JP7247584B2 - Polyester film for surface protection film of foldable display and its application - Google Patents

Description

The present invention relates to a polyester film for a surface protective film of a foldable display, a hard coat film for a surface protective film of a foldable display, a foldable display, and a mobile terminal device, and the deformation of the film located on the surface even after repeated folding. The present invention relates to a foldable display and a portable terminal device in which image disturbance is unlikely to occur, and a polyester film and a hard coat film for a surface protective film of the foldable display.

As mobile terminal devices become thinner and lighter, mobile terminal devices represented by smartphones are widely used. Mobile terminal devices are required to have various functions, but are also required to be convenient. For this reason, mobile terminal devices that are in widespread use need to have a small screen size of about 6 inches because it is assumed that they can be easily operated with one hand and stored in a pocket of clothes.

On the other hand, tablet terminals with a screen size of 7 inches to 10 inches are expected to be used not only for video content and music, but also for business use, drawing use, reading, etc., and have high functionality. However, it cannot be operated with one hand, is inferior in portability, and has a problem of convenience.

In order to achieve these, a method has been proposed to make it more compact by connecting multiple displays (Patent Document 1), but since the bezel part remains, the image is cut off and visibility is reduced. and has not spread.

Therefore, in recent years, mobile terminals incorporating flexible displays and foldable displays have been proposed. With this method, the image is not interrupted, and it can be conveniently carried as a mobile terminal equipped with a large-screen display.

Here, in conventional displays and portable terminal devices that do not have a folding structure, the surface of the display could be protected with a non-flexible material such as glass. In the case of a one-sided display, it is necessary to use a hard coat film or the like that is flexible and capable of protecting the surface. However, in the foldable display, since the portion corresponding to the certain folding portion is repeatedly folded, the film at the portion is deformed over time, and there is a problem such as distortion of the image displayed on the display.

Therefore, a method of partially changing the film thickness has been proposed (see Patent Document 2), but it has a problem of poor productivity.

JP 2010-228391 A JP 2016-155124 A

The present invention is intended to solve the problems of the conventional display surface protection member as described above. To provide a foldable display and a portable terminal equipped with such a foldable display, a polyester film for a surface protective film and a hard coat film for a surface protective film of the foldable display. It is.

That is, the present invention consists of the following configurations.
1. A foldable display characterized by being a polyester film having a thickness of 10 to 75 μm and having a 0.2% proof strain of 2.6 to 5.0% in at least one of the longitudinal direction and the width direction. polyester film for surface protection film.
2. 1. The polyester film for a surface protective film of a foldable display as described in 1 above, wherein the strain at the 0.2% proof stress point in the bending direction is 2.6 to 5.0%.
(Here, the bending direction refers to a direction orthogonal to the folded portion when folding the polyester film.)
3. 3. The polyester film for a surface protective film of a foldable display as described in 1 or 2 above, wherein the film has an intrinsic viscosity of 0.60 to 1.0 dl/g.
4. A surface protective film for a foldable display, comprising a hard coat layer having a thickness of 1 to 50 μm on at least one side of the polyester film for a surface protective film for a foldable display according to any one of the first to third above. for hard coat film.
5. The hard coat film for a surface protective film of a foldable display according to 4 above, wherein the hard coat layer has a pencil hardness of H or more measured with a load of 750 g according to JIS K5600-5-4:1999. .
6. A foldable display in which the hard coat film for a surface protective film of a foldable display according to the above fourth or fifth is arranged as a surface protective film so that the hard coat layer is positioned on the surface, and A foldable display having a bending radius of 5 mm or less.
7. 6. The foldable display according to the sixth aspect, wherein a continuous single hard coat film is arranged through the folded portion of the foldable display. 8. A mobile terminal device having the foldable display according to the sixth or seventh aspect.

The foldable display using the polyester film for the surface protection film of the foldable display and the hard coat film of the present invention maintains mass productivity, and the polyester film and the hard coat film do not cause deformation after repeated folding. Therefore, the image is not distorted when the display is folded. A portable terminal device equipped with a foldable display as described above provides beautiful images, is rich in functionality, and is excellent in convenience such as portability.

FIG. 4 is a schematic diagram showing measurement points of the bending radius when folded in the present invention. FIG. 3 is a schematic diagram showing the bending direction of the polyester film for the surface protection film of the foldable display of the present invention. It is a schematic diagram for explaining the 0.2% proof stress point strain in the present invention.

(display)
The display 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, and the like. , organic EL, and inorganic EL are preferred. Particularly preferred are organic EL and inorganic EL, which can reduce the number of layers, and more preferred is organic EL, which has a wide color gamut.

(Foldable display)
The foldable display preferably has a structure in which one continuous display is folded in half to reduce the size by half and improve portability. At the same time, it is desirable to be thin and lightweight. Therefore, the bend radius of the foldable display is preferably 5 mm or less, more preferably 3 mm or less. If the bending radius is 5 mm or less, it becomes possible to reduce the thickness in the folded state. Although it can be said that the smaller the bending radius is, the better it is, it may be 0.1 mm or more, or 0.5 mm or more. Even if it is 1 mm or more, practicality is sufficiently good as compared with conventional displays that do not have a folding structure. The bending radius when folded is measured at the point 11 in the schematic diagram of FIG. 1, and means the inside radius of the folded portion when folded. In addition, the surface protection film, which will be described later, may be positioned outside or inside when the foldable display is folded.

(Organic EL)
A general structure of an organic EL display comprises an organic EL layer composed of electrode/electron transport layer/light emitting layer/hole transport layer/transparent electrode, a retardation plate for improving image quality, and a polarizing plate.

(Mobile terminal device having a touch panel)
When an organic EL display is used for a mobile terminal device having a touch panel, a touch panel module is arranged above the organic EL display or between the organic EL layer/phase difference plate. At this time, if an impact is applied from above, the organic EL and touch panel circuits may be disconnected, so a surface protective film is required. A hard coat layer is preferably laminated on the side.

(Surface protection film for foldable displays)
As the surface protection film, polyimide film, polyester film, polycarbonate film, acrylic film, triacetyl cellulose film, cycloolefin polymer film, etc. can be used as long as they have high light transmittance and low haze. Polyimide films and polyester films having high impact resistance and sufficient pencil hardness are preferred, and polyester films that can be produced at low cost are particularly preferred.

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

Polyester resins include, for example, polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, and polyester films made of copolymers containing these resin components as main components. Among them, a 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 polyester 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; 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; and 1,4-cyclohexanedimethanol. and polyethylene glycol 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 impact resistance of the obtained film is improved, and disconnection of the internal circuit due to an external impact is less likely to occur, which is preferable. Moreover, it is preferable because it contributes to small deformation when repeatedly bent. 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. It is more preferably 0.62 dl/g or more. More preferably, it is 0.68 dl/g or more. If it is 0.60 dl/g or more, fatigue resistance can be imparted and a sufficient bending resistance effect can be obtained. On the other hand, a film having an intrinsic viscosity of 1.00 dl/g or less is preferable because it can be produced with good workability.

The thickness of the polyester film is preferably 10-75 μm, more preferably 25-75 μm. When the thickness is 10 μm or more, the effect of improving the pencil hardness is observed, and when the thickness is 75 μm or less, it is advantageous for weight reduction and is excellent in flexibility, workability, handleability, and the like.

The surface of the polyester film of the present invention may be smooth or uneven, but since it is used as a surface cover for displays, 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 one 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 projections on the film surface can be further reduced by a method of kneading and extruding the pellets of the polyester containing particles and pellets not containing particles (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, a method for producing a biaxially stretched polyester film will be described in detail by way of example in which pellets of polyethylene terephthalate (hereinafter referred to as PET) are used as the raw material for the base film, but the method is not limited to this. 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 when manufacturing a multi-layer film, two or more extruders, two or more layers of manifolds or confluence blocks (for example, a confluence with 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.

In addition, the 0.2% proof stress strain in at least one of the longitudinal direction (machine flow direction) and the width direction of the polyester film is preferably 2.6 to 5.0%, and 3.0 to 5.0%. 0% is more preferred. The 0.2% proof stress strain is a value used as a substitute for the yield point regardless of whether or not the yield point appears in the stress-strain curve, and can be used as an index of the elastic region. FIG. 3 shows a schematic diagram of a stress-strain curve for obtaining the 0.2% proof stress point strain. FIG. 3 shows an example in which the yield point does not appear. The 0.2% yield point strain is determined by a normal tensile test. In the stress-strain curve, a parallel line is drawn from the point Q where the stress is 0 MPa and the strain is 0.2% to OA where Hooke's law holds. The value of the strain at the intersection point H with the strain axis when the is lowered is the 0.2% yield point strain. Stress as described above - from the strain curve, it is possible to determine the 0.2% proof stress point strain, stress by a tensile tester (manufactured by Shimadzu Corporation AUTOGRAPH AG-X Plus 1kN) - when obtaining a strain curve, then, If the autograph software TRAPEZIUM X manufactured by Shimadzu Corp. is used, by setting the strain at 0.2%, the calculation result of the 0.2% proof point strain can be obtained directly.

When the 0.2% proof stress point strain is 2.6% or more, the deformation due to the strain generated during folding is less likely to occur, which is preferable. A content of 5.0% or less is preferable because good impact resistance can be obtained. The 0.2% proof stress strain in the bending direction of the polyester film is preferably 2.6 to 5.0%, more preferably 3.0 to 5.0%. Here, the bending direction is a direction orthogonal to the folded portion (reference numeral 21) assumed in the application of the surface protection film of the folding display, as indicated by reference numeral 22 on the polyester film (reference numeral 2) in FIG. pointing. The bending direction is not limited to either the longitudinal direction or the width direction of the film.

The 0.2% proof stress strain can be adjusted within the above range by adjusting the draw ratio, draw temperature, heat setting temperature, polyester raw material, and the like.

The 0.2% proof stress strain of the polyester film can be effectively adjusted by adjusting the draw ratio. The draw ratio of the unstretched polyester sheet in at least one of the longitudinal direction (machine direction) and width direction is preferably 1.0 to 3.4 times, more preferably 1.0 to 3.0 times. . By setting the draw ratio to 1.0 to 3.4 times, the 0.2% proof stress point strain can be adjusted to the above high range, and deformation when repeatedly folded is small. And it is preferable that the said extending|stretching direction is the said bending direction. The stretching temperature is preferably 80 to 130°C, more preferably 90 to 130°C. As a heating method during stretching, conventionally known means such as a hot air heating method, a roll heating method, and an infrared heating method can be employed. By setting the stretching temperature to 80 to 130° C., thickness unevenness due to stretching at the above stretching ratio can be prevented.

It is preferable from the mechanical properties of the film that the draw ratio in the direction perpendicular to the bending direction when folded (the direction of the folded portion) is larger than the bending direction, and the draw ratio in the direction perpendicular to the bending direction is 2.5 to 5. 0 times can be exemplified. Stable productivity can be obtained by setting the draw ratio to 2.5 times or more, and good impact resistance can be obtained by setting the draw ratio to 5.0 times or less.

The crystallinity of the polyester film is preferably 35-54%, more preferably 43-54%. By increasing the degree of crystallinity, the 0.2% proof strain and impact resistance can be improved. In order to effectively increase the degree of crystallinity, heat treatment at 180 to 250° C. is preferably performed after biaxial stretching. If it is 180° C. or higher, the degree of crystallinity can be effectively increased, and if it is 250° C. or lower, there is no possibility that the surface of the polyester film will melt, and the appearance can be kept good, which is preferable.

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 1.0 to 3.4 times in the longitudinal direction with rolls heated to 80 to 130° 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 180 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.

(Hard coat layer)
The polyester film placed on the surface of the foldable display to protect the display preferably has a hard coat layer on its surface. The hard coat layer is preferably positioned on the display surface side of the polyester film and used in the display. As the resin for forming the hard coat layer, acrylic resins, siloxane resins, inorganic hybrid resins, urethane acrylate resins, polyester acrylate resins, epoxy resins, 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.

(film thickness)
The thickness of the hard coat layer is preferably 1 to 50 μm. 1 to 40 μm is more preferable. 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 50 μm or less, curling due to curing shrinkage of the hard coat can be suppressed, and the handleability of the film can be improved. More preferably 2 to 25 μm, particularly preferably 2 to 20 μm, most preferably 3 to 15 μm.

(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.

(Curing conditions)
As a method for curing the hard coat layer, energy beams such as ultraviolet rays, electron beams, or heat can be used. In order to reduce damage to the film, curing methods using ultraviolet rays, electron beams, or the like are preferable.

(Pencil hardness)
The pencil hardness of the hard coat layer is preferably B or higher, more preferably H or higher, and particularly preferably 2H or higher. If it has a pencil hardness of B or higher, it will not be easily scratched and the 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) 0.2% proof stress strain The film was cut into a strip shape with a width of 10 mm in the measurement direction of 140 mm and used as a sample, and a tensile test was performed with a tensile tester (AUTOGRAPH AG-X Plus 1 kN manufactured by Shimadzu Corporation) at a tensile speed of 100 mm / min. to obtain the stress-strain curve. After that, using autograph software TRAPEZIUM X manufactured by Shimadzu Corporation, the strain was set to 0.2%, and the 0.2% proof stress point strain was calculated.

(2) 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.

(3) Crystallinity Density of the film sample was measured at three points. The average value was taken as the degree of 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. In addition, the following values were used for the density of each component alone (unit: g/cm3)
Polyethylene terephthalate resin: completely amorphous 1.34, completely crystalline 1.46
Polyethylene naphthalate resin: completely amorphous 1.32, completely crystalline 1.41

(4) Bending Resistance A polyester film was cut into a size of 200 mm (bending direction)×50 mm (orthogonal direction) to prepare a measurement sample. Spacers with different thicknesses were placed at the ends of two 5 mm-thick glass plates to form a space, and the film was sandwiched and held for 10 seconds. Immediately thereafter, the film was exposed to fluorescent light, the folds were observed, and the interval without fold marks was recorded.
◯: The interval with no folding marks is less than 6.5 mm.
Δ: The interval without folding marks is 6.5 mm or more and less than 7.0 mm ×: The interval without folding marks is 7.0 mm or more.

(5) Repeated bending resistance A sample having a size of 50 mm in the width direction and 100 mm in the machine direction is prepared. Using a no-load U-shaped stretching tester (manufactured by Yuasa System Co., Ltd., DLDMLH-FS), a bending radius of 3 mm was set, and bending was performed 50,000 times at a speed of 1 time/second. At that time, the sample was fixed at 10 mm on both ends on the long side, and the bent portion was 50 mm×80 mm. After the bending process was completed, the samples were placed on a flat surface with the inside of the bend facing down and visually inspected.
◯: No deformation of the sample, or even if the sample is deformed, the maximum floating height is less than 3 mm when placed horizontally.
Δ: The sample was deformed, and when placed horizontally, the maximum floating height was 3 mm or more and less than 5 mm.
x: The sample has creases or has a maximum floating height of 5 mm or more when placed horizontally.

(6) Pencil hardness The hardness of the hard-coated polyester film was measured according to JIS K 5600-5-4:1999 at a load of 750 g and a speed of 0.5 mm/s.

(Coating liquid 1 for forming hard coat layer)
0.1 part by weight of a leveling agent (BYK307, BYK Chemie Japan, concentration 100%) was added to 100 parts by weight of a hard coat material (manufactured by JSR, Opstar (registered trademark) Z7503, concentration 75%), followed by addition of methyl ethyl ketone. A hard coat coating liquid 1 having a solid concentration of 40% by weight was prepared by dilution.

(Coating liquid 2 for forming hard coat layer)
Urethane acrylate hard coating agent (manufactured by Arakawa Chemical Industries, Ltd., Beamset (registered trademark) 577, solid content concentration 100%) 95 parts by weight, photopolymerization initiator (manufactured by BASF Japan, Irgacure (registered trademark) 184, solid content Concentration 100%) 5 parts by weight, leveling agent (BYK307 manufactured by BYK Chemie Japan Co., Ltd., solid concentration 100%) 0.1 parts by weight, diluted with a solvent of toluene / MEK = 1/1, concentration 40 % coating liquid 2 was prepared.

(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% cut) to obtain polyethylene terephthalate pellets A having an intrinsic viscosity of 0.62 dl/g.

(Preparation of polyethylene terephthalate pellet B)
Polyethylene terephthalate pellets A were solid-phase polymerized at 220° C. for various times under a reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus to prepare polyethylene terephthalate pellets B having an intrinsic viscosity of 0.72 dL/g.

(Preparation of polyethylene-2,6-naphthalate pellets C)
100 parts of dimethyl 2,6-naphthalenedicarboxylate, 60 parts of ethylene glycol, and 0.03 parts of manganese acetate tetrahydrate as a transesterification catalyst were subjected to a transesterification reaction according to a conventional method, and then 0.0 part of triethylphosphonoacetate was added. 042 parts were added to substantially complete the transesterification reaction. Next, 0.024 parts of antimony trioxide was added, and polymerization was then carried out at high temperature under high vacuum in a conventional manner to obtain polyethylene-2,6-naphthalate (PEN) having an intrinsic viscosity of 0.60 dl/g. . Then, after pre-drying at 150 to 160° C. for 3 hours, solid-phase polymerization was performed at 210° C., 13 kPa, under a nitrogen gas atmosphere to obtain polyethylene-2,6-naphthalate pellets C having an intrinsic viscosity of 0.72 dl/g. Obtained.

(Example 1)
After drying the polyethylene terephthalate master pellets B at 150°C for 8 hours under reduced pressure (3 Torr), polyethylene terephthalate pellets B 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 uniformly heated to 75° C. using a heating roll, heated to 100° C. using a non-contact heater, and roll-stretched (longitudinal stretching) 3.4 times. The obtained uniaxially stretched film is guided to a tenter, heated to 140° C., transversely stretched to 4.0 times, fixed in width and subjected to heat treatment at 240° C. for 5 seconds, and further relaxed 4% in the width direction at 210° C. A polyethylene terephthalate film having a thickness of 50 μm was obtained.

(Examples 2-6)
A polyester film was obtained in the same manner as in Example 1 above, except that the stretch ratio and heat treatment temperature shown in Table 1 were changed.

(Examples 7 and 8)
A polyester film was obtained in the same manner as in Example 1 above, except that the thickness was changed to that shown in Table 1.

(Example 9)
A polyester film was obtained in the same manner as in Example 1 except that polyethylene-2,6-naphthalate pellets C shown in Table 1 were used and the temperature was adjusted.

(Example 10)
A polyester film was obtained in the same manner as in Example 1, except that the polyethylene terephthalate master pellets A were used.

(Comparative example 1)
A polyester film was obtained in the same manner as in Example 1, except that the longitudinal draw ratio was changed to 3.5 times.

(Comparative Examples 2 and 3)
A polyester film was obtained in the same manner as in Example 1 except that the heat setting temperature in Table 1 was changed.

Using a Meyer bar on one side of the polyester films obtained in Examples 1 to 10 and Comparative Examples 1 to 3, the hard coat layer forming coating liquid 1 was dried so that the film thickness after drying was 5.0 μm. and dried at 80° C. for 1 minute, followed by ultraviolet irradiation (high-pressure mercury lamp, cumulative light intensity 200 mJ/cm ² ) to obtain a hard coat film. In Example 11, a hard coat film was obtained in the same manner as in Example 1 except that the polyester film obtained in Example 1 was coated so as to have a thickness of 10.0 μm after drying. In Example 12, a hard coat film was obtained in the same manner as in Example 1, except that the hard coat layer-forming coating solution 2 was applied to the polyester film obtained in Example 1.

These hard coat films are attached to an organic EL module via a 25 μm thick adhesive layer, and a smartphone-type foldable display that can be folded in half at the center with a radius of 3 mm, which corresponds to the bending radius in Fig. 1. It was created. The hard coat film is arranged on the surface of one continuous display via the folded portion, and the hard coat layer is arranged so as to be positioned on the surface of the display. The smartphone using the hard coat film of each example satisfies the operation and visibility as a smartphone that can be folded in half at the center and carried. On the other hand, the foldable display using the hard coat film of each comparative example was not very preferable because it seemed that the folding portion of the display caused image distortion as the frequency of use increased.

According to the foldable display using the polyester film and the hard coat film for the surface protection film of the foldable display of the present invention, the polyester film and the hard coat film are positioned on the surface of the foldable display while maintaining mass productivity. Since the display does not deform after being repeatedly folded, there is no distortion of the image at the folded portion of the display. A portable terminal device equipped with a foldable display using the polyester film or hard-coated film of the present invention as a surface protective film provides beautiful images, is rich in functionality, and is excellent in convenience such as portability. .

1: folding display 11: bending radius 2: polyester film for surface protection film of folding display 21: folding part 22: bending direction (direction perpendicular to the folding part)
O: Origin A: The farthest point from O where Hooke's law holds P: The point where a parallel line is drawn from Q to the OA line and intersects the stress-strain curve Q: The point where stress is 0 MPa and elongation (strain) is 0.2% H: A point where a parallel line is drawn from P to the vertical axis and intersects with the horizontal axis (0.2% proof stress point strain (%))
σ _0.2 : Stress value of P (MPa)

Claims (7)

A polyester film having a thickness of 10 to 75 μm,
The degree of crystallinity is 35 to 54%,
a total light transmittance of 85% or more and 99% or less and a haze of 0.1% or more and 3% or less;
A polyester film for a surface protective film of a foldable display, characterized by having a 0.2% proof stress point strain of 2.6 to 5.0% in a bending direction.
(Here, the bending direction refers to a direction orthogonal to the folded portion when folding the polyester film.) 2. The polyester film for a surface protective film of a foldable display according to claim 1, wherein the polyester film has an intrinsic viscosity of 0.60 to 1.0 dl/g. 3. The polyester film for a surface protective film of a foldable display according to claim 1, wherein the 0.2% proof stress strain in the bending direction of the polyester film is 3.0 to 5.0%. In a no-load U-shaped stretching test in which the bending radius is 3 mm and the bending rate is 50,000 times per second, after bending is completed, when the inside of the bending is turned down and placed horizontally on a flat surface,
4. The polyester film for displays according to any one of claims 1 to 3, which has a maximum floating height of less than 5 mm or is not deformed. Having a hard coat layer with a thickness of 1 to 50 μm on at least one side of the polyester film for a surface protective film of a folding display according to any one of claims 1 to 4 ,
A hard coat film for a surface protective film of a foldable display, wherein the hard coat layer has a pencil hardness of H or more measured under a load of 750 g according to JIS K5600-5-4:1999. 6. The hard coat film for a surface protective film of a foldable display according to claim 5 is a foldable display arranged as a surface protective film so that the hard coat layer is positioned on the surface, and the bending radius when folded is 5 mm or less,
A foldable display characterized in that a single continuous hard coat film is arranged across the folded portion of the foldable display. A mobile terminal device comprising the foldable display according to claim 6 .

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