WO2024058059A1 - Polyester film and use thereof - Google Patents

Abstract

Provided is a polyester film for a folding-type display, the polyester film having excellent mass productivity and heat resistance, and having reduced occurrences of distortions in an image displayed on a folding portion after being repeatedly folded, and reduced occurrences of cracks in a folding section. The polyester film for a folding-type display has a thickness of 10-125 μm, wherein the high-temperature hold angle in a bending direction is 85° or greater, the refractive index in the bend direction of the polyester film is 1.610-1.710, and the refractive index in a direction orthogonal to the bending direction is 1.750-1.870. The high-temperature hold angle refers to an angle formed by an folded portion that is made after fixing the polyester film for 18 hours under heating at 85°C such that 1.7% strains are produced on both surfaces of a bending portion thereof.

Description

Polyester film and its uses

The present invention relates to a polyester film for foldable displays, a hard coat film for foldable displays, a foldable display, and a mobile terminal device that have excellent bending resistance, and in particular, image distortion due to film deformation is unlikely to occur even after repeated folding. , relates to a back protective film for a foldable display, a foldable display, and a mobile terminal device.

As mobile terminal devices become thinner and lighter, mobile terminal devices typified by smartphones are becoming more 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 are highly functional. 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 compact by connecting multiple displays (see Patent Document 1). However, in the invention of Patent Document 1, since the bezel portion remains, the image is cut off, resulting in a problem of reduced visibility, and it has not become popular.

On the other hand, 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 displays and mobile terminal devices that do not have a conventional folding structure, the surface of the display could be protected with a non-flexible material such as glass. However, in the case of a folding display, when a one-page display is created through the folding portion, it is necessary to use a hard coat film or the like that is flexible and capable of protecting the surface.

However, in a foldable display, since a certain folded part is repeatedly folded, the film in that part deforms over time, causing problems such as distorting the image of the folded part displayed on the display. In addition, films are used in various parts of foldable displays, including not only surface protection films, but also polarizing plates, retardation plates, touch panel base materials, display cell base materials such as organic EL, and back protection members. These films were also required to have durability against repeated folding.

Therefore, a method of partially changing the film thickness has also been proposed. (See Patent Document 2). However, in the invention of Patent Document 2, the manufacturing process is complicated because the film thickness is changed, and there is a problem that mass productivity is poor.

Japanese Patent Application Publication No. 2010-228391 Japanese Patent Application Publication No. 2016-155124 International Publication No. 2018/150940

Additionally, a method of adjusting the refractive index of a polyester film in the bending direction has also been proposed (see Patent Document 3). However, there was a concern that films using polyethylene terephthalate could not be used in applications that required higher reliability (high temperature range).

The present invention is an attempt to solve the above-mentioned problems with conventional display members, and is suitable for mass production and is less likely to cause disturbances in the image displayed at the folded portion after being repeatedly folded. In order to make it possible to provide a foldable display and a mobile terminal device equipped with such a foldable display, a polyester film for a foldable display is provided.
Alternatively, the present invention aims to provide a polyester film for foldable displays that does not easily cause creases in the folded portion even when folded in a high temperature range.

The present invention typically consists of the following configuration.
1. A polyester film with a thickness of 10 to 125 μm, a high temperature hold angle in the bending direction of 85° or more, a refractive index of the polyester film in the bending direction of 1.610 to 1.710, and a direction perpendicular to the bending direction. A polyester film for foldable displays having a refractive index of 1.750 to 1.870.
Here, the high-temperature hold angle is the angle formed by the folds that occur after the polyester film is fixed for 18 hours under heating at 85°C so that a strain of 1.7% is generated on both surfaces of the bent portion. refers to
2. 1. The polyester film for foldable displays according to 1, which has an intrinsic viscosity of 0.50 to 0.80 dl/g.
3. 3. The polyester film for foldable displays according to 1 or 2, which has a density of 1.353 g/cm ³ or more.
4. 4. The polyester film for foldable displays according to any one of 1 to 3, wherein the polyester is polyethylene naphthalate.
5. 5. The polyester film for foldable displays according to any one of 1 to 4, wherein the polyester film has a stretching ratio of 1.1 to 2.5 times in the longitudinal direction.
6. 6. The polyester film for foldable displays according to any one of 1 to 5, which has an easily adhesive layer on at least one side of the polyester film.
7. A foldable display, comprising the polyester film for foldable displays according to any one of items 1 to 5, wherein the polyester film for foldable displays has a single back surface that is continuous through the folded portion of the foldable display. Foldable display placed as a protective film.
8. 7. A mobile terminal device having a foldable display according to 7th.

The polyester film for foldable displays of the present invention maintains good mass productivity while suppressing deformation caused by folding under high temperature conditions, especially the generation of crease marks, and prevents cracks caused by repeated folding. Hard to occur. Furthermore, a foldable display using the film of the present invention is less likely to cause image distortion at the folded portion of the display. A mobile terminal device equipped with a foldable display using a polyester film as described above provides beautiful images even when folded repeatedly, is highly functional, and has excellent convenience such as portability. Moreover, the polyester film for foldable displays of the present invention has high adhesion to the hard coat layer.

FIG. 2 is a schematic diagram showing a foldable display and a bending radius when folded. It is a schematic diagram for showing the bending direction of the polyester film for foldable displays in the present invention. FIG. 3 is a schematic diagram for explaining a method of measuring a high temperature hold angle in a bending direction. An enlarged schematic diagram of a sample film (number 3) sandwiched between two PTFE plates is shown. A schematic diagram of an outward-bending foldable smartphone and a foldable display is shown.

(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. For example, preferred are LCDs, organic EL displays, and inorganic EL displays that have a bendable structure. Organic EL displays and inorganic EL displays are more preferred because they can reduce the layer structure, and organic EL displays are particularly preferred because they have a wide color gamut.

(Foldable display)
A foldable display is a single continuous display that can be folded into two for carrying. By folding it, you can reduce the size by half and improve portability. 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, it is possible to make the foldable display thinner in the folded state. It can be said that the smaller the bending radius, the better. The foldable display of the present invention can suppress the occurrence of crease marks in the polyester film even when used with such a bending radius. Further, in a certain embodiment of the foldable display of the present invention, even when used with such a bending radius, the occurrence of cracks in the polyester film can be suppressed.

The bending radius of the foldable display is preferably 0 to 5 mm, more preferably 0 to 3 mm. If the bending radius is 5 mm or less, the folded state can be made thinner. The smaller the bending radius is, the better, but it may be 0.1 to 5 mm, 0.1 to 3 mm, 0.5 to 3 mm, 0.5 to 5 mm, or 1 to 5 mm. Even if the bending radius is 1 to 5 mm, it is possible to achieve a practically sufficient thickness reduction in the folded form compared to a display without a folding structure.

The bending radius when folded is the value measured at the point 11 of the foldable display 1 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.

Further, the foldable display may be folded in three or four, or may be a roll-up type called a rollable display, and all of these fall within the scope of the foldable display in the present invention.

Furthermore, the polyester film of the present invention can be used for folding displays not only by folding in the longitudinal direction as shown in FIG. 1 but also by folding in the width direction.

The foldable display may be bent outward or bent inward. Inward-bending type foldable displays have a display located on the inside when folded, and normally the display cannot be viewed in the folded state. In a foldable display that bends outward, the display is located on the outside when folded, and the display can be viewed even in the folded state (Figure 5). The polyester film of the present invention can be used in any embodiment. Furthermore, in both outward-bending and inward-bending foldable displays, the polyester film of the present invention may be placed either on the outside or inside of the display, or may be placed on both the outside and inside of the display. The polyester film of the present invention does not easily leave fold marks even with a small bending radius, so it can be used in such various embodiments.

The polyester film for foldable displays of the present invention may be used in any component of foldable displays. FIG. 5 shows a diagram schematically showing the structure of a foldable display. Below, a typical structure of a foldable display and a portion to which the polyester film of the present invention can be used will be explained using an organic EL display as an example. In addition, hereinafter, the polyester film for foldable displays of the present invention may be simply referred to as the polyester film of the present invention.

(Foldable organic EL display)
A foldable organic EL display has an organic EL module as an essential component, but is further provided with a circularly polarizing plate, a touch panel module, a front protection film, a back protection film, etc., as necessary.

(Organic EL module)
The general structure of an organic EL module consists of an electrode/electron transport layer/light emitting layer/hole transport layer/transparent electrode. The polyester film of the present invention can be used as a base material on which an electrode is provided, and further an electron transport layer, a light emitting layer, a hole transport layer, and a transparent electrode are provided. In particular, it can be preferably used as a base material for transparent electrodes. In this case, since the base film is required to have high water vapor and oxygen barrier properties, the polyester film of the present invention is preferably provided with a barrier layer such as a metal oxide layer. In order to improve barrier properties, a plurality of barrier layers may be provided, or a plurality of polyester films each provided with a barrier layer may be used.

(touch panel module)
It is preferable that the mobile terminal device has a touch panel. When an organic EL display is used, it is preferable that a touch panel module is disposed above the organic EL display or between the organic EL module and the circularly polarizing plate. A touch panel module has a transparent base material such as a film and a transparent electrode disposed on the transparent base material. The polyester film of the present invention can be used as this transparent substrate. When used as a transparent base material for a touch panel, the polyester film is preferably provided with a hard coat layer or a refractive index adjusting layer.

(Circular polarizing plate)
The circularly polarizing plate prevents external light from being reflected by members inside the display and deteriorating image quality. The circularly polarizing plate includes a linearly polarizing plate and a retardation plate. The linearly polarizing plate has a protective film on at least the viewing side. A protective film may also be provided on the surface of the linearly polarizing plate opposite to the viewing side, or a retardation plate may be directly laminated on the linearly polarizing plate. The retardation plate used is a resin film having a retardation such as polycarbonate or cyclic olefin, or a resin film provided with a retardation layer made of a liquid crystal compound. The polyester film of the present invention can be used as a polarizer protective film or a resin film for a retardation plate. In these cases, it is preferable that the slow axis direction of the polyester film of the present invention be parallel or perpendicular to the absorption axis direction of the polarizer. Note that a deviation of up to 10 degrees, preferably up to 5 degrees with respect to this parallel or perpendicular direction is allowed.

(Surface protection film)
If a shock is applied to the display from above, there is a risk that the circuits of the organic EL module or touch panel module will break, and to prevent this, a surface protection film is often provided. The polyester film of the present invention can be used as this surface protection film. There are two types of surface protection films: a cover window built into the top surface of a display, and an after film. The polyester film of the present invention can be used as any film. The after-film is a film that is attached to the cover window by the manufacturer during the manufacturing process to protect the cover window, or a replaceable film that is attached to the cover window by the user. When the polyester film of the present invention is used as a surface protection film, it is preferable that a hard coat layer is laminated on at least the surface side of the polyester film. The polyester film is provided on the surface of the foldable display with the hard coat layer facing the viewing side. Note that the hard coat layer may be provided on both sides of the polyester film.

(back side protective film)
It is also preferable that a protective film is provided on the back side of the display. Specifically, an adhesive layer is provided on the non-visible side of the organic EL module, and a protective film is attached. The polyester film of the present invention can be used as a protective film on the back side.

The polyester film of the present invention may be used for purposes other than those described above, as long as it is used in a folded part of a foldable display component.
Among these, the polyester film of the present invention is preferably used for a cover window surface protection film, a surface protection film as an aftermarket product, a touch panel module base film, and a back surface protection film. It is preferable to use it as a cover window surface protection film or a surface protection film as an aftermarket product.

Furthermore, in a foldable display, the polyester film of the present invention does not need to be used for all of the above films. For foldable displays, in addition to the polyester film of the present invention, polyimide films, polyamide films, polyamide-imide films, polyester films other than the polyester film of the present invention, polycarbonate films, acrylic films, triacetyl cellulose films, cycloolefin polymer films, polyphenylene Sulfide films, polymethylpentene films, and the like can be appropriately selected and used depending on the purpose of use and the like.

(Polyester film)
The polyester film of the present invention may be a single layer film or a multilayer film having two or more layers, and each layer constituting the multilayer film may contain the same polyester resin or may contain different polyester resins. . Furthermore, the multilayer film may have a super multilayer structure in which different layers are alternately and repeatedly laminated. The multilayer film may have a structure of 2 to 30 layers, or a structure of 2 to 20 layers, for example. The polyester film may be a single layer film made of one or more types of polyester resin. When two or more types of polyester resins are used, the polyester film may be a multilayer film having a super multilayer structure, or may be another multilayer film.

Examples of the polyester resin used in the polyester film include polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene-2,6-naphthalate, or copolymers containing the constituent components of these resins as main components. Can be mentioned. Among these, polyethylene naphthalate film, particularly stretched polyethylene naphthalate film, is preferred from the viewpoint of mechanical properties, heat resistance, transparency, and the like.

One embodiment of the polyester film contains polyethylene naphthalate resin as a main component (for example, more than 50% by mass). The polyester film may further contain other polyester resins, and the content thereof is, for example, less than 50% by mass, 40% by mass or less, and 10% by mass or less, based on 100% by mass of the polyester film. Generally, it is preferably 5% by weight or less, and more preferably less than 5% by weight. When the content of other polyester resins is less than 5% by mass, the crystallinity of the polyester film can be maintained highly, and the high temperature hold angle can be maintained well. When the polyester film contains other polyester resin, the content thereof is, for example, 0.1% by mass or more and less than 50% by mass, 0.1 to 40% by mass, 0.1% by mass, based on 100% by mass of the polyester film. It can be 10% by mass, 0.1% by mass to 5% by mass, 0.1% by mass or more and less than 5% by mass, etc.

On the other hand, the content of polyethylene naphthalate resin in the polyester film may be more than 50% by mass, 60% by mass or more, or 90% by mass or more, preferably 95% by mass or more, based on 100% by mass of the polyester film. , more preferably more than 95% by mass.

In one embodiment, the proportion of polyethylene naphthalate in the raw material ratio of the polyester film is 100% by weight.
In addition, in this invention, a polyester film may contain multiple types of polyethylene naphthalates with different characteristics.
By increasing the proportion of polyethylene naphthalate, deformation of the polyester film after folding in a high temperature region is suppressed, and image disturbance at the folded portion of the display can be suppressed. Furthermore, a mobile terminal device equipped with a foldable display using the polyester film of the present invention provides beautiful images, is rich in functionality, and has excellent convenience such as portability.

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. In addition, the glycol components of polyester include, for example, fatty acid glycols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, and neopentyl glycol; aromatic glycols such as p-xylene glycol; Examples include alicyclic glycols such as methanol; polyethylene glycols having an average molecular weight of 150 to 20,000. The preferred mass ratio of the copolymer components in the polyester copolymer is less than 3% by mass. When the amount is less than 3% by mass, film strength, transparency, and heat resistance are maintained, which is preferable.

Further, the intrinsic viscosity of the resin pellets as a raw material for the polyester film may be 0.40 to 1.0 dl/g, preferably 0.40 to 0.80 dl/g, and 0.40 to 0.70 dl/g. is more preferable. When the intrinsic viscosity is 0.40 dl/g or more, (1) the impact resistance of the obtained film is improved, making it difficult for the internal circuit of the display to break due to external impact, and (2) the unstretched sheet is It is preferable because it can be stably molded. It is preferable that the intrinsic viscosity is 1.00 dl/g or less, particularly 0.80 dl/g or less, because it facilitates stable film production without increasing the filtration pressure of the molten fluid too much.
For example, when the polyester film includes multiple types of resin pellets, the intrinsic viscosity of all of these multiple types of resin pellets may be within the above range, for example, within the range of 0.40 to 1.0 dl/g. , at least one type of pellet may have an intrinsic viscosity within the above range, for example within the range of 0.40 to 1.0 dl/g. In one aspect, when the polyester film includes multiple types of resin pellets, the resin pellets that are the main component (for example, more than 50% by mass) have an intrinsic viscosity of 0.40 to 1.0 dl/g, 0.40 to 0.80 dl /g, or within the range of 0.40 to 0.70 dl/g.

The intrinsic viscosity of the polyester film may be 0.40 to 0.95 dl/g. The intrinsic viscosity of the polyester film is preferably 0.50 to 0.80 dl/g, more preferably 0.50 to 0.75 dl/g, and even more preferably 0.53 to 0.75 dl/g, in order to enhance adhesion to a hard coat layer formed on the film. When the intrinsic viscosity of the film is 0.40 dl/g or more, and particularly 0.50 dl/g or more, it is preferable that (1) the impact resistance of the obtained film is improved, and disconnection of the internal circuit of the display due to external impact is unlikely to occur, (2) the surface strength of the film is increased, and peeling of a part of the film from the inside of the film by tape or the like can be suppressed, and (3) the breaking elongation of the polyester film is improved, and cracking when continuously bent can be suppressed, etc. If the intrinsic viscosity is 0.95 dl/g or less, and particularly 0.80 dl/g or less, the filtration pressure of the molten fluid does not increase too much during the polyester film formation process, making it easier to operate the film production stably, which is preferable.

The thickness of the polyester film is, for example, 10 to 125 μm, preferably 25 to 100 μm. When the thickness is 10 μm or more, the effect of improving pencil hardness and impact resistance is improved, and when the thickness is 125 μm or less, it is advantageous for weight reduction, and also has excellent flexibility, workability, and handling properties.

The surface of the polyester film may be smooth or uneven. When used as a display cover window, it is preferable to have a smooth film surface. The haze of the polyester film is preferably 3% or less, more preferably 2.1% or less, even more preferably 2% or less, and particularly preferably 1% or less. When the haze is 3% or less, image visibility can be improved. The lower limit of haze is better, but from the viewpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more. Haze is, for example, 0.1-3%, 0.1-2.1%, 0.1-2%, 0.1-1%, 0.3-3%, 0.3-2.1%, It can be 0.3-2%, 0.5-2.1%, 0.5-2%, etc.

As mentioned above, for the purpose of reducing haze, it is better not to have large irregularities on the surface of the film, but in order to provide a certain degree of slipperiness from the viewpoint of handling, the film may have irregularities. Such irregularities can be formed by adding particles to the polyester film disposed on the surface layer, by coating the polyester film with a coating layer containing particles during film formation, and the like.

As a method for blending particles into a polyester film, a known method can be adopted. For example, particles can be added at any stage of manufacturing the polyester. Preferably, at the stage of esterification, or after the end of the transesterification reaction and before the start of the polycondensation reaction, a slurry of particles dispersed in ethylene glycol or the like is added, and the polycondensation reaction is allowed to proceed. good. 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 Particles can be mixed using methods such as

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. The addition method is preferred as a method for blending particles. 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 homogeneously disperse inorganic particles in the remainder of the raw material in a low temperature state before the esterification reaction, and then add the resultant mixture after filtration.

In addition, the number of protrusions on the film surface can be further reduced by a method (masterbatch method) in which a polyester containing particles is obtained in advance and then the pellets are kneaded and extruded with pellets not containing particles.

Additionally, the polyester film may contain various additives within the range that maintains the 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 from the standpoint of stable production, it is preferably 99% or less, and may be 97% or less. The total light transmittance of the polyester film may be 85-99%, 85-97%, 87-99%, 87-97%.

The maximum thermal shrinkage rate of the polyester film after heat treatment at 150° C. for 30 minutes may be 2% or less, preferably 1.5% or less, and more preferably 1.2% or less. If the maximum thermal shrinkage rate is 2% or less, dimensional changes due to heat generation of the organic EL display itself can be suppressed. It can be said that the lower the maximum thermal shrinkage rate, the better, but it is preferably −1% or more, and preferably 0% or more. The negative value here means that the material expanded after heating, and if it is less than -1%, flatness may be defective. The maximum heat shrinkage rate of polyester film after heat treatment at 150°C for 30 minutes is -1 to 2%, -1 to 1.5%, -1 to 1.2%, 0 to 2%, 0 to 1.5%, It may be 0 to 1.2%.

One or both sides of the polyester film of the present invention can be treated to improve adhesion with the resin forming the hard coat layer or the like.

The treatment for improving adhesion may be, for example, a method of treating the surface of the polyester film, a method of providing an adhesion-improving layer on the surface of the polyester film, 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 and ultraviolet irradiation treatment, flame treatment, chromic acid treatment, hot air treatment, etc. can be used without particular limitation.

Furthermore, by interposing an adhesion-improving layer such as an easy-adhesion layer between the polyester film and the hard coat layer, it is also possible to improve the adhesion with the resin forming the hard coat layer. As the resin for forming the easily adhesive layer, acrylic resin, polyester resin, polyurethane resin, polyether resin, etc. can be used without particular limitation, and the easily adhesive layer can be formed by a general coating method, preferably a so-called in-line coating method.

The above-mentioned polyester film is produced by, for example, homogeneously dispersing inorganic particles in a monomer liquid that is part of the polyester raw material, filtering it, and adding the inorganic particles in the form of a slurry to the remainder of the polyester raw material to polymerize the polyester. It can be manufactured through a polymerization step and a film forming step in which the polyester is melt-extruded into a sheet through a filter, cooled, and stretched to form a base film.

Next, a method for producing a biaxially stretched polyester film will be explained in detail using an example in which pellets of polyethylene terephthalate (hereinafter sometimes referred to as PET) are used as the raw material for the base film, but the method is not limited to this. . Further, the number of layers is not limited, such as a single layer structure or a multilayer structure.
Note that even in an embodiment in which a polyethylene naphthalate (PEN) film is used instead of a PET film, the polyester film according to the present invention can be manufactured by the same method.

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. . When producing a film with a single layer structure, an unstretched film can be formed using one extruder, but when producing a film with a multilayer structure, two or more extruders and a manifold with two or more layers are required. Alternatively, use a merging block (for example, a merging block with a square merging part) to laminate multiple film layers constituting each outermost layer, extrude two or more layers of sheets from a die, cool them with a casting roll, and leave them unused. A stretched film can be formed.

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 300° 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.

Further, 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.

(About the refractive index in the bending direction)
In the present invention, the refractive index of the polyester film in at least one of the longitudinal direction (machine flow direction) and the width direction is preferably 1.610 to 1.710, for example, 1.620 to 1.700, More preferably, it is 1.630 to 1.690. It is preferable that the refractive index in at least one of the longitudinal direction and the width direction is 1.610 to 1.710, since there is little deformation when repeatedly folded and there is no risk of degrading the image quality of the foldable display.

In one embodiment, when the refractive index in the longitudinal direction of the polyester film is 1.610 or more, the degree of crystallinity can be efficiently improved, and the high-temperature hold angle can be improved. If it is 1.710 or less, stress during bending can be lowered, and both the hold angle at room temperature and the hold angle at high temperature can be improved.

Additionally, apart from this aspect, when the refractive index in the width direction of the polyester film is within the above range, it is desirable that the refractive index in the longitudinal direction of the polyester film is higher than the refractive index in the width direction of the polyester film.

The refractive index of the polyester film in the bending direction is preferably 1.610 to 1.710, for example, 1.620 to 1.700, and more preferably 1.630 to 1.690. When the refractive index in the bending direction is 1.610 or more, the degree of crystallinity can be efficiently improved and the high temperature hold angle can be improved. When the refractive index in the bending direction is 1.710 or less, stress during bending can be lowered, and both the hold angle at room temperature and the hold angle at high temperatures can be improved.

Here, the bending direction is the direction indicated by the reference numeral 22 on the polyester film (reference numeral 2) in FIG. There is.

The refractive index of the polyester film can be effectively adjusted by adjusting the stretching ratio and stretching temperature. Furthermore, a relaxation step in the stretching direction or multistage stretching may be used to adjust the refractive index. When performing multi-stage stretching, it is preferable that the stretching ratio in the second and subsequent stages is higher than the stretching ratio in the first stage.

By controlling the refractive index in at least one of the longitudinal direction (machine flow direction) and the width direction of the polyester film within the above range, and more preferably controlling the refractive index in the bending direction within the above range, the folding effect can be improved during folding. Fatigue due to compressive stress applied to the inside can be reduced. Fatigue due to compressive stress is thought to occur mainly in crystal parts, and the fewer crystals in the bending direction, the less fatigue occurs. Therefore, since the refractive index in the bending direction is smaller than the refractive index in the direction perpendicular to the bending direction, the amount of oriented crystals in the bending direction is reduced, and fatigue due to compressive stress is suppressed.

Furthermore, the creep phenomenon caused by the tensile stress applied to the outside of the fold during folding can be suppressed by reducing the refractive index. Fatigue due to tensile stress is thought to occur mainly in the amorphous region, and repeated stress causes molecular chains to be aligned, resulting in deformation. It can be inferred that the fewer molecular chains are aligned in the bending direction, the less deformation will occur due to alignment. Furthermore, since fatigue due to tension can be suppressed when the amorphous portion is small, it is preferable that the crystallinity, that is, the density is high.

In the present invention, 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, and 1.4 to 2.3 times. is even more preferable. The stretching direction in which the magnification is given is preferably the bending direction when the polyester film is used in a folding display. It is preferable that the stretching ratio is 3.4 times or less because uneven thickness of the film does not occur. The stretching temperature is preferably 120 to 150°C, more preferably 125 to 145°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 125 to 145°C, large thickness unevenness due to stretching at the above-mentioned stretching ratio can be prevented.

(About the refractive index in the direction of the folded part)
The refractive index in the direction perpendicular to the direction in which the polyester film has a refractive index of 1.610 to 1.710 is preferably 1.750 to 1.870. For example, it is preferable that the polyester film has a refractive index of 1.610 to 1.710 in the bending direction, and a refractive index of 1.750 to 1.870 in a direction perpendicular to the bending direction (folding direction). By setting the refractive index in the folding direction to 1.750 to 1.870, deformation when folded in the bending direction can be reduced. By setting the refractive index in the direction of the folded portion to 1.870 or less, it is possible to suppress the occurrence of cracks in the direction of the folded portion, and further suppress the occurrence of breakage. Moreover, breakage in the winding process after stretching can be suppressed. By setting the refractive index in the direction of the folded portion to 1.750 or more, the density of the polyester film can be increased and the high temperature hold angle can be improved.

For example, when the longitudinal direction of the polyester film is the bending direction, the direction perpendicular to the bending direction (folding direction) corresponds to the width direction of the polyester film.

The refractive index in the direction orthogonal to the bending direction is more preferably 1.770 to 1.830, and even more preferably 1.800 to 1.830.
Further, when comparing the refractive index in the bending direction and the refractive index in a direction perpendicular to the bending direction (folding section direction), it is desirable that the refractive index in the bending direction is low.
The polyester film has a refractive index of 1.610 to 1.710 in a direction and a refractive index of 1.750 to 1.870 in a direction perpendicular to that direction, which reduces deformation when folded in the bending direction. can do. Moreover, it is possible to suppress the occurrence of cracks in the direction of the folded portion, and it is also possible to suppress the occurrence of breakage. Moreover, breakage in the winding process after stretching can be suppressed. In addition, the density can be increased and the high temperature hold angle can be improved.
Examples of methods for adjusting the refractive index in the direction orthogonal to the bending direction include methods for adjusting the stretching ratio, stretching preheating temperature, stretching temperature, multistage stretching, film relaxation, etc. in the direction. The stretching ratio in the direction perpendicular to the bending direction is preferably 3.3 to 5.0 times, more preferably 3.5 to 4.5 times. Further, the stretching preheating temperature in the direction perpendicular to the bending direction is preferably 125 to 145°C. In the case of multi-stage stretching in a direction perpendicular to the bending direction, it is preferable that the stretching ratio in the second stage and subsequent stages is higher than that in the first stage. The film may be relaxed by 0 to 10% in either the machine flow direction (longitudinal direction) or the vertical direction (width direction).

(About the refractive index in the thickness direction)
The refractive index in the thickness direction is preferably 1.520 or less. It is more preferably 1.515 or less, still more preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. The refractive index in the thickness direction is preferably low, but in terms of stable production, it is preferably 1.300 or more, more preferably 1.400 or more, particularly preferably 1.410 or more. The refractive index in the thickness direction is 1.300-1.520, 1.300-1.515, 1.300-1.510, 1.300-1.500, 1.400-1.520, 1.400- 1.515, 1.400-1.510, 1.400-1.500, 1.410-1.520, 1.410-1.515, 1.410-1.510, 1.410-1. It may be 500.

(About the density of polyester film)
It is preferable that the density of the polyester film is 1.353 g/cm ³ or more. More preferably, it is 1.355 g/cm ³ or more. The high temperature hold angle can be improved by setting it to 1.353 g/cm ³ or more. Although the density depends somewhat on the presence or absence of particles in the film, it is preferably 1.400 g/cm ³ or less, and more preferably 1.395 g/cm ³ or less. The density of the polyester film is 1.353-1.400g/cm ³ , 1.353-1.395g/cm ³ , 1.349-1.395g/cm ³ , 1.350-1.400g/cm ³ , It may be 1.350 to 1.395 g/cm ³ .
When the density of the polyester film is 1.353 g/cm ³ or more, the polyester film of the present invention can be sufficiently crystallized and deformation at 85° C. can be suppressed. Further, it is possible to suppress an increase in the thermal shrinkage rate, and it is possible to suppress dimensional changes due to heat generation of the device.
By setting the heat setting temperature at 210 to 270° C. during film formation, crystallization can proceed and the density can be effectively increased within the above range.

It is preferable that the bending direction of the polyester film corresponds to the longitudinal direction (machine flow direction). By doing so, it is easy to lower the refractive index in the bending direction at the biaxial stretching stitches, and it is easy to improve the flexibility. That is, it is preferable to stretch the unstretched polyester sheet in the longitudinal direction at a stretching ratio of 1.1 to 2.5 times, preferably 1.4 to 2.3 times, more preferably 1.6 to 2.1 times. A polyester film can be obtained. It can be said that a preferred embodiment is stretching in the width direction at a stretching ratio of 3.3 to 5.0 times, more preferably 3.5 to 4.5 times.

The polyester film of the present invention has a high temperature hold angle of 85° or more in the bending direction. Here, the high temperature hold angle refers to the bending angle that occurs after the polyester film is fixed for 18 hours under heating at a predetermined temperature (for example, 85°C) so that a strain of 1.7% is generated on both surfaces of the bent portion. Points to the angle formed afterward. Further, the bending direction refers to a direction perpendicular to the folded portion.
The high temperature hold angle in the bending direction may be 87° or more, 89° or more, 90° or more, or 100° or more. The high temperature hold angle in the bending direction is preferably large, and most preferably 180°. The high-temperature hold angle in the bending direction may be 180° or less, and even if it is 170° or less, for example, it has sufficient functionality. High temperature hold angles are 87-180°, 89-180°, 90-180°, 100-180°, 87-170°, 89-170°, 90-170°, 100-170°, 87-160°, 89 ~160°, 90-160°, 100-160°, 87-150°, 89-150°, 90-150°, 100-150°, 87-120°, 89-120°, 90-120°, 100 ~120°.

By setting the high temperature hold angle in the bending direction within the above range, deformation of the polyester film due to bending at 85° C. can be suppressed. Further, it is possible to suppress the thermal shrinkage rate from increasing, and it is possible to suppress dimensional changes in the polyester film due to heat generation of the device. Therefore, according to the present invention, deformation after repeated folding is less likely to occur even in a high temperature region, and image disturbance at the folded portion of the display can be suppressed. Furthermore, a mobile terminal device equipped with a foldable display using the polyester film of the present invention provides beautiful images, is rich in functionality, and has excellent convenience such as portability.
Note that a method for measuring the high temperature hold angle in the bending direction will be exemplified in Examples.

The high temperature hold angle is set to 85° or more by adjusting the refractive index in the bending direction to 1.610 to 1.710 and the refractive index in the direction perpendicular to the bending direction to within the range of 1.750 to 1.870. be able to. By setting the density of the polyester film to 1.353 g/cm ³ or more, the high temperature hold angle can easily be set to 85° C. or more. It is possible to increase the high temperature hold angle to 85° or more by increasing the density, lowering the refractive index in the bending direction, and increasing the refractive index in the direction perpendicular to the bending direction, but it is extremely difficult to maintain a balance within the above range. is important. For example, if the density is high and the refractive index in the direction perpendicular to the direction of bending is high, the film will become brittle in one direction, and repeated bending at 25°C (room temperature) will easily cause cracks. . Therefore, cracking can be suppressed by stretching so that the refractive index in the bending direction is 1.610 or more, and by stretching so that the refractive index in the direction perpendicular to the bending direction is 1.870 or less. In this way, by adjusting and balancing the density, the refractive index in the bending direction, and the refractive index in the direction perpendicular to the bending direction, it is possible to achieve resistance to bending in high temperature regions and suppress cracking when repeatedly bent at room temperature. It becomes possible to achieve both.

The produced film may be collected, crushed and melt-extruded recycled resin may be added to the film. The intrinsic viscosity of the recycled resin is preferably 0.45 to 0.60 dl/g.

(About breaking elongation of polyester film)
The tensile elongation at break in the bending direction of the polyester film is preferably 5% or more. More preferably, it is 10% or more, and still more preferably 20% or more. The higher the tensile elongation at break in the bending direction, the better; for example, it may be 200% or more. The tensile elongation at break in the bending direction may be 5 to 250%, 10 to 250, or 20 to 250. By setting the tensile elongation at break in the bending direction to 5% or more, it is possible to prevent the film from breaking during handling during processing. Moreover, cracking when repeatedly bent can be suppressed. The elongation at break can be satisfied by adjusting the refractive index in the bending direction, for example, by adjusting the stretching conditions so that the refractive index in the bending direction is 1.610 or more (preferably 1.610 to 1.710). .

(About the elastic modulus of polyester film)
The elastic modulus of the polyester film in the bending direction may be 4500 to 6500 MPa. More preferably 4,750 to 6,250 MPa, still more preferably 5,000 to 6,000 MPa. By setting the elastic modulus in the bending direction to 4,500 MPa or more, it is possible to laminate without distortion when pasting with other materials, and by setting it to 6,500 MPa or less, the stress applied when bending can be suppressed and the high-temperature hold angle can be improved. can.

(Easy adhesive layer)
In order to improve the adhesion between the polyester film and the hard coat layer, it is also preferable to laminate an easily adhesive layer on at least one side of the polyester film of the present invention. The easy-adhesion layer is formed by applying a coating solution for forming an easy-adhesion layer to one or both sides of an unstretched or longitudinally uniaxially stretched film, then subjecting it to heat treatment and drying as necessary, and then applying the coating solution to at least one unstretched direction. It can be obtained by stretching. Heat treatment and drying can also be performed after biaxial stretching. The final coating amount of the adhesive layer is preferably controlled to 0.005 to 0.20 g/m ² . It is preferable that the coating amount is 0.005 g/m ² or more because adhesiveness is improved. On the other hand, it is preferable that the coating amount is 0.20 g/m ² or less because blocking resistance can be obtained.

As the resin to be contained in the coating liquid used for laminating the easily adhesive layer, for example, polyester resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc. can be used without particular limitation. Examples of the crosslinking agent to be included in the coating solution for forming an easily adhesive layer include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, and carbodiimide compounds. It is also possible to use a mixture of two or more of each. Due to the nature of the in-line coating, these are preferably applied using a water-based coating liquid. The resin and crosslinking agent described above are preferably water-soluble or water-dispersible resins or compounds.

It is preferable to add particles to the easy-adhesion layer in order to impart slipperiness. The average particle size of the particles is preferably 2 μm or less. If the average particle diameter of the particles is 2 μm or less, the particles will be difficult to fall off from the adhesive layer. Examples of particles to be included in the adhesive layer include 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 to the adhesive layer singly or in combination of two or more.

Moreover, a known method can be used as a method for applying the coating liquid for forming an easily adhesive layer. For example, reverse roll coating method, gravure coating method, kiss coating method, roll brushing method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, etc. Or it can be done in combination.

(Hard coat layer)
When the polyester film of the present invention is placed on the surface of a folding display and used as a surface protection film for protecting the display, it is preferable that at least one surface thereof has a hard coat layer. In a display, the hard coat layer is preferably used by being located on the display surface side on the polyester film. As the resin forming the hard coat layer, resins such as acrylic, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, and epoxy 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.

(Thickness of hard coat layer)
The thickness of the hard coat layer is preferably 1 to 50 μm. It is preferable that the thickness is 1 μm or more because it will be sufficiently cured and the pencil hardness will be high. 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.

(Coating method)
As a coating method for the hard coat layer, a Meyer bar, a gravure coater, a die coater, a knife coater, etc. can be used 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 rays such as ultraviolet rays and electron beams, and heat curing methods can be used. As a curing method, curing with energy rays such as ultraviolet rays or electron beams is preferred in order to reduce damage to the film.

(Pencil hardness)
The pencil hardness of the hard coat layer is preferably 3H or higher, more preferably 4H or higher. If it has a pencil hardness of 3H or higher, it will not be easily scratched and will not reduce visibility. Generally, the pencil hardness of the hard coat layer is preferably higher, but it may be 9H or less, 8H or less, and 6H or less can be used practically without any problem. The pencil hardness of the hard coat layer may be 3H to 9H, 3H to 8H, or 3H to 6H.

(Characteristics of hard coat layer)
The hard coat layer in the present invention can be used for the purpose of protecting the display by increasing the pencil hardness of the surface as described above, and preferably has high transmittance. The total light transmittance of a film provided with a hard coat layer (sometimes referred to as a hard coat film) is preferably 87% or more, more preferably 88% or more. If the total light transmittance is 87% or more, sufficient visibility can be obtained. Generally, the higher the total light transmittance of the hard coat film is, the more preferable it is, but from the viewpoint of stable production, it may be 99% or less, or 97% or less. The total light transmittance of the hard coat film may be 87 to 99%, 88 to 99%, 87 to 98%, or 88 to 98%. Further, the haze of the hard coat film is generally preferably low, and preferably 3% or less. The haze of the hard coat film is more preferably 2% or less, most preferably 1% or less. When the haze is 3% or less, image visibility can be improved. From the viewpoint of stable production, the haze is preferably 0.1% or more, and may be 0.3% or more. The haze of the hard coat film is 0.1-3%, 0.1-2%, 0.1-1%, 0.3-3%, 0.3-2%, 0.3-1%. You can.

The hard coat layer may further have other functions added to it. 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.

A hard coat layer may also be provided when used as a base film for a touch panel module. For example, when an ITO layer is used as the transparent electrode layer of the touch panel module, a refractive index adjustment layer is preferably provided between the base film and the transparent electrode layer in order to make the electrode pattern less visible. In this case, the hard coat layer itself may also serve as a refractive index adjusting layer, or a separate refractive index adjusting layer may be laminated.

In another embodiment, the polyester film for foldable displays of the present invention can be used as a back protection film for foldable displays. For example, the polyester film for a foldable display of the present invention can be disposed as a single back protective film continuous through the folded portion of the foldable display.

In another aspect, a mobile terminal device having the polyester foldable display of the present invention is provided.

Next, the present invention will be explained using Examples and Comparative Examples. First, the method of determining characteristic values carried out in the following example will be described below.

(1) 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. The same calculation formula was used to evaluate both polyethylene terephthalate (PET) and polyethylene naphthalate (PEN).

(2) Refractive index of polyester film Using a laser refractometer manufactured by Metricon (model 2010 prism coupler), a sample film was held between the built-in pressure gauges at 40 scales and measured using a laser beam with a wavelength of 633 nm. , got a spectrum chart. On the obtained spectrum chart, the point where the detector output suddenly decreased was read, and this value was taken as the refractive index. The refractive index in the longitudinal direction and the width direction was measured in measurement mode TE, and the refractive index in the thickness direction was measured in TM.

(3) Total light transmittance and haze of polyester film The total light transmittance and haze of the polyester film were measured using a haze meter (manufactured by Nippon Denshoku Industries, Ltd., NDH5000).

(4) Density of polyester film The density of the polyester film was measured according to a method based on JIS K 7112:1999 (density gradient tube method). (Unit: g/cm ³ ).

(5) Maximum heat shrinkage rate of polyester film Cut the sample film to 10 mm vertically x 250 mm horizontally, make marks at 200 mm intervals on the long side in the direction you want to measure, and under a constant tension of 5 g, measure the distance between the marks. A (mm) was measured. Subsequently, the sample film was left unloaded in an oven at 150° C. for 30 minutes, then taken out from the oven and cooled to room temperature. Thereafter, the distance B (mm) between the marks was determined under a constant tension of 5 g, and the heat shrinkage rate (%) was determined using the following formula. The distance A and the distance B were measured at positions divided into three equal parts in the width direction of the sample film, and the average value of the distance A and the distance B at the three points was taken as the heat shrinkage rate (%).
Heat shrinkage rate (%) = [(A-B) x 100]/A
For the same film, measure the heat shrinkage rate of the sample film with the bending direction as the long side and the heat shrinkage rate of the sample film with the fold direction (direction perpendicular to the bending direction) as the long side, and calculate the larger one. The heat shrinkage rate was defined as the maximum heat shrinkage rate (%).

(6) High temperature hold angle (85℃ hold angle)
The angle formed by the folds formed when the polyester film was fixed so that a strain of 1.7% was applied to both surfaces of the bent portion was measured.

FIG. 3 is a schematic diagram for explaining a method of measuring the high temperature hold angle in the bending direction. A sample film (number 3) was cut to a width of 10 mm and a length of 50 mm in the machine flow direction. A space was formed by sandwiching a PTFE plate (32) as a spacer between two PTFE plates (31). In the case of a 50 μm thick sample film, the spacer thickness was 3 mm. Apply double-sided tape to both ends of the sample film in the machine flow direction, bend it and sandwich it between the 3 mm gap formed between two PTFE plates, and attach both ends to two PTFE plates (code 31) with double-sided tape. Fixed. After leaving it in a dry environment at 85° C. for 18 hours, it was taken out from between two PTFE plates (number 31), and after 5 minutes, the angle (number 33) formed by the fold marks on the film was measured. This angle was defined as the high temperature hold angle.

In order to keep the strain constant, the thickness of the PTFE plate used as a spacer (code 32) was changed depending on the thickness of the film.

FIG. 4 shows an enlarged schematic diagram of a sample film (number 3) sandwiched between two PTFE plates (number 31 in FIG. 3). In addition, in FIG. 4, numeral 41 is the diameter of the semicircle formed by the outermost surface of the sample film, numeral 42 is the diameter of the semicircle formed by the neutral surface of the sample film, and numeral 43 is the diameter of the semicircle formed by the outermost surface of the sample film. It shows the diameter of the semicircle formed by the innermost surface of . The neutral plane on which neither the compressive stress nor the tensile stress is applied is defined as the center in the thickness direction (broken line in the figure), and the difference between the neutral plane and both surfaces is defined as the strain.

That is, in evaluating the high temperature hold angle, strain (1.7%) can be expressed in the following manner.
Strain (1.7%)
= (|Semicircle of the outermost or innermost surface - Semicircle of the neutral surface | / Semicircle of the neutral surface) x 100 Here, the semicircle is the thickness t (mm) of the sample film, the bending diameter (The diameter of the outermost surface, that is, the thickness of the spacer used) d (mm), each can be determined by the following formula. Semicircumference of outermost surface = d×π/2
Semi-circumference of neutral plane = (d-t) x π/2
Innermost semicircle = (d-2t) x π/2
From the above, when the strain is set at 1.7%, the spacer thickness (mm) is determined from the sample film thickness t (mm) and the bending diameter (spacer thickness) d (mm) using the following formula. do. Spacer thicknesses relative to typical film thicknesses are shown, for example, as follows.
Spacer thickness d (mm) = film thickness (mm) x 60

For example, in the case of the above-mentioned sample film having a thickness of 50 μm, the diameter of the outermost surface (reference numeral 41) is the same as the thickness d of the spacer, which is 3 mm. The diameter of the innermost surface (numeral 43) is 2.9 mm, and the diameter of the neutral surface (numeral 42) is 2.95 mm. Here, in the equation representing the strain described above, the semicircumference of the outermost surface and the semicircumference of the innermost surface can be selected as appropriate.
The spacer thickness (d) relative to the typical film thickness (t) is shown below.
Film thickness (t) Spacer thickness (d)
38μm 2.3mm
50μm 3.0mm
75μm 4.5mm
100μm 6.0mm

(7) Tensile elongation at break and tensile modulus of polyester film (Young's modulus (unit: MPa)) Measure the tensile modulus of polyester film in the bending direction and folding direction at 23°C in accordance with JIS K7127:1999. did.

(8) Continuous bending resistance of polyester film sample (bending radius 1.5 mm)
A polyester film sample with a size of 20 mm in the width direction (TD) x 110 mm in the machine flow direction (MD) was prepared. Using a no-load U-shaped stretch tester (manufactured by Yuasa System Equipment Co., Ltd., DLDMLH-FS), the bending radius was set to 1.5 mm, and it was bent 200,000 times at a rate of 1 turn/second. At that time, the sample was fixed at a position 10 mm from both ends on the long side, and the bent portion was 20 mm x 90 mm. In this test, it was assumed that in the foldable display shown in Figure 1, a polyfilm film was placed on the folded inner surface, the display bending radius was set to 1.5 mm, and the display was repeatedly folded. There is. After bending the sample a predetermined number of times, the sample was placed on a flat surface with the inner side of the bend facing down, visually observed, and evaluated using the following criteria.
○: No cracks can be seen in the sample ×: Cracks can be seen in the sample

(9) Adhesion to hard coat layer A coating solution for forming a hard coat layer having the following composition was applied to one side of a polyester film using a #5 wire bar, dried at 80°C for 1 minute, and the solvent was removed. Next, the film coated with the hard coat layer was irradiated with ultraviolet rays of 300 mJ/cm ² using a high-pressure mercury lamp to obtain a hard coat film in which the hard coat layer was laminated on one side of the polyester film.
(Coating liquid for forming hard coat layer)
Methyl ethyl ketone 36.00% by mass
Toluene 18.00% by mass
Cyclohexanone 6.00% by mass
Urethane acrylate 40.00% by mass
(BS577, manufactured by Arakawa Chemical Co., Ltd.)
Surfactant 0.10% by mass
Photoinitiator 2.00% by mass
(Irgacure 184 manufactured by Ciba Specialty Chemicals)

Next, using a cutter guide with a gap interval of 2 mm, 100 square cuts were made on the surface of the hard coat layer, penetrating the hard coat layer and reaching the film base material. Next, cellophane adhesive tape (manufactured by Nichiban, No. 405; 24 mm width) was pasted on the square cut surface, and an eraser was rubbed over it to ensure complete adhesion. Then, peel off the cellophane adhesive tape in the vertical direction from the hard coat layer surface of the hard coat laminated film, visually count the number of squares peeled off from the hard coat layer surface of the hard coat laminated film, and calculate the hard coat layer using the following formula. Adhesion to the film base material was determined. Note that squares that were partially peeled off were also counted as peeled squares. Hard coat adhesion (HC adhesion) was evaluated as 95 (%) as passing.
Hard coat adhesion (%) = 100 - (number of peeled squares)

(Preparation of polyethylene naphthalate (PEN) pellets)
100 parts of dimethyl naphthalene-2,6-dicarboxylate and 60 parts of ethylene glycol were esterified for 120 minutes while gradually increasing the temperature from 150°C to 238°C using 0.03 part of manganese acetate tetrahydrate as a transesterification catalyst. An exchange reaction was performed. During the reaction, when the reaction temperature reached 170°C, trimethyl phosphate (added as a solution heated in ethylene glycol at 135°C for 5 hours under a pressure of 0.11 to 0.16 MPa: in terms of trimethyl phosphate) After the transesterification reaction was completed, 0.024 part of antimony trioxide was added. Thereafter, the reaction product is transferred to a polymerization reactor, heated to 290°C, and subjected to a polycondensation reaction under high vacuum of 27 Pa or less, and has an intrinsic viscosity of 0.48 dl/g and substantially does not contain particles. Polyethylene-2,6-naphthalene dicarboxylate (a) was obtained.

Next, the obtained polyethylene-2,6-naphthalene dicarboxylate was subjected to solid phase polymerization at 220° C. using a vacuum dryer while maintaining the degree of vacuum at 133 Pa or less to obtain polyethylene-2,6-naphthalene dicarboxylate having an intrinsic viscosity of 0.75 dl/g. A polyethylene-2,6-naphthalene dicarboxylate (b) substantially free of particles was obtained.

(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 parts of dimethylolpropionic acid were added. parts by mass, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of polycarbonate diol with a number average molecular weight of 2000, and 85.00 parts by mass of acetonitrile and 5.00 parts by mass of N-methylpyrrolidone as solvents, and a nitrogen atmosphere was added. The mixture was stirred at 75° C. for 3 hours, and it was confirmed that the reaction solution had reached a predetermined amine equivalent. Next, the temperature of this reaction solution was lowered to 40° C., and then 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, the temperature was adjusted to 25°C, and while stirring and mixing at 2000 min ^-1 , the isocyanate group-terminated prepolymer was added and dispersed in water. did. Thereafter, acetonitrile and part of the water were removed under reduced pressure to prepare a water-soluble polyurethane resin (A) with a solid content of 35% by mass.

(Polymerization of water-soluble carbodiimide compound)
In a flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer, 200 parts by mass of isophorone diisocyanate and 4 parts by mass of 3-methyl-1-phenyl-2-phospholene-1-oxide as a carbodiimidization catalyst were added. was added and stirred at 180° C. for 10 hours under a nitrogen atmosphere to obtain 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 to this at 50° C. to obtain a yellow transparent water-soluble carbodiimide compound (B) with a solid content of 40% by mass.

(Preparation of coating liquid for forming easy adhesive layer)
A coating solution was prepared by mixing the following coating materials.
Water 16.97 parts by mass Isopropanol 21.96 parts by mass Polyurethane resin (A) 3.27 parts by mass Water-soluble carbodiimide compound (B) 1.22 parts by mass Particles 0.51 parts by mass (Silica sol with an average particle size of 40 nm, solid content Concentration 40% by mass)
Surfactant 0.05 parts by mass (silicone type, solid content concentration 100% by mass)

(Example 1)
Pellets of polyethylene naphthalate a were fed into an extruder and melted at 310°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 60°C using an electrostatic casting method. They were brought into contact and cooled to solidify, producing an unstretched film. The unstretched film was uniformly heated to 120° C. using a heating roll, heated to 135° C. using a non-contact heater, and rolled stretched to 1.7 times (longitudinal direction stretching (MD stretching)). The above coating solution for forming an easily bonding layer was applied to both sides of the uniaxially stretched film by a roll coating method, and then dried at 80° C. for 20 seconds. The final (after biaxial stretching) dry coating amount was adjusted to 0.06 g/m ² . After that, it was introduced into a tenter and preheated at 130°C, then horizontally stretched to 4.2 times at 135°C (TD stretching), the width was fixed, heat set at 240°C for 5 seconds, and then 180°C in the width direction. % relaxation, a polyethylene naphthalate film having a thickness of 50 μm was obtained. This film was subjected to various tests. The films obtained in Examples 2 to 5 and Comparative Examples 1 to 5 were similarly subjected to various tests (Tables 1 and 2).

(Example 2)
A polyester film was obtained in the same manner as in Example 1 except that the heat setting temperature was changed to 250°C.

(Examples 3 to 5)
A polyester film was obtained in the same manner as in Example 1, except that the heat setting temperature was changed to 260° C. and the stretching ratio was changed to the one shown in Table 1.

(Comparative example 1)
An unstretched film was obtained in the same manner as in Example 1. The above-mentioned coating solution for forming an easily adhesive layer was applied to both sides of the unstretched film by a roll coating method, and then dried at 80° C. for 20 seconds. The coating amount after drying after stretching was adjusted to 0.06 g/m ² . After that, it is introduced into a tenter and preheated at 130°C, then laterally stretched to 4.2 times at 135°C, fixed in width, heat set at 240°C for 5 seconds, and further relaxed by 1% in the width direction at 180°C. As a result, a polyethylene naphthalate film having a thickness of 50 μm was obtained.

(Comparative example 2)
A polyester film was obtained in the same manner as in Example 1 except that the heat setting temperature was changed to 230°C.

(Comparative example 3)
A polyester film was obtained in the same manner as in Example 1, except that the heat setting temperature was changed to 230° C. and the stretching ratio was changed to the one shown in Table 1.

(Comparative example 4)
A polyester film was obtained in the same manner as in Example 1, except that the stretching ratio shown in Table 1 was changed and the longitudinal stretching temperature was changed to 140°C.

(Comparative example 5)
A polyester film was obtained in the same manner as in Comparative Example 4 except that the heat setting temperature was changed to 260°C.

The polyester films obtained in Examples 1 to 5 and Comparative Examples 1 to 5 are laminated to the non-visible side of an organic EL module via a 25 μm thick adhesive layer, and the smartphone type can be folded in half at the center of the entire module. created a foldable display. The bending radius of this foldable display, which corresponds to numeral 11 in FIG. 1, was 3 mm. The polyester film is placed on the non-visible side of one continuous display via the folded portion as a back protection film, and is laminated to a polyimide film having a barrier layer, which is an organic EL substrate. The foldable displays using the polyester films of Examples 1 to 5 had satisfactory operation and visibility as smartphones that could be folded in two at the center and carried. Furthermore, there were no problems with operation or visibility even under high temperatures.
On the other hand, in the foldable displays in which the polyester films of Comparative Examples 1 to 5 were used in the same manner, as the frequency of use at high temperatures increases, it seems that image distortion occurs at the folded part of the display, which is less desirable. It wasn't something. In addition, there were some cases where dents and scratches were observed on the surface of the folding part. In particular, in the foldable display in which the polyester film of Comparative Example 1 was used, cracking occurred as the number of folds increased.

(Example 6)
Polyethylene naphthalate pellets a and b were mixed at a ratio of 60% and 40%, respectively, and fed to an extruder and melted at 310°C. The molten blended polymer is filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% of particles cut out at 10 μm), extruded into a sheet from a nozzle, and then cast using an electrostatic casting method at a surface temperature of 60°C. It was brought into contact with a drum and cooled and solidified to produce an unstretched film. The unstretched film was uniformly heated to 120° C. using a heating roll, heated to 135° C. using a non-contact heater, and rolled stretched to 1.7 times (longitudinal direction stretching (MD stretching)). The above coating solution for forming an easily bonding layer was applied to both sides of the uniaxially stretched film by a roll coating method, and then dried at 80° C. for 20 seconds. Note that the final coated amount after drying (after biaxial stretching) was adjusted to be 0.06 g/m ² . After that, it was introduced into a tenter and preheated at 130°C, then horizontally stretched to 4.2 times at 135°C (TD stretching), the width was fixed and heat set at 260°C for 5 seconds, and then 180°C in the width direction. % relaxation, a polyethylene naphthalate film having a thickness of 50 μm was obtained. This film was subjected to various tests. The films obtained in Examples 7 to 9 and Comparative Example 6 were also subjected to various tests (Tables 3 and 4).

(Examples 7-8)
A polyester film was obtained in the same manner as in Example 6 except that the resin ratios shown in Table 3 were changed.

(Example 9)
A polyester film was obtained in the same manner as in Example 7 except that the longitudinal direction stretching ratio (MD ratio) was changed to 1.4 times.

(Comparative example 6)
A polyester film was obtained in the same manner as in Example 9 except that the resin ratio was 100% polyethylene naphthalate a.

The polyester films obtained in Examples 6 to 9 and Comparative Example 6 were bonded to the non-visible side of an organic EL module via a 25 μm thick adhesive layer to create a smartphone-type folding device that could be folded in half at the center of the entire module. Created a type display. The bending radius of this foldable display, which corresponds to numeral 11 in FIG. 1, was 3 mm. The polyester film is placed as a back protection film on the non-viewing side of one continuous display through the folded portion, and is laminated to a polyimide film having a barrier layer, which is an organic EL substrate. The foldable displays in which the polyester films of Examples 6 to 9 were used had satisfactory operation and visibility as smartphones that could be folded in two at the center and carried. Furthermore, there were no problems with operation or visibility even under high temperatures.
On the other hand, with the foldable display using the polyester film of Comparative Example 6 for the same purpose, as the frequency of use under high temperatures increases, it seems that image distortion occurs at the folded part of the display, which is not very desirable. Ta.

The polyester films of Examples 3 to 9 and Comparative Example 6 were evaluated for adhesion to the hard coat layer. The results are shown in Table 5. Films with intrinsic viscosities of 0.54, 0.58, and 0.70 dl/g had high adhesion to the hard coat layer.

The foldable display using the polyester film for foldable displays of the present invention maintains mass productivity and, for example, does not cause deformation after the polyester film located on the back side of the foldable display is repeatedly folded. , there will be no image disturbance at the folded portion of the display. In particular, a mobile terminal device or image display device equipped with a foldable display using the polyester film of the present invention as a back protection film provides beautiful images, is rich in functionality, has excellent portability and other convenience, and is reliable. is high.

1: Folding display 11: Bending radius 2: Polyester film for folding display 21: Folding section 22: Bending direction (direction perpendicular to the folding section)
3: Sample film 31: PTFE board 32: Spacer 33: Hold angle 41: Diameter of outermost surface 42: Diameter of neutral plane 43: Diameter of innermost surface 5: Foldable display 51: After film 52: Cover window (surface protection film)
53: Polarizing plate/anti-reflection member 54: Touch panel module 55: Organic EL module 56: Back protective film 6: Outward bending foldable smartphone

Claims (8)

A polyester film with a thickness of 10 to 125 μm, a high temperature hold angle in the bending direction of 85° or more, a refractive index of the polyester film in the bending direction of 1.610 to 1.710, and a direction perpendicular to the bending direction. A polyester film for foldable displays having a refractive index of 1.750 to 1.870.
Here, the high-temperature hold angle is the angle formed by the folds that occur after the polyester film is fixed for 18 hours under heating at 85°C so that a strain of 1.7% is generated on both surfaces of the bent portion. refers to The polyester film for foldable displays according to claim 1, which has an intrinsic viscosity of 0.50 to 0.80 dl/g. The polyester film for foldable displays according to claim 1, having a density of 1.353 g/cm ³ or more. The polyester film for foldable displays according to claim 1, wherein the polyester is polyethylene naphthalate. The polyester film for a foldable display according to claim 1, wherein the stretching ratio in the longitudinal direction of the polyester film is 1.1 to 2.5 times. The polyester film for a foldable display according to claim 1, which has an easily adhesive layer on at least one side of the polyester film. A foldable display, comprising the polyester film for foldable displays according to any one of claims 1 to 5, wherein the polyester film for foldable displays comprises a single continuous polyester film through the folded portion of the foldable display. A foldable display with a protective film on the back. A mobile terminal device comprising the foldable display according to claim 7.

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