JP2023153164A - Polyester film serving as surface protective film for foldable display and application thereof - Google Patents

Abstract

To provide a foldable display which is well-suited for mass production and which causes little distortion to an image displayed on a folded portion even after repeated folding, and a portable terminal device with the foldable display installed thereon, and to provide a polyester film serving as a surface protective film for the foldable display, and a hard coat film serving as a surface protective film therefor.SOLUTION: A polyester film serving as a surface protective film for a foldable display is a polyester film with thickness of 10 to 75 μm and a deformation at 0.2% proof stress of 2.6 to 5.0% at least in either the lengthwise or widthwise direction. A hard coat film, the foldable display, and a portable terminal device use the polyester film serving as a surface protective film for the foldable display.SELECTED DRAWING: Figure 1

Description

The present invention relates to a polyester film for a surface protection film of a folding display, a hard coat film for a surface protection film of a folding display, a folding display, and a mobile terminal device. The present invention relates to a foldable display and a mobile terminal device in which image distortion is less likely to occur, and to a polyester film and a hard coat film for a surface protection film of the foldable display.

BACKGROUND OF THE INVENTION As mobile terminal devices become thinner and lighter, mobile terminal devices typified by smartphones are becoming widespread. While mobile terminal devices are required to have various functions, they are also required to be convenient. Therefore, the popular mobile terminal devices must have a small screen size of about 6 inches because they can be easily operated with one hand and are intended to be stored in clothing pockets.

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 purposes, drawing purposes, reading, etc., and have high functionality. However, it cannot be operated with one hand, has poor portability, and has problems in terms of convenience.

In order to achieve these, a method has been proposed to make the display more compact by connecting multiple displays together (Patent Document 1), but since the bezel remains, the image becomes cut off, resulting in reduced visibility. It is not widespread.

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

Here, for conventional displays and mobile 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. When creating a one-page display, it is necessary to use a hard coat film that is flexible and can protect the surface. However, in a foldable display, since the portion corresponding to a certain folding portion is repeatedly folded, the film at that portion deforms over time, causing problems such as distorting the image displayed on the display.

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

Japanese Patent Application Publication No. 2010-228391 Japanese Patent Application Publication No. 2016-155124

The present invention aims to solve the above-mentioned problems with conventional display surface protection members, and is suitable for mass production.The present invention is also intended to solve the problems described above with respect to the surface protection members of conventional displays, and is suitable for mass production. In order to be able to provide foldable displays that do not require a foldable display and mobile terminal devices equipped with such foldable displays, we aim to provide polyester films for surface protection films and hard coat films for surface protection films for foldable displays. It is something.

That is, the present invention has the following configuration.
1. A foldable display comprising a polyester film having a thickness of 10 to 75 μm and having a 0.2% yield point 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 use as a surface protection film for a folding display as described in the above item 1, which has a 0.2% yield point strain in the bending direction of 2.6 to 5.0%.
(Here, the bending direction refers to the direction perpendicular to the folded part when folding the polyester film.)
3. The polyester film for a surface protection film of a foldable display as described in the first or second item above, wherein the film has an intrinsic viscosity of 0.60 to 1.0 dl/g.
4. A surface protection film for a folding 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 protection film for a folding display according to any one of the above first to third aspects. hard coat film.
5. Hard coat film for a surface protection film of a folding display according to item 4 above, wherein the hard coat layer has a pencil hardness of H or higher when measured at a load of 750 g in accordance with JIS K5600-5-4:1999. .
6. A folding display in which the hard coat film for a surface protection film of a folding display according to the fourth or fifth item above is disposed as a surface protection film so that the hard coat layer is located on the surface, and when folded. A foldable display characterized by a bending radius of 5 mm or less.
7. 6. The foldable display according to the above item 6, wherein a single continuous hard coat film is disposed 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 or hard coat film for the surface protection film of the foldable display of the present invention maintains mass productivity, and the polyester film or hard coat film does not cause deformation after repeated folding. Therefore, image distortion does not occur at the folded portion of the display. A mobile terminal device equipped with a foldable display as described above provides beautiful images, is rich in functionality, and has excellent convenience such as portability.

It is a schematic diagram for showing the measuring point of the bending radius when folded in the present invention. It is a schematic diagram for showing the bending direction of the polyester film for the surface protection film of the folding display in this invention. It is a schematic diagram for explaining 0.2% yield stress point strain in the present invention.

(display)
The term "display" used in the present invention generally refers to display devices, and types of displays include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs. , organic EL, and inorganic EL are preferred. Particularly preferred are organic EL and inorganic EL, which can reduce the layer structure, and organic EL, which has a wide color gamut, is even more preferred.

(Foldable display)
The foldable display preferably has a structure in which one continuous display is folded in half when carried, reducing the size by half and improving portability. At the same time, it is desirable that the device be thin and lightweight. Therefore, the bending 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, the folded state can be made thinner. It can be said that the smaller the bending radius, the better, but it may be 0.1 mm or more, or 0.5 mm or more. Even if it is 1 mm or more, it is sufficiently practical compared to 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 inner radius of the folded portion when folded. Note that the surface protection film, which will be described later, may be located on the outside of the folded display, or may be located on the inside of the folded display.

(Organic EL)
The general structure of an organic EL display includes an organic EL layer consisting of an 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 with touch panel)
When an organic EL display is used in a mobile terminal device having a touch panel, a touch panel module is placed above the organic EL display or between the organic EL layer/retardation plate. At this time, if a shock is applied from above, the organic EL and touch panel circuits may be disconnected, so a surface protection film is required. Preferably, a hard coat layer is laminated on the side.

(Surface protection film for folding displays)
As a surface protection film, any film with high light transmittance and low haze can be used, such as polyimide film, polyester film, polycarbonate film, acrylic film, triacetyl cellulose film, and cycloolefin polymer film. Polyimide films and polyester films that have high impact resistance and sufficient pencil hardness are preferred, and polyester films that can be manufactured 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, a multilayer film when two or more types of polyester are used, or a super multilayer laminated film with a repeated structure. good.

Examples of the polyester resin include polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or a polyester film made of a copolymer mainly composed of constituent components of these resins. Among these, stretched polyethylene terephthalate film is particularly preferred in terms of mechanical properties, heat resistance, transparency, cost, and the like.

When using a polyester copolymer 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-naphthalene dicarboxylic acid. aromatic dicarboxylic acids such as; 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, and 1,4-cyclohexanedimethanol. and polyethylene glycol having an average molecular weight of 150 to 20,000. The mass ratio of the copolymerized components of the preferred copolymer is less than 20% by mass. When the amount is less than 20% by mass, film strength, transparency, and heat resistance are maintained, which is preferable.

Further, in the production of polyester film, the intrinsic viscosity of at least one type of resin pellet is preferably in the range of 0.60 to 1.0 dl/g. It is preferable that the intrinsic viscosity is 0.60 dl/g or more because the impact resistance of the obtained film is improved and the internal circuit is less likely to be disconnected due to external impact. It is also preferable because it contributes to reducing deformation when repeatedly bent. On the other hand, it is preferable that the intrinsic viscosity is 1.00 dl/g or less, since the filtration pressure of the molten fluid will not increase too much and it will be easy to stably operate the film production.

Regardless of whether the film has a single layer structure or a laminated structure, the intrinsic viscosity of the film is preferably 0.60 dl/g or more. More preferably, it is 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 preferred because it can be manufactured with good operability.

The thickness of the polyester film is preferably 10 to 75 μm, more preferably 25 to 75 μm. When the thickness is 10 μm or more, the effect of improving pencil hardness is observed, and when the thickness is 75 μm or less, it is advantageous for weight reduction and also has excellent flexibility, workability, and handling properties.

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 a display, deterioration in optical properties due to unevenness is not preferable. 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, image visibility can be improved. The lower limit of the haze is preferably as small as possible, but it may be 0.1% or more, or 0.3% or more.

As mentioned above, for the purpose of reducing haze, it is better that the unevenness on the film surface is not too large, but in order to give a certain degree of slipperiness from the viewpoint of handling, the method of forming unevenness is to use polyester resin on the surface layer. It can be formed by adding a filler to the layer or by coating a filler-containing coating layer during film formation.

As a method for blending particles into the base film, a known method can be adopted. For example, it can be added at any stage of producing polyester, but is preferably added as a slurry dispersed in ethylene glycol or the like during the esterification stage, or after the end of the transesterification reaction and before the start of the polycondensation reaction. However, 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 This can be done by, for example.

In particular, after homogeneously dispersing aggregate inorganic particles in a monomer liquid that will become a part of the polyester raw material, the filtered particles are 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 particles and high-precision filtration of slurry can be easily performed. Aggregates are also less likely to occur. From this point of view, it is particularly preferable to add it to the remainder of the raw material in a low temperature state before the esterification reaction.

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

Moreover, the polyester film may contain various additives within a range that maintains a preferable range of total light transmittance. Examples of additives include antistatic agents, UV absorbers, and stabilizers.

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, visibility can be sufficiently ensured. It can be said that 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 treated to improve its adhesion to the resin forming the hard coat layer or the like.

Examples of surface treatment methods include sandblasting, unevenness treatment using solvent treatment, corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone/ultraviolet irradiation treatment, flame treatment, chromic acid treatment, hot air treatment, etc. Examples include oxidation treatment, which can be used without particular limitation.

Furthermore, adhesion can be improved by using an adhesion-improving layer such as an easy-adhesion layer. As the easily adhesive layer, acrylic resin, polyester resin, polyurethane resin, polyether resin, etc. can be used without particular limitation, and it can be formed by a general coating method, preferably by a so-called in-line coating formulation.

The above-mentioned polyester film is produced by, for example, a polymerization process in which inorganic particles are homogeneously dispersed in a monomer liquid that becomes a part of the polyester raw material, filtered, and then added to the remainder of the polyester raw material to polymerize the polyester. It can be manufactured through a film forming process in which a sheet is melt-extruded through a filter, cooled, and stretched to form a base film.

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

After PET pellets are mixed at a predetermined ratio and dried, they are fed to a known extruder for melt lamination, extruded into a sheet through a slit-shaped die, and cooled and solidified on a casting roll to form an unstretched film. . In the case of a single layer, one extruder is sufficient, but when producing a multilayer film, two or more extruders, a manifold with two or more layers, or a merging block (for example, a merging block with a rectangular merging section) are required. It is possible to laminate a plurality of film layers constituting each outermost layer using a block), extrude two or more layers of sheets from a die, and cool them with a casting roll to form an unstretched film.

In this case, during melt extrusion, it is preferable to perform high-precision filtration at any location 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 molten resin is not particularly limited, but a filter medium of stainless steel sintered body has excellent ability to remove aggregates 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. When the filtration particle size (initial filtration efficiency of 95%) of the filter medium exceeds 20 μm, foreign matter having a size of 20 μm or more cannot be sufficiently removed. By performing high-precision filtration of molten resin using a filter medium with a filter particle size (initial filtration efficiency of 95%) of 20 μm or less, productivity may decrease, but a film with fewer protrusions due to coarse particles can be obtained. preferred above.

Further, the 0.2% yield point strain in at least one of the longitudinal direction (machine flow direction) and width direction of the polyester film is preferably 2.6 to 5.0%, and 3.0 to 5.0%. More preferably, it is 0%. The 0.2% yield point strain is a value used as a substitute for the yield point, whether or not the yield point appears in the stress-strain curve, and can be used as an index of the elastic region. Figure 3 shows a schematic diagram of the stress-strain curve for determining the strain at the 0.2% yield strength point. FIG. 3 is an example where the yield point does not appear. The 0.2% yield point strain is determined by a normal tensile test. On the stress-strain curve, draw a parallel line from point Q where the stress is 0 MPa and strain 0.2% to OA where Hooke's law holds, the point where it intersects with the stress-strain curve is the proof point P, and from there, draw a parallel line on the vertical axis. The value of the strain at the intersection H with the strain axis when it is lowered is the 0.2% yield stress point strain. The strain at the 0.2% proof stress point can be determined from the stress-strain curve as described above, but if the stress-strain curve is obtained using a tensile testing machine (Shimadzu AUTOGRAPH AG-X Plus 1kN), then By using the autograph software TRAPEZIUM

When the strain at the 0.2% proof stress point is 2.6% or more, deformation is less likely to occur due to strain generated during folding, which is preferable. When it is 5.0% or less, good impact resistance can be obtained, which is preferable. The 0.2% yield point 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 refers to the direction perpendicular to the folded portion (code 21), which is assumed to be used as a surface protection film for folding displays, as shown by code 22 on the polyester film (code 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% yield stress point strain can be adjusted within the above range by adjusting the stretching ratio, stretching temperature, heat setting temperature, polyester raw material, etc.

The 0.2% yield point strain of the polyester film can be effectively adjusted by adjusting the stretching ratio. The stretching ratio of the unstretched polyester sheet in at least one of the longitudinal direction (machine flow direction) and the width direction is preferably 1.0 to 3.4 times, more preferably 1.0 to 3.0 times. . By increasing the stretching ratio from 1.0 to 3.4 times, the strain at the 0.2% proof stress point can be adjusted to the above-mentioned high range, resulting in less deformation when repeatedly folded. The stretching direction is preferably the above-mentioned 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, an infrared heating method, etc. can be employed. By setting the stretching temperature to 80 to 130° C., thickness unevenness due to stretching at the above-mentioned stretching ratio can be prevented.

From the mechanical properties of the film, it is preferable that the stretching ratio in the direction perpendicular to the bending direction when folded (direction of the folded part) is larger than the bending direction, and the stretching ratio in the direction perpendicular to the bending direction is 2.5 to 5. I can give an example of .0 times. 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.

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

Specifically, for example, after sufficiently vacuum-drying PET pellets, they are fed to an extruder, melted and extruded at about 280° C. into a sheet, 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 using rolls heated to 80 to 130°C to obtain a uniaxially oriented PET film. Furthermore, the ends of the film are held with clips and introduced into a hot air zone heated to 80 to 180°C, and after drying, the film is stretched 2.5 to 5.0 times in the width direction. Subsequently, the material is introduced into a heat treatment zone at 180 to 250° C., and heat treatment is performed for 1 to 60 seconds to complete crystal orientation. During this heat treatment step, a relaxation treatment of 1 to 12% may be performed in the width direction or length direction, if necessary.

(Hard coat layer)
The polyester film placed on the surface of the folding display to protect the display preferably has a hard coat layer on its surface. The hard coat layer is preferably used in a display by being positioned on the display surface side on the polyester film. As the resin forming the hard coat layer, acrylic, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, and epoxy resins can be used without particular limitation. Moreover, two or more types of materials can be mixed and used, and particles such as inorganic filler or organic filler can also be added.

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

(Coating method)
As the coating method for the hard coat layer, Meyer bar, gravure coat, die coater, knife coater, etc. can be used without particular limitation, and can be selected as appropriate depending on the viscosity and film thickness.

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

(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 the pencil hardness is B or higher, it will not be easily scratched and visibility will not be reduced. Generally, the pencil hardness of the hard coat layer is preferably higher, but it may be less than 10H, it may be less than 8H, and even less than 6H can be used practically without any problem.

(Type of hard coat layer)
The hard coat layer in the present invention may be added with 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, hard coat layers with added functionality such as anti-glare layers, anti-glare anti-reflection layers, anti-reflection layers, low reflection layers and antistatic layers having a certain pencil hardness as described above can also be used in the present invention. is preferably applied.

Next, the effects of the present invention will be explained using Examples and Comparative Examples. First, the method for evaluating characteristic values used in the present invention is shown below.

(1) 0.2% yield strength point strain The film was cut into a rectangular sample with a width of 10 mm in the measuring direction of 140 mm, and a tensile test was performed at a tensile speed of 100 mm/min using a tensile tester (Shimadzu Corporation AUTOGRAPH AG-X Plus 1 kN). to obtain a stress-strain curve. Thereafter, using autograph software TRAPEZIUM

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

(3) Crystallinity The density of the film sample was measured at three points. The average value was defined as the degree of crystallinity. The resin component is a mixture of completely amorphous and completely crystalline, and its density is as described below, and the density of the sample is the value obtained by dividing the sum of the masses of each component making up the sample by the sum of the volumes of each component. Based on the assumption that , the crystallinity (weight ratio) of each resin was estimated. The sample density was determined in accordance with a 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) Flexibility A polyester film was cut into a size of 200 mm (bending direction) x 50 mm (orthogonal direction) to prepare a measurement sample. Spacers of each thickness were placed at the ends of two 5 mm thick glass plates to create 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 at which no folding marks were left was recorded.
○: The interval at which no folding marks were left was less than 6.5 mm.
Δ: The interval at which no folding marks were left was 6.5 mm or more and less than 7.0 mm. ×: The interval at which no folding marks were left was at least 7.0 mm.

(5) Repeated bending resistance A sample with a size of 50 mm in the width direction x 100 mm in the flow direction is prepared. Using a no-load U-shaped stretch tester (manufactured by Yuasa System Equipment Co., Ltd., DLDMLH-FS), a bending radius of 3 mm was set, and the film was bent 50,000 times at a rate of 1 turn/second. At that time, the sample was fixed at a position of 10 mm at both long side ends, and the bent portion was 50 mm x 80 mm. After the bending process was completed, the sample was placed on a flat surface with the inside of the bend facing down, and a visual inspection was performed.
○: There is no deformation of the sample, or even if there is deformation, the maximum height of lifting is less than 3 mm when placed horizontally.
△: The sample is deformed and when placed horizontally, the maximum height is 3 mm or more and 5 mm.
less than.
×: The sample has creases or the maximum height of lifting is 5 mm or more when placed horizontally.

(6) Pencil hardness The prepared hard coated polyester film was measured in accordance with 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, manufactured by BYK Chemie Japan, concentration 100%) was added to 100 parts by weight of a hard coat material (manufactured by JSR Corporation, Opstar (registered trademark) Z7503, concentration 75%), and the mixture was mixed with methyl ethyl ketone. A hard coat coating liquid 1 having a solid content concentration of 40% by weight was prepared by diluting it.

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

(Preparation of polyethylene terephthalate pellets A)
As the esterification reactor, a continuous esterification reactor consisting of a three-stage complete mixing tank having a stirrer, a partial condenser, a raw material inlet, and a product outlet was used. The amount of antimony trioxide was set at 2 mol to mol, and the amount of antimony trioxide was set so that Sb atoms were 160 ppm with respect to the produced PET, and the slurry was continuously supplied to the first esterification reactor of the esterification reactor and heated 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 and supplied to the second esterification reactor, and the reaction product is distilled from the first esterification reactor into the second esterification reactor. In addition, an EG solution containing magnesium acetate in an amount such that Mg atoms are 65 ppm with respect to the generated PET, and 20 ppm of P atoms with respect to the generated PET is supplied. An EG solution containing an amount of TMPA was added, and the reaction was carried out at 260° C. at normal pressure 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 P atoms are 20 ppm with respect to the produced PET. The EG solution was added and reacted at 260° C. with an average residence time of 0.5 hours at normal pressure. The esterification reaction product produced in the third esterification reactor is continuously supplied to a three-stage continuous polycondensation reactor for polycondensation, and a stainless steel sintered filter medium (nominal filtration accuracy of 5 μm 90% of the particles were cut) to obtain polyethylene terephthalate pellets A having an intrinsic viscosity of 0.62 dl/g.

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

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

(Example 1)
After drying the above polyethylene terephthalate master pellets B under reduced pressure (3 Torr) at 150°C for 8 hours, the polyethylene terephthalate pellets B were each supplied to an extruder and melted at 285°C. This polymer is filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% of particles cut out at 10 μm), extruded from a die in the form of a sheet, and then 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, producing an unstretched film. This unstretched film was uniformly heated to 75° C. using a heating roll, and heated to 100° C. using a non-contact heater to perform roll stretching (longitudinal stretching) by 3.4 times. The obtained uniaxially stretched film was introduced into a tenter, heated to 140°C, stretched horizontally by 4.0 times, fixed width, heat treated at 240°C for 5 seconds, and further relaxed by 4% in the width direction at 210°C. By doing so, a polyethylene terephthalate film having a thickness of 50 μm was obtained.

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

(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 the one shown in Table 1.

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

(Example 10)
A polyester film was obtained in the same manner as in Example 1 except that 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 stretching 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 shown in Table 1 was changed.

A Mayer bar was used on one side of the polyester films obtained in Examples 1 to 10 and Comparative Examples 1 to 3 to apply coating solution 1 for forming hard coat layer so that the film thickness after drying would be 5.0 μm. After drying at 80° C. for 1 minute, ultraviolet rays were irradiated (high-pressure mercury lamp, cumulative light amount 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 that the film thickness after drying was 10.0 μm. In Example 12, a hard coat film was obtained in the same manner as in Example 1, except that coating liquid 2 for forming a hard coat layer was applied to the polyester film obtained in Example 1.

These hard coat films are laminated 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 of the whole with a radius of 3 mm, which corresponds to the bending radius in Figure 1. It was created. The hard coat film is disposed on the surface of one continuous display via the folded portion, and the hard coat layer is disposed on the surface of the display. The devices using the hard coat film of each example had satisfactory operation and visibility as a smartphone that could be folded in two at the center and carried. On the other hand, as the folding type displays using the hard coat films of the comparative examples were used more frequently, it seemed that image distortion occurred at the folded portion of the display, which was not very desirable.

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

1: Folding display 11: Bending radius 2: Polyester film for surface protection film of folding display 21: Folding portion 22: Bending direction (direction perpendicular to the folding portion)
O: Origin A: Point farthest from O where Hooke's law holds P: Point where a parallel line is drawn from Q to the OA line and intersects the stress-strain curve Q: Point where stress is 0 MPa and elongation (strain) is 0.2% H: Draw a parallel line from P to the vertical axis, and the point where it intersects with the horizontal axis (0.2% yield strength point strain (%))
σ0.2: Stress value of P (MPa)

Claims (8)

Crystallinity is 35-54%,
The total light transmittance is 85% or more and 99% or less,
A polyester film for displays, characterized in that the strain at the 0.2% yield point in the machine flow direction is 2.6 to 5.0%. 2. The polyester film for display according to claim 1 , wherein the mechanical flow direction is a bending direction perpendicular to a folded portion when the polyester film is folded . The polyester film for display according to claim 1 , having a thickness of 10 to 75 μm and an intrinsic viscosity of 0.60 to 1.0 dl/g. The polyester film for display according to claim 1 , having a haze of 0.1% or more and 3% or less . In a no-load U-shaped stretch test in which the product was bent 50,000 times at a bending radius of 3 mm and a speed of 1 time/second, after the bending was completed, when the product was placed horizontally on a flat surface with the inside of the bend facing down,
2. The polyester film for display according to claim 1, wherein the polyester film has a maximum lifting height of less than 5 mm or does not deform. The polyester film for display according to any one of claims 1 to 5 has a hard coat layer having a thickness of 1 to 50 μm on at least one side,
A hard coat film for displays, characterized in that the hard coat layer has a pencil hardness of H or higher when measured at a load of 750 g in accordance with JIS K5600-5-4:1999. The hard coat film for display according to claim 6 is a foldable display arranged as a surface protection film so that the hard coat layer is located 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 disposed through a folding portion of the display. A mobile terminal device comprising the foldable display according to claim 7.