazoline-based, and epoxy-based crosslinking agents. Two or more kinds may be used in combination. These are preferably applied with an aqueous coating solution in view of the properties of the in-line coating, and the resin or the crosslinking agent is preferably a water-soluble or water-dispersible resin or compound.

  It is preferable to add particles to the easily adhesive layer in order to impart lubricity. The average particle size of the fine particles is preferably 2 μm or less. When the average particle diameter of the particles exceeds 2 μm, the particles are likely to fall off the easy-adhesion layer. Examples of the 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. These may be added alone to the easy-adhesion layer, or two or more of them may be added in combination.

  In addition, as a method of applying the coating liquid, a known method can be used similarly to the above-described coating layer. For example, a reverse roll coating method, a gravure coating method, a kiss coating method, a roll brush method, a spray coating method, an air knife coating method, a wire bar coating method, a pipe doctor method, and the like, and these methods can be used alone. Alternatively, they can be performed in combination.

(Hard coat layer)
As the resin for forming the hard coat layer, a (meth) acrylate, siloxane-based, inorganic hybrid-based, urethane acrylate-based, polyester acrylate-based, epoxy-based resin can be used without any particular limitation. Further, two or more kinds of materials can be used as a mixture, and particles such as an inorganic filler and an organic filler can be added. Among them, (meth) acrylates and acrylate resins are preferable, and resins having three or more reactive groups in the molecule as essential components are preferable.

(Resin having three or more reactive groups)
Examples of the resin having three or more reactive groups include tris (2-acryloyloxyethyl) isocyanurate, tris (3-acryloyloxypropyl) isocyanurate, tris (2-methacryloyloxyethyl) isocyanurate, and tris (3- Tris [(meth) acryloyloxyalkyl] isocyanurate such as (methacryloyloxypropyl) isocyanurate, and further trimethylolpropane tri (meth) acrylate, dipentaerythritol tri (meth) acrylate, 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 dipen Erythritol penta (meth) acrylate, dipentaerythritol hexa (meth) acrylate, (meth) acrylates such as caprolactone-modified dipentaerythritol hexa (meth) acrylate. These may be used alone or in combination of two or more.

  In addition, commercially available hard coat agents and the like can be used. For example, a beam set (registered trademark) series manufactured by Arakawa Chemical Industry Co., Ltd., an Aika Itron (registered trademark) series manufactured by Aika Kogyo Co., Ltd., and a Lioduras (registered trademark) series manufactured by Toyo Ink Co., Ltd. ) It can also be used by mixing with acrylic ester compounds.

(Photopolymerization initiator)
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 the photopolymerization initiator 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, and 2,4. -Diethylthioxanthone, 1-hydroxycyclohexylphenylketone, benzyldiphenylsulfide, tetramethylthiurammonosulfide, 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 considered 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 preferable, and 2-hydroxy-2-methyl-1-phenyl-propan-1-one and 2-methyl-1- [4- (methylthio) phenyl] -2-morpholinopropan-1-one are particularly preferable. preferable. These may be used alone or in combination of two or more.

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

(Thickness)
The thickness of the hard coat layer is preferably from 1 to 55 μm. If it is thicker than 1 μm, it is sufficiently cured, and good pencil hardness is obtained. By setting the thickness to 55 μm or less, curling due to curing shrinkage of the hard coat can be suppressed, and the handleability of the film can be improved.

(Coating method)
As a method of applying the hard coat layer, a Meyer bar, a gravure coat, a die coater, a knife coater, or the like can be used without any particular limitation, and can be appropriately selected according to the viscosity and the film thickness. The drying temperature is preferably from 60 to 100 ° C, more preferably from 60 to 90 ° C. When the temperature is 60 ° C. or higher, drying can be sufficiently performed in a short time, and when the temperature is 100 ° C. or lower, deformation of the polyester film or the like does not occur, which is preferable.

(Curing conditions)
As a method for curing the hard coat layer, an energy beam such as an ultraviolet ray or an electron beam, or a curing method using heat can be used, but an ultraviolet ray or an electron beam is preferable in order to reduce damage to the film.

(UV)
As the ultraviolet lamp, a high-pressure mercury lamp, an electrodeless lamp, and the like can be used as appropriate. The integrated quantity of ultraviolet light is preferably 50 to 500 mJ / cm ^2, more preferably 100~200mJ / cm ^2. Irradiation with an integrated light quantity of 50 mJ / cm ² or more can provide a sufficient pencil hardness, and with an integrated light quantity of 500 mJ / cm ² or less, the processing speed can be increased, and the economic efficiency is improved, which is preferable. Irradiation with ultraviolet rays in an inert gas such as nitrogen, argon, or carbon dioxide can improve the curability, and can be appropriately selected.

(Electron beam)
The electron beam irradiation apparatus is of an area type, a scanning type or the like, and can be appropriately selected and used. The integrated irradiation amount of the electron beam is preferably 25 to 500 kGy, more preferably 50 to 300 kGy. Irradiation with an integrated irradiation amount of 25 kGy or more can provide a sufficient pencil hardness, and when it is 500 kGy or less, the processing speed can be increased, and economy 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, ozone generation can be suppressed, and safety during work can be improved.

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

(Type of hard coat layer)
The hard coat layer in the present invention may be provided with another function 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, an antiglare layer having a certain pencil hardness as described above, an antiglare antireflection layer, an antireflection layer, a hard coat layer to which additional functions such as a low reflection layer and an antistatic layer are added in the present invention. Is preferably applied.

  Next, the effects of the present invention will be described using examples and comparative examples. First, a method for evaluating characteristic values used in the present invention will be described below.

(1) Intrinsic viscosity After the film or polyester resin was pulverized and dried, it was dissolved in a mixed solvent of phenol / tetrachloroethane = 60/40 (mass ratio). After the solution was subjected to centrifugation to remove inorganic particles, the flow time of the solution having a concentration of 0.4 (g / dl) at 30 ° C. and the flow time of the solvent alone were measured at 30 ° C. using an Ubbelohde viscometer. From these time ratios, the intrinsic viscosity was calculated using the Huggins equation, assuming that the Huggins constant was 0.38. In the case of a laminated film, the limiting viscosity of each layer alone was evaluated by shaving off the corresponding polyester layer of the film according to the laminated thickness.

(2) Test force Depressed depth after unloading The sample was cut into a square of about 2 cm, and the opposite side of the measurement surface was put on an adhesive (Cemedine High Super 30) on a micro cover glass 18 × 18 mm (Matsunami Kalas). Fixed. After sticking and fixing, the sample 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 after unloading the test force ( μm) was measured.
≪Measurement conditions≫
Test mode: Load-unload test
Indenter used: 115 degrees between edges, triangular pyramid indenter
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
A 5-point sample was collected, and the refractive index in the longitudinal direction of the film (Nx), the refractive index in the width direction (Ny), and the refractive index in the width direction were measured with an Abbe refractometer using sodium D line as a light source according to JIS K7142: 5.1 (Method A). The refractive index (Nz) in the thickness direction was measured, and the plane orientation coefficient (ΔP) was calculated by the following equation. The average value of the obtained plane orientation coefficients was defined as the plane orientation coefficient.
ΔP = (Nx + Ny) / 2−Nz
(4) Pencil hardness According to JIS K 5600-5-4: 1999, the 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 without deformation were OK. The pencil hardness was measured 5 times and the evaluation of OK was 4 times or more.

(Polymerization of urethane resin)
In a four-necked flask equipped with a stirrer, a Dimroth condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer, 72.96 parts by mass of 1,3-bis (isocyanatomethyl) cyclohexane and 12.60 dimethylolpropionic acid. Parts by mass, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of a polycarbonate diol having a number average molecular weight of 2,000, 85.00 parts by mass of acetonitrile as a solvent, and 5.00 parts by mass of N-methylpyrrolidone, and nitrogen atmosphere Under stirring at 75 ° C. for 3 hours, it was confirmed that the reaction solution reached a predetermined amine equivalent. Next, the reaction solution was cooled to 40 ° C., and 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer D solution. Next, 450 g of water was added to a reaction vessel equipped with a homodisper capable of high-speed stirring, and isocyanate-terminated prepolymer was added and dispersed in water while stirring and mixing at 25 ° C. Thereafter, acetonitrile and a part of 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)
In a flask equipped with a thermometer, a nitrogen gas inlet tube, a reflux condenser, a dropping funnel, and a stirrer, 200 parts by mass of isophorone diisocyanate, and 4 parts by mass of 3-methyl-1-phenyl-2-phospholene-1-oxide as a carbodiimidation catalyst. And stirred at 180 ° C. for 10 hours under a nitrogen atmosphere to obtain an 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 thereto at 50 ° C. to obtain a yellow transparent water-soluble carbodiimide compound (B) having a solid content of 40% by mass.
(Preparation of coating solution for easy adhesion layer)

The following coating agents were mixed to prepare a 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 part by mass (silica sol having an average particle size of 40 nm) (Solids concentration 40 mass%)
Surfactant 0.05 parts by mass (silicone, solid content concentration 100% by mass)

(Preparation of hard coat coating liquid 10)
95 parts by weight of pentaerythritol triacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., A-TMM-3, solid content concentration: 100%), photopolymerization initiator (manufactured by BASF Japan, Irgacure (registered trademark) 907, solid content concentration: 100%) ) 5 parts by weight, 0.1 parts by weight of a leveling agent (BYK307, manufactured by BYK Japan KK, solid content concentration 100%) were mixed, diluted with a solvent of toluene / MEK = 1/1, and coated at a concentration of 40%. Liquid 10 was prepared.

(Preparation of hard coat coating solution 11)
50 parts by weight of urethane acrylate (manufactured by Kyoeisha Chemical Co., Ltd., UA-306H, solid content concentration 100%), 45 parts by weight of pentaerythritol triacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., A-TMM-3, solid content concentration 100%), 5 parts by weight of a photopolymerization initiator (Irgacure (registered trademark) 907, manufactured by BASF Japan, solid content concentration: 100%) and 0.1 part by weight of a leveling agent (BYK307, manufactured by BYK Japan, 100% solid content). The mixture was mixed and diluted with a solvent of toluene / MEK = 1/1 to prepare a coating solution 11 having a concentration of 40%.

(Preparation of polyethylene terephthalate pellets)
As the esterification reactor, a continuous esterification reactor consisting of a three-stage complete mixing tank having a stirrer, a decomposer, a raw material charging port, and a product outlet was used, TPA was set at 2 tons / hr, EG was TPA1 The amount of antimony trioxide was set to 2 mol with respect to the mol, and the amount of Sb atoms was 160 ppm with respect to the produced PET. These slurries were continuously supplied to the first esterification reactor of the esterification reactor, and at normal pressure. The reaction was carried out at 255 ° C. with an average residence time of 4 hours. Next, the reaction product in the first esterification reactor is continuously taken out of the system, supplied to the second esterification reactor, and distilled out of the first esterification reactor in the second esterification reactor. An EG solution containing 8% by mass of EG to be produced with respect to the produced polymer (produced PET), and further containing magnesium acetate in an amount of 65 ppm of Mg atoms with respect to the produced PET, and 20 ppm of P atoms with respect to the produced PET An EG solution containing the following amount of TMPA was added, and reacted at 260 ° C. under normal pressure for an average residence time of 1.5 hours. Next, the reaction product in the second esterification reaction vessel is continuously taken out of the system and supplied to the third esterification reaction vessel, and further contains TMPA in such an amount that P atoms become 20 ppm with respect to the produced PET. The EG solution was added and reacted at 260 ° C. at normal pressure for an average residence time of 0.5 hour. The esterification reaction product generated in the third esterification reactor is continuously supplied to a three-stage continuous polycondensation reaction apparatus to perform polycondensation, and further, a stainless steel sintered material filter material (nominal filtration accuracy of 5 μm (90% particle cut) 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 is filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 10 μm, particles cut by 95%), extruded in a sheet shape from a die, and then applied to a casting drum having a surface temperature of 30 ° C. by using an electrostatic application casting method. It was contacted and cooled and solidified to form an unstretched film. This unstretched film was stretched 3.4 times at 85 ° C. in the longitudinal direction. The coating solution for easy adhesion layer was applied on one side of a PET film by a roll coating method, and then dried at 80 ° C. for 20 seconds. In addition, it adjusted so that the coating amount after drying of the last (after biaxial stretching) might be set 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 on the easy-adhesion layer of the polyethylene terephthalate film, a hard coat coating solution 10 was applied so that the thickness after drying was 5.0 μm, dried at 80 ° C. for 1 minute, and then exposed to ultraviolet light. (A cumulative light amount of 200 mJ / cm ² ) to obtain a hard coat film.

(Examples 2 to 4, Comparative Examples 1 to 3)
Stretching was performed in the same manner as in Example 1 except that the stretching 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)
Stretching was performed 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)
Stretching was carried out in the same manner as in Example 1 except that the stretching ratio was changed using polyethylene-2,6-naphthalate (PEN) having an intrinsic viscosity of 0.62 dl / g to obtain a polyethylene naphthalate film and a hard coat film.

(Examples 8 and 9)
A hard coat film was produced using the same polyethylene terephthalate film as in Example 1 and employing the hard coat conditions described in Table 1.

  The produced hard coat film was bonded to an organic EL module via a 25-μm-thick adhesive layer to produce a flexible display. In each of the examples, the organic EL module could be protected from external impact by the hard coat film, and good visibility could be obtained.

  According to the present invention, it is possible to provide a polyester film having a high surface hardness when a hard coat layer or the like is laminated, and a hard coat film having a hard coat layer laminated on the surface of the polyester film of the present invention can be used for display such as a flexible display. By being used as a body surface protection film, it has become possible to provide a display body that is resistant to scratches, impacts, deformations, and the like on the surface of the display body and has excellent design.

Claims (5)

  A polyester film for a surface protective film of a flexible display having a thickness of 25 to 100 μm and a pressing depth after unloading a test force of 1.4 μm or less.   The polyester film for a surface protective film of a flexible display according to claim 1, wherein the plane orientation coefficient ΔP is 0.160 or more.   The polyester film for a surface protective film of a flexible display according to claim 1 or 2, wherein the polyester film is a biaxially stretched polyester film, and has an easily adhesive layer on at least one surface of the biaxially stretched polyester film.   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 claim 1.   A display using the hard coat film for a surface protection film of a flexible display according to claim 4 as a surface protection film.