Polyethylene terephthalate film, laminated film, and foldable display

Abstract

A purpose of the present invention is to provide a polyethylene terephthalate film for foldable displays and the like that have a sufficient elastic modulus in tension required of films for foldable displays and that has excellent flexural rigidity. Portable terminals or image display devices including, mounted therein, the polyethylene terephthalate film for foldable displays provide beautiful images, are highly functional, and have excellent convenience including portability and the like.　The polyethylene terephthalate film for foldable displays has an MD flexural rigidity Fa and a TD flexural rigidity Fb in terms of flexural rigidity for 15-μm thickness, the sum of Fa and Fb being 0.05 gf·cm2 / cm or greater, and has a longitudinal-direction elongation at break of 10% or higher and a width-direction elongation at break of 10% or higher. The poly ethylene terephthalate film can be folded along both a first bending direction and a second bending direction, which is orthogonal to the first bending direction.

Description

Polyethylene terephthalate film, laminated film, and foldable display

[0001] This invention relates to a polyethylene terephthalate film for foldable displays that has excellent flexibility in both the longitudinal and transverse directions.

[0002] As devices equipped with foldable displays become thinner, the polyethylene terephthalate film used in these displays also needs to be made thinner. However, thinning the polyethylene terephthalate film reduces its strength.

[0003] As a method for increasing the strength of polyethylene terephthalate film, for example, Patent Document 1 discloses a method for improving strength without impairing moldability by stretching the film in multiple stages in the longitudinal and width directions.

[0004] Japanese Patent Publication No. 2002-011786

[0005] The biaxially oriented polyethylene terephthalate film manufactured by the method described in Patent Document 1 suppresses the elongation deformation due to tension required for its application (magnetic tape), but it has the problem of having a low tensile modulus.

[0006] There was a need for a polyethylene terephthalate film for foldable displays that had sufficient elongation at break in both the longitudinal and width directions (directions perpendicular to the longitudinal and thickness directions), and that maintained high rigidity even when thinned.

[0007] The object of the present invention is to provide a polyethylene terephthalate film for foldable displays that has sufficient elongation at break and bending rigidity as a film for foldable displays in both the first bending direction and the second bending direction perpendicular to the first bending direction.

[0008] As a result of diligent research by the inventors of the present invention regarding polyethylene terephthalate film, they have found that by stretching polyester film multiple times at a high temperature below its melting point (Tm), the crystallinity can be increased while extending the internal molecular chains, thereby obtaining high elongation at break, high bending rigidity, and high bending resistance.

[0009] This disclosure typically includes the following aspects: [Section 1] The sum of the longitudinal bending stiffness Fa and the widthwise bending stiffness Fb, converted to a thickness of 15 μm, is 0.05 gf·cm ２[Item 2] A polyethylene terephthalate film for foldable displays that can be folded in both directions, a first bending direction and a second bending direction perpendicular to the first bending direction, wherein the elongation at break in the longitudinal direction and the elongation at break in the width direction are both 10% or more, and the elongation at break in the longitudinal direction and the width direction are both 10% or more. [Item 2] The polyethylene terephthalate film according to Item 1, wherein the size of the crystals formed by the polymer molecules constituting the film is 3.0 nm or more as determined by small-angle X-ray scattering. [Item 3] The polyethylene terephthalate film according to Item 1 or 2, wherein the transmittance of light at a wavelength of 380 nm is 20% or less and the b value is 3.5 or less. [Item 4] The polyethylene terephthalate film according to any one of Items 1 to 3, wherein the total light transmittance is 85% or more and the haze is 3% or less. [Item 5] The polyethylene terephthalate film according to any one of Items 1 to 4, characterized in that no cracks or breaks occur when a test of folding 180° so that the distance between opposing sides is 0.5 mm is repeated 200,000 times. [Item 6] A laminated film for a foldable display that can be folded in both a first bending direction and a second bending direction perpendicular to the first bending direction, comprising a base layer and an easy-adhesion layer, wherein the base layer is a polyethylene terephthalate film according to any one of items 1 to 5, and the easy-adhesion layer is laminated on at least one surface of the base layer. [Item 7] The laminated film according to item 6, wherein a hard coat layer is laminated on the easy-adhesion layer. [Item 8] A foldable display that can be folded in both a first bending direction and a second bending direction perpendicular to the first bending direction, comprising a polyethylene terephthalate film according to any one of items 1 to 5 as a surface protective film. [Item 9] A foldable display that can be folded in both a first bending direction and a second bending direction perpendicular to the first bending direction, comprising a laminated film according to item 6 or 7 as a surface protective film. [Item 10] A portable terminal device that can be folded in both a first bending direction and a second bending direction perpendicular to the first bending direction, comprising the foldable display according to item 8 or 9.

[0010] The polyethylene terephthalate film of the present invention has sufficient elongation at break and bending rigidity for use as a film for foldable displays in both the first bending direction and the second bending direction perpendicular to the first bending direction. Furthermore, because of its excellent elongation at break and bending rigidity, it maintains good stiffness even when thinned. A film with good stiffness can, for example, be a film that does not break when bent and maintains deformation due to bending. In addition, because the polyethylene terephthalate film of the present invention has a high elastic modulus, it can suppress the occurrence of deformation and the occurrence of scratches caused by foreign matter. For this reason, it is suitable as a film for surface protection films for foldable displays and other applications where high quality is required during processing. Furthermore, the polyethylene terephthalate film of the present invention can protect organic EL from ultraviolet rays. Moreover, in the present invention, it is possible to give the polyethylene terephthalate film itself UV absorption performance, and compared to coating the surface of the polyethylene terephthalate film with a UV absorber, the layer structure does not increase and the light transmittance of visible light can be maintained. Furthermore, because the polyethylene terephthalate film of the present invention has high elongation at break in both the longitudinal and width directions, when used in a foldable display having a structure that folds in a first bending direction (e.g., the longitudinal direction) and a structure that folds in a second bending direction perpendicular to the first bending direction (e.g., the transverse direction), it can suppress cracking and rupture due to repeated bending in both the first bending direction and the second bending direction perpendicular to the first bending direction.

[0011] Figure 1 is a schematic diagram illustrating a method for measuring bending stiffness using a pure bending tester. Figure 2 shows an example of a measurement curve used to illustrate a method for measuring crystal size obtained by small-angle X-ray scattering. Figure 3 is a schematic diagram illustrating the bending radius when the foldable display in the present invention is folded. Figure 4 is a schematic diagram illustrating the first bending direction of the polyester film constituting the surface protective film of the foldable display in the present invention. Figure 5 is a schematic diagram illustrating the second bending direction of the polyester film constituting the surface protective film of the foldable display in the present invention.

[0012] In this specification, the phrase "contains" is used to include the phrases "essentially consist of" and "consist of." In this specification, the phrase "PET" is used to mean "polyethylene terephthalate."

[0013] The numerical ranges described herein may be arbitrarily combined with the upper or lower limits of other numerical ranges of the same type. Furthermore, in the numerical ranges described herein, the upper or lower limits of the numerical range may be replaced with values ​​shown in the examples or values ​​that can be uniquely derived from the examples.

[0014] In this specification, with respect to numerical ranges, "~" means greater than or equal to the leftmost number and less than or equal to the rightmost number. For example, both "1~10 mass%" and "1 mass%~10 mass%" mean "1 mass% or more and 10 mass% or less."

[0015] In this specification, with respect to numerical ranges, "greater than or equal to" means "the same as or greater than," and "less than or equal to" means "the same as or less than." For example, "1% by mass or more" is synonymous with "1% by mass or greater than 1% by mass."

[0016] (Display) In the present invention, the term "display" can encompass all types of display devices. Examples of display types include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs, but flexible LCDs, organic EL displays, or inorganic EL displays are preferred. Flexible organic EL displays and inorganic EL displays are even more preferred because they reduce the number of layers required for the display, and flexible organic EL displays are even more preferred because they offer a wide color gamut.

[0017] (Foldable Display) A foldable display may encompass a single continuous display that can be folded in half or in other ways when necessary, such as when carrying the device. Folding it reduces the size of the display by half, improving the portability of the terminal equipped with this display. The bending radius of the foldable display is preferably 5 mm or less, and more preferably 3 mm or less. A bending radius of 5 mm or less allows for a thinner design when folded. A smaller bending radius is better, but a smaller bending radius makes it easier for creases to form. A bending radius of 0.1 mm or more is preferred, but it may also be 0.25 mm or more, 0.5 mm or more, or 1 mm or more. Even with a bending radius of 1 mm, a sufficiently thin design can be achieved when folded. The bending radius is measured at the location indicated by reference numeral 11 in the schematic diagram of Figure 3, and represents the inner radius of the folded portion. The surface protection film, described later, may be located on the outer surface or the inner surface of the folded display. Furthermore, the foldable display may be a tri-fold type, a quad-fold type, or even a rollable type. The foldable display may also be a foldable display that can be folded in both directions: a first bending direction (e.g., vertical direction) and a second bending direction perpendicular to the first bending direction (e.g., horizontal direction). For example, a foldable display having a structure that folds in the first bending direction (e.g., vertical direction) and a structure that folds in the second bending direction perpendicular to the first bending direction (e.g., horizontal direction). All of these are included in the definition of a foldable display as defined in this invention. The polyethylene terephthalate film and laminated film for foldable displays of this invention may be used in any part of the foldable display. Below, using an organic EL display as an example, a typical configuration of a foldable display and the parts in which the polyethylene terephthalate film and laminated film of this invention may be used will be described.In this specification, the polyethylene terephthalate film for foldable displays of the present invention may be simply referred to as a polyester film, and the laminated film for foldable displays of the present invention may be simply referred to as a laminated film.

[0018] (Foldable Organic EL Display) A foldable organic EL display includes an organic EL module as an essential component, and may also include a circular polarizer, touch panel module, surface protective film, back protective film, etc., as needed.

[0019] (Organic EL Module) The general configuration of an organic EL module consists of electrodes, an electron transport layer, an emissive layer, a hole transport layer, and a transparent electrode. The polyester film and laminated film of the present invention can be used as a substrate for which electrodes are provided, and further for which an electron transport layer, an emissive layer, a hole transport layer, and a transparent electrode are provided. The polyester film and laminated film of the present invention can be used particularly as a substrate for the transparent electrode. Since the substrate film is required to have high barrier properties against water vapor and oxygen, it is preferable that the polyester film and laminated film of the present invention be provided with a barrier layer such as a metal oxide layer. To improve barrier properties, multiple barrier layers may be provided in the laminated film. Multiple polyester films with barrier layers may be used as the substrate film.

[0020] (Protective film for organic EL module) It is preferable that a protective film be provided on the visible side of the organic EL module. It is also preferable that a protective film be provided on the non-visible side of the organic EL module. Organic EL modules are generally formed on a glass substrate. The glass substrate is a component that supports and protects the organic EL module. However, since glass substrates cannot be bent, they cannot be used in flexible displays such as foldable displays, and a foldable film is used as a protective component instead. The polyester film and laminated film of the present invention can be used as protective films for organic EL modules. A hard coat layer may be provided on the protective film to prevent scratches.

[0021] (Touch Panel Module) Mobile terminal devices preferably have a touch panel. In the case of an organic EL display, the touch panel module is preferably located on top of the organic EL display or between the organic EL module and the circular polarizer. The touch panel module may have a transparent substrate such as a film and transparent electrodes placed on it. The polyester film and laminated film of the present invention can be used as this transparent substrate. When the polyester film and laminated film of the present invention are used as transparent substrates included in a touch panel module, it is preferable to provide the polyester film and laminated film with a hard coat layer, a refractive index adjustment layer, etc.

[0022] (Circular Polarizer) A circular polarizer can suppress the degradation of image quality caused by the reflection of external light by internal components of the display. A circular polarizer may have a linear polarizer and a phase difference plate. A linear polarizer may have a protective film on at least the viewing side of the polarizer. A linear polarizer may also have a protective film on the side opposite to the viewing side of the polarizer, or a phase difference plate may be directly laminated on the polarizer. The phase difference plate may be a film in which a phase difference layer made of a liquid crystal compound is provided on a resin film having a phase difference. The polyester film of the present invention can be used as a polarizer protective film, a resin film for a phase difference plate, etc. In these cases, it is preferable that the slow phase axis direction of the polyester film in the laminated film of the present invention is 45 degrees parallel or perpendicular to the absorption axis direction of the polarizer. A range of up to 10 degrees, preferably 5 degrees, from this 45 degrees parallel or perpendicular (also called a deviation) is acceptable.

[0023] (Surface protection film) When an impact is applied to the display from above, there is a risk that the circuits of the organic EL module and the touch panel module may be disconnected. Therefore, in many cases, a surface protection film is provided on the display. The polyester film and the laminated film of the present invention can be used as this surface protection film. The surface protection film includes those called cover windows incorporated on the outermost surface of the display, and those called after-films that can be self-attached, peeled off, and replaced by the user. Both the polyester film and the laminated film of the present invention can be used. When the polyester film and the laminated film of the present invention are used as the surface protection film, the laminated film with a hard coat layer can be provided on the surface of the foldable display such that the side with the hard coat layer laminated thereon faces the viewing side of the display. Note that the hard coat layer may be provided on both sides of the laminated film. The polyester film and the laminated film of the present invention are preferably used as the surface protection film for the organic EL module of the foldable display.

[0024] The polyester film of the present invention contains polyethylene terephthalate resin as the main resin component. The content ratio of the polyethylene terephthalate resin in the polyester film of the present invention may be 70 to 100% by mass, 80 to 100% by mass, etc., preferably 90 to 100% by mass, and more preferably 95 to 100% by mass.

[0025] The polyethylene terephthalate resin can be a polymer containing ethylene glycol and terephthalic acid as main constituent components. Specifically, the polyethylene terephthalate resin can be a resin in which 80 mol% or more of the repeating units are composed of ethylene terephthalate. In the polyethylene terephthalate resin, other dicarboxylic acid components and glycol components may be copolymerized within a range that does not inhibit the object of the present invention. Examples of the above other dicarboxylic acid components include 2,6-naphthalenedicarboxylic acid and the like. Examples of the above other glycol components include propylene glycol, butanediol, neopentyl glycol, and the like.

[0026] Polyethylene terephthalate resin can be produced, for example, by any production method such as the direct polymerization method, the transesterification method, or the like.

[0027] The intrinsic viscosity of the resin pellets as a raw material for the polyethylene terephthalate resin is preferably in the range of 0.57 dl / g to 0.7 dl / g, and more preferably in the range of 0.58 dl / g to 0.65 dl / g. The intrinsic viscosity of the polyethylene terephthalate film is preferably in the range of 0.55 dl / g to 0.69 dl / g, and more preferably in the range of 0.58 dl / g to 0.65 dl / g.

[0028] In addition to the polyethylene terephthalate resin, the polyester film of the present invention may contain various additives as needed. Examples of the additives include inorganic lubricants such as titanium dioxide, fine particle silica, kaolin, calcium carbonate, and organic lubricants such as acrylic acid and methacrylic acid. Other additives include stabilizers, colorants, antioxidants, defoaming agents, antistatic agents, ultraviolet absorbers, and the like.

[0029] Further, for the purpose of suppressing the deterioration of optical functional dyes such as iodine-based dyes, the film of the present invention preferably has a light transmittance of 20% or less at a wavelength of 380 nm. The light transmittance at a wavelength of 380 nm is more preferably 17% or less, even more preferably 16% or less, and particularly preferably 15% or less. If the light transmittance is 20% or less, the alteration of the optical functional dye by ultraviolet rays can be suppressed. From the viewpoint of production cost, the light transmittance at a wavelength of 380 nm is preferably at least 0.1%.

[0030] Here, when the film of the present invention (meaning polyethylene terephthalate film and laminated film; the same applies hereinafter) is used as a surface protective film for a foldable display, it is preferable from the viewpoint of visibility that it be colorless and transparent (i.e., has low absorption in the visible light region). However, if an ultraviolet absorber having strong absorption around a wavelength of 380 nm is used, the surface protective film may turn yellow. Therefore, the light transmittance at a wavelength of 380 nm is more preferably 0.2% or more, even more preferably 0.3% or more, and particularly preferably 0.4% or more. The light transmittance of the film of the present invention at a wavelength of 380 nm can be 0.1-20%, 0.1-17%, 0.1-16%, 0.1-15%, 0.2-20%, 0.2-17%, 0.2-16%, 0.2-15%, 0.3-20%, 0.3-17%, 0.3-16%, 0.3-15%, 0.4-20%, 0.4-17%, 0.4-16%, or 0.4-15%.

[0031] The light transmittance at a wavelength of 380 nm in the film of the present invention is measured perpendicular to the plane of the film and can be measured using a spectrophotometer (for example, a Shimadzu UV1800). Examples of ultraviolet absorbers include triazine-based ultraviolet absorbers, benzotriazole-based ultraviolet absorbers, benzophenone-based ultraviolet absorbers, and benzoxazinon-based ultraviolet absorbers. An example of such an ultraviolet absorber is 2,2'-(1,4-phenylene)bis(4H-3,1-benzoxazine-4-one).

[0032] The above-mentioned UV absorbers are divided into UV absorbers with a molecular weight of less than 1,000 (hereinafter also called "low-molecular-weight UV absorbers") and UV absorbers with a weight-average molecular weight of 1,000 or more (hereinafter also called "high-molecular-weight UV absorbers"). Low-molecular-weight UV absorbers have high UV absorption capacity, so relatively small amounts are sufficient, but many of them have strong absorption in the visible light region, especially around wavelengths of 390-420 nm, which can easily cause surface protective films to turn yellow. In contrast, high-molecular-weight UV absorbers cause less discoloration, but their UV absorption capacity is lower than that of low-molecular-weight UV absorbers, and adding large amounts to compensate for this can lead to a decrease in film hardness.

[0033] The concentration of the ultraviolet absorber in polyethylene terephthalate film is preferably, for example, 0.1% by mass or more, and more preferably 1 to 8% by mass, based on 100% by mass of the film. When it is 0.1% by mass or more, a sufficient ultraviolet absorption effect can be obtained. The concentration of the ultraviolet absorber is preferably 0.1 to 8% by mass, more preferably 0.1 to 6% by mass, more preferably 0.1 to 5% by mass, more preferably 0.1 to 4% by mass, and particularly preferably 0.1 to 3% by mass, based on 100% by mass of the film.

[0034] Particles can be incorporated into polyethylene terephthalate film. Known methods can be used to incorporate particles into the film.

[0035] The total light transmittance of polyethylene terephthalate film is preferably 85% or higher, and more preferably 87% or higher. Similarly, for laminated films in which an easy-adhesion layer and a hard coat layer are provided on the surface of the polyethylene terephthalate film, the total light transmittance is preferably 85% or higher, and more preferably 87% or higher. A transmittance of 85% or higher is sufficient to ensure adequate visibility. While a higher total light transmittance is preferable, from the standpoint of manufacturing costs, it is acceptable to have a transmittance of 99% or less, or even 97% or less. The total light transmittance of polyethylene terephthalate film and laminated film may be 85-99%, 85-97%, 87-99%, 87-97%, etc.

[0036] The polyethylene terephthalate film of the present invention may contain an ultraviolet absorber. However, it is preferable that the b value of the polyethylene terephthalate film, which indicates yellow-blue tint, is near zero. The b value is preferably between -1.0 and 3.5, and more preferably between -0.5 and 3.0. In particular, when the polyethylene terephthalate film is used as a surface protection film for foldable displays, it is preferable that the b value is between -0.5 and 3.0. A b value of 3.5 or less prevents the film from appearing yellowish, and is preferable because it avoids problems such as deterioration of image quality or discoloration when applied to a film for foldable displays. Furthermore, a b value of -1.0 or higher is preferable because it prevents the film from appearing blue, thus preventing it from giving the user a dark impression. The b value can be determined in accordance with JIS K 7373-2006, and is preferably determined by the method described in the examples.

[0037] (Method for manufacturing stretched film) The polyethylene terephthalate film of the present invention is preferably an oriented film, and more preferably a biaxially oriented film, from the viewpoint of mechanical strength, chemical resistance, heat resistance, etc.

[0038] Polyethylene terephthalate resin, with crosslinking agents or other additives added as needed, can be processed into unstretched sheets by various methods and then obtained by biaxial stretching. Methods for manufacturing the unstretched sheets include solution casting and melt extrusion.

[0039] The melting temperature of the resin composition in the present invention is preferably 280 to 300°C, and more preferably 280°C. When the melting temperature is within this range, the decrease in resin molecular weight is suppressed, it melts sufficiently, the increase in melt viscosity is suppressed, and subsequent extrusion becomes easier.

[0040] The die temperature for melt extrusion is the same as described above, but 280 to 300°C is preferred, and more preferably 280°C. When the die temperature during melt extrusion is 280°C or higher, the melt viscosity falls within a suitable range, allowing for stable extrusion. By keeping the temperature below 300°C, thermal decomposition of the resin can be suppressed.

[0041] Next, the unstretched film obtained by the above method is stretched biaxially in the width direction (also called TD) and the length direction (also called MD), either sequentially or simultaneously. Then, it is essential to stretch it multiple times in the width direction and / or length direction (multi-stage stretching) at a temperature in the range of 100°C to 250°C.

[0042] In conventional sequential biaxial stretching methods, in order to suppress thickness variations in the width direction or to obtain dimensional stability, it was preferable to set the stretching temperature in both the longitudinal and width directions to a range of 10°C higher (Tg + 10°C) to 50°C higher (Tg + 50°C) than the glass transition temperature of polyethylene terephthalate resin, and to set the stretching ratio in both the longitudinal and width directions to a range of 2 to 5 times. After stretching, in order to reduce the thermal shrinkage rate of the polyester film, it is recommended to perform a heat setting treatment (heat setting process) within 30 seconds, preferably within 15 seconds, and to perform a longitudinal relaxation treatment of 0.5 to 10%, a transverse relaxation treatment, etc.

[0043] However, conventional stretching temperatures induce film breakage due to insufficient heat during high-magnification stretching. Therefore, in the present invention, for example, a uniaxially oriented film stretched longitudinally at (Tg-10°C) to (Tg+50°C) is stretched at least once in the width direction at (Tg+10°C) to (Tg+50°C) to orient the molecular chains to some extent in the width direction, and then stretched at least once at an even higher temperature of (Tg+60°C) to (Tg+180°C). In this specification, Tg refers to the glass transition temperature of polyethylene terephthalate resin. By such multi-stage stretching, the present invention can suppress the frequency of breakage during high-magnification stretching and further obtain the rigidity of the film. The total stretching ratio in the multi-stage stretched direction can be 9.0 times or less, preferably 1.1 to 9.0 times, more preferably 3.0 to 9.0 times, and even more preferably 6.0 to 9.0 times. By keeping the total stretching ratio in the multi-stage stretching direction within the aforementioned range, the frequency of film breakage is reduced, which is advantageous.

[0044] (1) Sequential biaxial stretching In the case of sequential biaxial stretching, an unstretched film is stretched longitudinally using the difference in speed of the rolls at a temperature above the glass transition temperature (Tg; 78°C) of polyethylene terephthalate resin to produce a uniaxially stretched film. For example, a sheet is heated and stretched to 1.1 to 6 times its original length between two or more rolls with different peripheral speeds. The heating method at this time may be a method using a heating roll, a method using a non-contact heating medium, or a combination of both. In this case, it is preferable to set the film temperature to, for example, 65 to 130°C, preferably (Tg - 10°C) to (Tg + 50°C).

[0045] Next, the uniaxially oriented film is stretched in multiple stages in the width direction using a tenter. Specifically, for example, when stretching in two stages, the first stage film temperature is set to, for example, 85 to 130°C, preferably (Tg + 10°C) to (Tg + 50°C), and the film is stretched 1.1 to 4.0 times in the width direction. Then, for the second stage, the film temperature is set to, for example, 135 to 260°C, preferably (Tg + 60°C) to (Tg + 180°C), and the film is stretched 1.1 to 3.0 times in the width direction. When stretching in two stages, the total stretching ratio in the width direction can be 9.0 times or less, preferably 1.1 to 9.0 times, more preferably 3.0 to 9.0 times, and even more preferably 6.0 to 9.0 times. Keeping the total stretching ratio in the multi-stage stretched direction within the above range is advantageous in that it reduces the frequency of film breakage.

[0046] For example, when stretching in three stages, in the first stage of stretching, the film temperature is set to, for example, 85 to 120°C, preferably (Tg + 10°C) to (Tg + 40°C), and the film is stretched by 1.1 to 3.0 times in the width direction. Next, in the second stage of stretching, the film temperature is set to, for example, 125 to 190°C, preferably (Tg + 50°C) to (Tg + 110°C), and it is preferable to stretch the film by 1.1 to 3.0 times in the width direction. Next, in the third stage of stretching, the film temperature is set to, for example, 195 to 260°C, preferably (Tg + 120°C) to (Tg + 180°C), and it is preferable to stretch the film by 1.1 to 3.0 times in the width direction. When stretching in three stages, the total stretching ratio in the width direction can be 9.0 times or less, preferably 1.1 to 9.0 times, more preferably 3.0 to 9.0 times, and even more preferably 6.0 to 9.0 times. By keeping the total stretching ratio in the multi-stage stretching direction within the aforementioned range, it is advantageous in that the frequency of film breakage is reduced.

[0047] For example, when stretching in four stages, in the first stage of stretching, it is preferable to set the film temperature to, for example, 85 to 130°C, preferably (Tg + 10°C) to (Tg + 50°C), and stretch the film by 1.1 to 3.0 times in the width direction. Next, in the second stage of stretching, it is preferable to set the film temperature to, for example, 125 to 150°C, preferably (Tg + 50°C) to (Tg + 70°C), and stretch the film by 1.1 to 3.0 times in the width direction. Next, in the third stage of stretching, it is preferable to set the film temperature to, for example, 155 to 190°C, preferably (Tg + 80°C) to (Tg + 110°C), and stretch the film by 1.1 to 3.0 times in the width direction. Next, in the fourth stretching stage, the film temperature is preferably set to 195 to 260°C, more preferably (Tg + 120°C) to (Tg + 180°C), and more preferably (Tg + 140°C) to (Tg + 180°C), and the film is stretched to 1.1 to 3.0 times its original width. When stretching in four stages, the total stretching ratio in the width direction can be 9.0 times or less, preferably 1.1 to 9.0 times, more preferably 3.0 to 9.0 times, and even more preferably 6.0 to 9.0 times. Keeping the total stretching ratio in the multi-stage stretched direction within the above range is advantageous in that it reduces the frequency of film breakage.

[0048] After multi-stage stretching, it is preferable to heat-treat (heat-fix) the film by passing it through a heat treatment zone adjusted to (Tg + 10°C) to (Tg + 50°C) (for example, by passing the film in the longitudinal direction). The heat-fixing treatment time (the time the film passes through the heat treatment zone) is, for example, 1 to 30 seconds, preferably 8 to 15 seconds. By rapidly cooling, it is possible to suppress the relaxation of the molecular chains in the width direction, which have been tensed to the extreme by the final stretching, due to cooling. The heat-fixing temperature may be 90°C to 130°C, and more preferably 90 to 110°C. The heat-fixing treatment improves the flatness of the film and suppresses the decrease in strength due to cooling. Furthermore, in this heat-fixing treatment, it is also preferable to apply a relaxation treatment in the longitudinal and / or width direction by, for example, 0.5 to 7.0%, preferably 0.5 to 6.5%. Applying the relaxation treatment is advantageous from the viewpoint of improving the planar stability of the film or from the viewpoint of preventing film shrinkage during storage or processing.

[0049] (2) Simultaneous biaxial stretching When increasing the rigidity in the longitudinal direction by the simultaneous biaxial stretching method, the following control is preferable. For example, the unstretched film is held in clips and the film temperature is set in a heating furnace to, for example, 85 to 130°C, preferably (Tg + 10°C) to (Tg + 50°C), and the film is simultaneously stretched by 1.1 to 4.0 times in the longitudinal direction and by 1.1 to 2.0 times in the width direction. Then, it is preferable to stretch it again in the longitudinal direction by 1.1 to 1.5 times at, for example, 225 to 260°C, preferably (Tg + 150°C) to (Tg + 180°C). In the case of two-stage stretching, it is preferable to keep the total stretching ratio in the multi-stage stretched direction to 8.0 times or less, as this reduces the frequency of breakage.

[0050] (Physical properties of the film) The thickness of the film of the present invention is preferably 2 to 500 μm, more preferably 10 to 400 μm, even more preferably 10 to 250 μm, and even more preferably 20 to 100 μm. When the film thickness is 2 μm or more, it is advantageous in that the film has sufficient rigidity, is easy to handle, and can protect foldable display components such as organic EL modules and touch panels. When the film thickness is 500 μm or less, it is advantageous in that the film can be transported on multiple rolls, the manufactured film is easy to handle, and the occurrence of cracks due to folding is suppressed.

[0051] The tensile strength of the film of the present invention may be 100 MPa or more, 200 MPa or more, or 350 MPa or more in both the longitudinal and width directions. The tensile strength in one direction may be 100 to 1000 MPa, 250 to 1000 MPa, 300 to 1000 MPa, or 350 to 1000 MPa, with 250 to 600 MPa, 300 to 600 MPa, or 350 to 600 MPa being preferred, and 250 to 500 MPa, 300 to 500 MPa, or 350 to 500 MPa being more preferred. The tensile strength in the other direction may be 100 MPa or more, or 130 MPa, with 100 to 300 MPa being preferred, and 130 to 250 MPa being more preferred. When the breaking strength is within the aforementioned range, it is preferable because the breaking strength is high in both the longitudinal and width directions, and therefore, when laminating the polyester film during the processing of the foldable display, problems such as stretching and misalignment of the film are suppressed regardless of the direction of tension applied.

[0052] The elongation at break of the film of the present invention is preferably 10% or more in both the longitudinal and width directions, more preferably 20% or more, more preferably 30% or more, more preferably 40% or more, more preferably 50% or more, even more preferably 100% or more, and even more preferably 200% or more. It is preferable that the elongation at break in both directions is above the above lower limit because the high elongation at break in both the longitudinal and width directions suppresses cracking and breaking of the film when it is bent as a foldable display. The elongation at break in both directions may be 400% or less and 300% or less. The elongation at break in one direction (longitudinal or width direction) may be 20-100%, 30-100%, 40-100%, etc., and the elongation at break in the other direction (width direction or longitudinal direction) may be 100-400%, 100-300%, 200-400%, 200-300%, etc.

[0053] The sum of the tensile modulus Ea in the longitudinal direction (MD) and the tensile modulus Eb in the width direction (TD) of the film of the present invention may be 5.0 GPa or more. A preferred lower limit for the sum of the tensile moduli is 5.0 GPa, a more preferred lower limit is 7.0 GPa, an even more preferred lower limit is 8.0 GPa, and an even more preferred lower limit is 10.0 GPa. A preferred upper limit for the sum of the tensile moduli is 20.0 GPa, a more preferred upper limit is 15.0 GPa, and an even more preferred upper limit is 10.0 GPa. These lower and upper limits can be appropriately combined to form a numerical range. The sum of the tensile moduli can be 5.0–10.0 GPa, 5.0–15.0 GPa, 5.0–20.0 GPa, 7.0–10.0 GPa, 7.0–15.0 GPa, 7.0–20.0 GPa, 8.0–10.0 GPa, 8.0–15.0 GPa, 8.0–20.0 GPa, 10.0–15.0 GPa, or 10.0–20.0 GPa. When the sum of the tensile moduli is equal to or greater than the lower limit, the rigidity of the film is sufficient, preventing wrinkles and warping, allowing for sufficient stiffness even in thin films, and suppressing scratches and deformation caused by foreign matter.

[0054] The sum of the longitudinal bending stiffness Fa and the widthwise bending stiffness Fb, converted to a film thickness of 15 μm, is 0.05 gf·cm. ２ Preferably, it is 0.06 gf·cm or more. More preferably, 0.06 gf·cm.２ / cm or more, more preferably 0.07 gf·cm ２ / cm or more. The preferable lower limit of the sum of the flexural rigidities is 0.05 gf·cm ２ / cm, a more preferable lower limit is 0.06 gf·cm ２ / cm, and a further preferable lower limit is 0.07 gf·cm ２ / cm. The preferable upper limit of the sum of the flexural rigidities is 0.10 gf·cm ２ / cm, a more preferable upper limit is 0.09 gf·cm ２ / cm, and a further preferable upper limit is 0.08 gf·cm ２ / cm. When the sum of the flexural rigidities is not less than the lower limit value, the film has sufficient stiffness, and it is preferable because it can suppress the occurrence of wrinkles and warpage when the film thickness is reduced from the conventional film thickness. Also, since it has excellent flexural rigidity, it can suppress deformation of the film due to pressing by foreign matter or fingers even after being mounted on a foldable display. These lower and upper limits can be appropriately combined to form a numerical range. The sum of the flexural rigidities is 0.05 - 0.10 gf·cm ２ / cm, 0.06 - 0.10 gf·cm ２ / cm, 0.07 - 0.10 gf·cm ２ / cm, 0.05 - 0.09 gf·cm ２ / cm, 0.06 - 0.09 gf·cm ２ / cm, 0.07 - 0.09 gf·cm ２ / cm, 0.05 - 0.08 gf·cm ２ / cm, 0.06 - 0.08 gf·cm ２ / cm, 0.07 - 0.08 gf·cm ２ / cm can be possible.

[0055] The total light transmittance of the film of the present invention is preferably 85% or higher, and more preferably 87% or higher. A transmittance of 85% or higher is sufficient to ensure the visibility of the foldable display. While a higher total light transmittance of the polyester film is desirable, from the standpoint of manufacturing cost and stable production, it is preferably 99% or lower, and may also be 97% or lower. These lower and upper limits can be appropriately combined to form a numerical range. The total light transmittance of the film of the present invention may be 85-97%, 85-99%, 87-97%, or 87-99%.

[0056] The surface of the polyethylene terephthalate film of the present invention may be smooth or uneven, but since it is used as a surface protection film, a decrease in optical properties due to unevenness is undesirable. The haze of the film of the present invention is preferably 3.5% or less, more preferably 3% or less, and most preferably 2% or less. If the haze of the film of the present invention is 3.5% or less, the visibility of the image can be improved. The lower limit of the haze is better, but from the viewpoint of stable production, it is preferably 0.1% or more, and may be 0.3% or more. These lower and upper limits can be appropriately combined to form a numerical range. The haze of the film of the present invention may be 0.1 to 3.5%, 0.1 to 3%, 0.1 to 2%, 0.3 to 3.5%, 0.3 to 3%, or 0.3 to 2%.

[0057] When the film of the present invention is heated at 150°C for 30 minutes, the thermal shrinkage rate is preferably 20% or less in both the longitudinal and width directions, more preferably 10% or less in both directions, and even more preferably 7% or less. Having the thermal shrinkage rate within these ranges is advantageous in that it suppresses appearance defects due to deformation of the film under high heat during post-processing, and suppresses flatness defects such as curling and waviness caused by heating during processing when mounting on a foldable display. While a low thermal shrinkage rate is preferred, from a manufacturing standpoint, the lower limit is preferably 0.01%. These lower and upper limits can be appropriately combined to form a numerical range. The thermal shrinkage rate can be 0.01 to 20%, 0.01 to 10%, or 0.01 to 7%.

[0058] The refractive index Nx in the longitudinal direction and the refractive index Ny in the width direction of the film of the present invention may be 1.50 or higher, preferably 1.60 or higher, more preferably 1.61 or higher, and even more preferably 1.62 or higher. A higher refractive index is preferable because it allows the molecular chains to be aligned in the stretching direction, resulting in sufficient mechanical strength and suppressing defects such as elongation and displacement during processing. Furthermore, because the mechanical strength is high in both the longitudinal and width directions even after mounting on a foldable display, cracks can be suppressed in the folded part regardless of the direction of folding, and the visibility of the display can be maintained well. From a manufacturing standpoint, the upper limit is preferably 1.75. These lower and upper limits can be appropriately combined to form a numerical range.

[0059] The refractive index Nx in the longitudinal direction is preferably 1.500 or higher, more preferably 1.600 or higher, and even more preferably 1.620 or higher. The refractive index Ny in the width direction is preferably 1.600 or higher, more preferably 1.610 or higher. The refractive index Nz in the thickness direction is preferably 1.600 or lower, and more preferably 1.550 or lower. It is preferable that the refractive index in the longitudinal and width directions is high and the refractive index in the thickness direction is low, as this allows the molecular chains to be aligned in the stretching direction, resulting in sufficient mechanical strength and suppressing defects such as elongation and displacement during the processing. Furthermore, because the mechanical strength is high even after mounting the foldable display, it is possible to suppress crack formation in the bent part regardless of the direction in which it is bent, and the visibility of the display can be maintained well. From a manufacturing standpoint, the upper limit of the refractive index in the longitudinal and width directions is preferably 1.750. The lower limit of the refractive index Nz in the thickness direction is preferably 1.400. These lower and upper limits can be appropriately combined to form a numerical range. The refractive index Nx in the longitudinal direction can be 1.500 to 1.750, 1.600 to 1.750, or 1.620 to 1.750; the refractive index Ny in the width direction can be 1.600 to 1.750 or 1.610 to 1.750; and the refractive index Nz in the thickness direction can be 1.400 to 1.600 or 1.400 to 1.550.

[0060] The minimum bending radius at which deformation or cracking does not occur in the bent portion when the film of the present invention is continuously folded is preferably 5.0 mm, and more preferably 3.0 mm. The smaller the minimum bending diameter at which deformation or cracking does not occur, the less likely deformation or cracking will occur even after mounting a foldable display, and the better the visibility of the display can be maintained.

[0061] The minimum bending radius at which cracks or breaks do not occur when the film of the present invention is continuously folded is preferably 0.5 mm, and more preferably 0.25 mm. The smaller the minimum bending diameter at which cracks or breaks do not occur, the less likely cracks or breaks are to occur even after mounting on a foldable display, and the better the durability of the display can be maintained.

[0062] The long period of the film of the present invention, as analyzed by small-angle X-ray scattering, is preferably 10 nm or more, more preferably 12 nm or more, and even more preferably 15 nm or more. A larger long period is preferable because it allows the molecular chains connecting the amorphous and crystalline structures of the higher-order structure to elongate, resulting in sufficient mechanical strength and suppressing defects such as elongation and displacement during processing. From a manufacturing standpoint, the upper limit is preferably 20 nm. These lower and upper limits can be appropriately combined to form a numerical range. The long period can be 10-20 nm, 12-20 nm, or 15-20 nm.

[0063] Regarding the crystals formed by the polymer molecules constituting the polyethylene terephthalate film, the crystal size of the film analyzed by small-angle X-ray scattering is preferably 3 nm or larger, more preferably 4 nm or larger, and even more preferably 5 nm or larger. A larger crystal size is preferable because it allows for a larger crystal structure in the higher-order structure, suppressing relaxation of the higher-order structure during film heating, resulting in sufficient thermal dimensional stability and suppressing deformation of the film under high temperatures during post-processing. From a manufacturing standpoint, an upper limit of 6 nm is preferred. These lower and upper limits can be appropriately combined to form a numerical range. The crystal size can be 3-6 nm, 4-6 nm, or 5-6 nm.

[0064] (Laminated film for foldable displays) The laminated film of the present invention comprises a base layer and an easy-adhesion layer. The base layer is preferably the polyethylene terephthalate film of the present invention.

[0065] (Easy-Adhesion Layer) An adhesion-enhancing layer, such as an easy-adhesion layer, may be formed on at least one surface of the polyethylene terephthalate film (base layer) of the present invention. By interposing this layer, the adhesion between the polyethylene terephthalate film and other layers is improved. The easy-adhesion layer can be obtained by applying the coating liquid for forming the easy-adhesion layer, described later, to one or both sides of an unstretched film or a uniaxially oriented film stretched in the longitudinal direction, drying it at 75 to 150°C, and then stretching it in one or two directions. The final amount of easy-adhesion layer applied is 0.05 to 0.20 g / m². ２ It is preferable to manage the amount to 0.05 g / m². ２ The above conditions are preferable as they satisfy the adhesive requirements. On the other hand, a coating amount of 0.20 g / m² is preferable. ２ The following conditions are preferable as they provide resistance to blocking.

[0066] The resin to be included in the coating solution used to form the easy-adhesion layer can be any, without particular limitation, such as polyester resins, polyurethane resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc. Examples of crosslinking agents to be included in the coating solution for forming the easy-adhesion layer include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, and carbodiimide compounds. Two or more of these resins or crosslinking agents can also be used in mixture form. In forming the easy-adhesion layer, due to the properties of inline coating, it is preferable to apply the coating with a water-based coating solution. Therefore, the resins and crosslinking agents are preferably water-soluble or water-dispersible resins or compounds.

[0067] It is preferable to add particles to the easy-adhesion layer to impart slipperiness. The average particle size is preferably 1.1 to 5.0 times the thickness of the easy-adhesion layer. When the average particle size is within this range, it is advantageous in that slipperiness is improved and the particles are less likely to fall off during handling. The thickness of the easy-adhesion layer is preferably 10 to 1000 nm, more preferably 20 to 500 nm, and even more preferably 50 to 200 nm.

[0068] Examples of particles to be included in the easy-adhesion layer include inorganic particles such as titanium dioxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, and calcium fluoride, as well as organic polymer particles such as styrene polymers, acrylic polymers, melamine polymers, benzoguanamine polymers, and silicone polymers.

[0069] One example of a method for applying a coating solution for forming an easily adhesive layer is the reverse roll coating method.

[0070] (Hard coat layer) In the laminated film of the present invention, it is preferable that a hard coat layer is laminated on an easily adhesive layer provided on the surface of the polyethylene terephthalate film (base layer).

[0071] The resin used to form the hard coat layer can be any resin without particular limitation, including (meth)acrylic acid ester-based, siloxane-based, inorganic hybrid-based, acrylate-based (urethane acrylate-based, polyester acrylate-based, etc.), and epoxy-based resins. A mixture of two or more resins can also be used. The hard coat layer may also contain particles such as inorganic fillers or organic fillers.

[0072] (Photopolymerization initiator) The hard coat layer of the present invention can be formed by curing the resin in the coating solution for hard coat formation. When curing using ultraviolet light, it is necessary to add a photopolymerization initiator to the coating solution.

[0073] (Film Thickness) The film thickness of the hard coat layer is preferably 1 to 55 μm. A thickness of 1 μm or more ensures sufficient curing and provides good pencil hardness. By limiting the thickness to 55 μm or less, curling of the film due to hard coat curing shrinkage is suppressed, improving the handling properties of the film.

[0074] (Pencil Hardness) The pencil hardness of the hard coat layer is preferably 2H or higher, more preferably 3H or higher, and particularly preferably 4H or higher. The pencil hardness of the hard coat layer may be 2H to 9H, 2H to 8H, 2H to 6H, 3H to 9H, 3H to 8H, or 3H to 6H.

[0075] (Types of hard coat layers) The hard coat layer in the present invention may have other functions added, 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 functions such as an anti-glare layer, anti-glare anti-reflective layer, anti-reflective layer, low-reflection layer, and anti-static layer having a certain pencil hardness as described above are also preferably applied in the present invention.

[0076] In another embodiment, the present invention is a foldable display comprising the polyethylene terephthalate film of the present invention or the laminated film of the present invention. Preferably, the polyethylene terephthalate film of the present invention and the laminated film of the present invention are included in this foldable display as a surface protective film.

[0077] Next, the effects of the present invention will be explained using examples and comparative examples. First, the evaluation method for characteristic values ​​used in the present invention is shown below.

[0078] (Evaluation Method) (1) Breaking Strength and Elongation at Breaking The breaking strength and elongation at breaking of polyethylene terephthalate film were measured in accordance with JIS-C-2318. Samples were cut into strips of 120 mm in length and 10 mm in width in the MD and TD directions of the film using a single-edged razor. Next, the breaking strength (MPa) and elongation at breaking (%) in each direction were determined from the load-strain curves obtained from the strip-shaped samples at a test speed of 5 mm / min and a chuck distance of 100 mm using an Autograph AG-IS-Loadcell.1KN manufactured by Shimadzu Corporation.

[0079] (2) Tensile modulus The tensile modulus of polyethylene terephthalate film was measured in accordance with JIS-K-7161 and ISO 527-1 and 527-3. Strips of film measuring 200 mm in length and 15 mm in width were cut from the longitudinal and width directions of the film using a single-edged razor, and two parallel gauge marks were made 50 mm apart in the center of the test piece. Then, using an Autograph AGS-IS-Loadcell. 1KN manufactured by Shimadzu Corporation, the tensile modulus (GPa) in each direction was determined from the load-strain curves obtained from 0.1-0.3% of the strip-shaped samples at a test speed of 0.5 mm / min, a chuck distance of 100 mm, and a gauge distance of 50 mm.

[0080] (3) Bending stiffness (calculated for a thickness of 15 μm) The bending stiffness of polyethylene terephthalate film is a constant curvature (maximum curvature K = ±2.5 cm) in accordance with the pure bending theory. －１ The bending moment (stiffness) was measured automatically while applying a pressure. The film was cut to a length of 100 mm in the stretching direction and 150 mm in the perpendicular direction. Next, the bending stiffness was calculated by performing automatic measurements using a pure bending tester (model number: KES-FB2A) manufactured by Kato Tech Co., Ltd. An overview of the pure bending tester is shown in Figure 1. For the evaluation method in all directions, the longitudinal direction was set to 0° and the width direction to 90°, and the bending stiffness at 5° intervals was determined according to the measurement method described above. The minimum value was taken as Fa and the maximum value as Fb. The bending stiffness converted to a thickness of 15 μm was obtained from the following formula: Bending stiffness measurement value (gf・cm) ２( / cm) × 15 (μm) / Thickness of the film being measured (μm) For example, in Example 4, the thickness of the film is 25 μm, and the bending stiffness obtained by the above measurement method is 0.080 gf·cm ２ Since the value is per cm, the bending stiffness converted to a thickness of 15 μm is calculated to be 0.048 from the formula 0.080 × 15 / 25.

[0081] (4) Total light transmittance and haze The total light transmittance and haze of polyethylene terephthalate film were measured using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS-7136.

[0082] (5) Flexural resistance (bending radius 3.0 mm) A polyethylene terephthalate film measuring 110 mm in width and 110 mm in length was prepared as a sample. This film was bent 200,000 times at a speed of 1 time / second using an unloaded U-shaped stretch tester (Yuasa System Equipment Co., Ltd., DLDMLH-FS) with a bending radius of 3.0 mm, so that the folds were parallel to the TD direction of the film (see Figures 3 and 4). Next, the same film, i.e., a polyethylene terephthalate film measuring 110 mm in width and 110 mm in length, was rotated 90 degrees to prepare a sample. This film was bent 200,000 times at a speed of 1 time / second using an unloaded U-shaped stretch tester (Yuasa System Equipment Co., Ltd., DLDMLH-FS) with a bending radius of 3.0 mm, so that the folds were parallel to the MD direction of the film (see Figure 5). For the above samples, after the bending process was completed, the samples were placed on a flat surface with the inner bent surface facing downwards and visually observed and evaluated according to the following criteria: ○: No cracks or deformation were observed in the sample. ×: Cracks or fold marks were present in the sample, or the maximum height of the lift from the flat surface was 5 mm or more.

[0083] (6) Heat shrinkage rate at 150°C The heat shrinkage rate at 150°C was measured in accordance with JIS-2318-1997-5.3.4 (dimensional change). When the measurement direction was the width direction (TD), a polyethylene terephthalate film with dimensions of 190 mm in the width direction and 10 mm in the length direction was prepared as a sample. Next, two gauge marks were made on the surface of this film at intervals of 50 mm to the left and right in the width direction from the center (100 mm between the gauge marks), and the distance between the gauge marks (A) was measured. Next, the film was placed in an oven in a 150°C atmosphere and heated at 150°C ± 3°C for 30 minutes under no load, and then the distance between the gauge marks (B) was measured. The heat shrinkage rate at 150°C was calculated using the following formula: Heat shrinkage rate (%) = [(A - B) / A] × 100 On the other hand, when the measurement direction was the length direction (MD), a polyethylene terephthalate film with dimensions of 190 mm in the length direction and 10 mm in the width direction was prepared as a sample. Next, two gauge marks were made on the surface of the film, 50 mm apart from the center in the longitudinal direction (100 mm between the marks), and the distance between the marks (C) was measured. Then, the film was placed in an oven in a 150°C atmosphere and heated at 150°C ± 3°C for 30 minutes under no load, and the distance between the marks (D) was measured. The thermal shrinkage rate at 150°C was calculated using the following formula: Thermal shrinkage rate (%) = [(C - D) / C] × 100

[0084] (7) Refractive index The refractive index was measured in accordance with JIS-K-7142. The refractive index was measured using an Abbe refractometer with NaD light. Methylene iodide was used as the mounting solution, and the refractive index in the longitudinal direction (Nx), the refractive index in the width direction (Ny), and the refractive index in the thickness direction (Nz) were measured. The measurements were taken at the center of the film in the width direction.

[0085] (8) Long Period The long period was calculated from the diffraction intensity profile obtained by small-angle X-ray scattering (SAXS) measurement using an X-ray diffractometer. For X-ray evaluation, a film sample was cut to 20-30 mm in the multi-stage stretching direction and 8 mm in the direction perpendicular to this direction, and several sheets were stacked to a thickness of approximately 100 μm. Silver behenate was used as the standard calibration sample. Using the beamline (BL03XU) of the synchrotron radiation facility SPring8, X-rays were incident from the direction normal to the film surface at a wavelength of 1.2 A and a camera length of 1700 mm, and measured by small-angle X-ray scattering (SAXS). The long period (Lp) was calculated using the analysis software Fit2D and Fityk. From the diffraction pattern obtained with X-rays, the value of profile I(q) (I: scattering intensity, q: scattering vector) was obtained, and the long period (d) was calculated from the following formula.

[0086]

[0087] (9) Crystal size Using the profile q2I(q) obtained by small-angle X-ray diffraction as described in (8) above, a Fourier transform is performed using the following formula to obtain the correlation function γ(r) (r = distance), thereby determining the crystal size (r ｃ ) was calculated.

[0088] Below, the crystal size (r ｃ The method for calculating ) will be described in detail. As shown in Figure 2, the smallest r ≥ 0 １ The local minimum value at γ(r １ ) = γ ｍｉｎ１ ... (A) Next, 0 ≤ r ≤ r １ In this case, the inflection point r of γ(r) is such that the second derivative of y = γ(r) is zero. ２ as, r ２ Let y = ar + b ... (B) be the tangent line to γ(r) at (A). At the intersection of equations (A) and (B), r = r c This is the crystal size that should be sought, γ min１ =ar c +b, r c = (γ min1 -b) / a, r ｃ The result was calculated.

[0089] (10) Color tone (b value) was measured using a colorimeter (ZE2000, manufactured by Nippon Denshoku) in accordance with JIS K 7373-2006 "Plastics - Method for determining yellowness and degree of yellowing".

[0090] (11) Flexural resistance (bending radius 0.25 mm) A polyethylene terephthalate film measuring 110 mm in width and 110 mm in length was prepared as a sample. This film was bent 200,000 times at a speed of 1 time / second using an unloaded U-shaped stretch tester (Yuasa System Equipment Co., Ltd., DLDMLH-FS) with a bending radius of 0.25 mm, so that the folds were parallel to the TD direction of the film (see Figures 3 and 4). Next, the same film, i.e., a polyethylene terephthalate film measuring 110 mm in width and 110 mm in length, was rotated 90 degrees to prepare a sample. This film was bent 200,000 times at a speed of 1 time / second using an unloaded U-shaped stretch tester (Yuasa System Equipment Co., Ltd., DLDMLH-FS) with a bending radius of 0.25 mm, so that the folds were parallel to the MD direction of the film (see Figure 5). For the above samples, after the bending process was completed, the samples were placed on a flat surface with the inner bent surface facing downwards and visually observed and evaluated according to the following criteria: ○: No cracks or fractures were observed in the sample. ×: Cracks or fractures were present in the sample.

[0091] (12) UV absorption performance A polyethylene terephthalate film sample with dimensions of 8 mm in the width direction (TD) and 40 mm in the mechanical flow direction (MD) was prepared. Using a spectrophotometer (Shimadzu Corporation, UV1800), the sample was positioned so that the mechanical flow direction (MD) and the direction of gravity were parallel, and the transmittance at a wavelength of 380 nm was measured.

[0092] (Raw materials for coating solution for easy adhesion layer formation: polymerization of urethane resin) A four-necked flask equipped with a stirrer, a Liebig condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer was prepared. 72.96 parts by mass of 1,3-bis(isocyanatemethyl)cyclohexane, 12.60 parts by mass of dimethylolpropionic acid, 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 were added to this four-necked flask. These were then stirred in the four-necked flask under a nitrogen atmosphere at 75°C for 3 hours, and it was confirmed that the reaction solution in the four-necked flask reached the predetermined amine equivalent. Next, after the reaction solution was cooled to 40°C, 9.03 parts by mass of triethylamine was added to obtain polyurethane prepolymer D solution. Next, 450 g of water was added to a reaction vessel equipped with a homodisperser capable of high-speed stirring, and the temperature was adjusted to 25°C. The isocyanate-terminated prepolymer was then added and dispersed in water while stirring. Subsequently, a water-soluble polyurethane resin (A) with a solid content of 35% by mass was prepared by removing acetonitrile and some of the water under reduced pressure.

[0093] (Raw materials for coating solution for easy adhesion layer formation; polymerization of water-soluble carbodiimide compounds) A flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer was prepared. 200 parts by mass of isophorone diisocyanate and 4 parts by mass of the carbodiimide catalyst 3-methyl-1-phenyl-2-phosphorene-1-oxide were added to this flask 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.

[0094] (Preparation of coating solution for easy adhesion layer formation) The following coating agents were mixed to prepare the coating solution: Water 16.97 parts by mass Isopropanol 21.96 parts by mass Water-soluble polyurethane resin (A) 3.27 parts by mass Water-soluble carbodiimide compound (B) 1.22 parts by mass Particles 0.51 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-based, solid content concentration 100% by mass)

[0095] (Preparation of polyethylene terephthalate pellets) A continuous esterification reactor consisting of a three-stage complete mixing tank equipped with a stirrer, a partial condenser, a raw material inlet, and a product outlet was used as the esterification reactor. Terephthalic acid (TPA) was added at a rate of 2 tons / hr, ethylene glycol (EG) at a rate of 2 moles per mole of TPA, and antimony trioxide was added in an amount such that the Sb atoms in the generated PET amounted to 160 ppm. This slurry was continuously supplied to the first esterification reactor of the esterification reactor and reacted at atmospheric pressure at 255°C with an average residence time of 4 hours. Subsequently, the reaction product in the first esterification reactor was continuously removed from the system and supplied to the second esterification reactor. Furthermore, EG distilled off from the first esterification reaction vessel was supplied to the second esterification reaction vessel at an amount of 8% by mass relative to the generated polymer (generated PET). Then, an EG solution containing magnesium acetate in an amount that results in 65 ppm of Mg atoms relative to the generated PET, and an EG solution containing trimethyl phosphate (TMPA) in an amount that results in 20 ppm of P atoms relative to the generated PET were added, and the reaction was carried out at atmospheric pressure at 260°C for an average residence time of 1.5 hours. Subsequently, the reaction product in the second esterification reaction vessel was continuously removed from the system and supplied to the third esterification reaction vessel, and an EG solution containing TMPA in an amount that results in 20 ppm of P atoms relative to the generated PET was added, and the reaction was carried out at atmospheric pressure at 260°C for an average residence time of 0.5 hours. The esterification reaction product generated in the third esterification reaction vessel was continuously supplied to a three-stage continuous polycondensation reactor for polycondensation. The product was then filtered using a stainless steel sintered filter material (nominal filtration accuracy: 90% reduction of 5 μm particles) to obtain polyethylene terephthalate pellets with an intrinsic viscosity of 0.60 dl / g.

[0096] (Example 1) Polyethylene terephthalate pellets (Tg 78°C) were dried under reduced pressure at 135°C for 8 hours (3 Torr). After that, these polyethylene terephthalate pellets (hereinafter sometimes referred to as PET pellets) were mixed with ultraviolet absorbing pellets (UN101 (manufactured by Nippon Pigment Co., Ltd.)) and supplied to an extruder, where they were melted at 285°C. These ultraviolet absorbing pellets mainly consist of polyethylene terephthalate and contain 2,2'-(1,4-phenylene)bis(4H-3,1-benzoxazine-4-one) as an ultraviolet absorber. The raw materials supplied to the extruder were melt-extruded into a sheet from the T-die by setting the resin temperature of the extruder's melting section, mixing section, polymer tube, gear pump, polymer tube, and T-die to 280°C, and then a sheet was produced using a cooling roll at 20°C. The sheet was stretched 3.0 times in the longitudinal direction (MD) using a heated roll (105°C). The coating solution for forming the easy-adhesion layer was applied to one side of a uniaxially oriented film by a roll-coating method, and then dried at 80°C for 20 seconds to obtain a uniaxially oriented film having an easy-adhesion layer on one side. The amount of coating solution used for forming the easy-adhesion layer was 0.06 g / m² after the final (biaxially oriented) drying. ２ The film was then adjusted to the following conditions. Next, the uniaxially oriented film was led to a tenter, preheated to 90°C, and stretched to 3.0 times its width in the TD direction at 100°C to produce a film. The film was then led back to the tenter, preheated to 120°C, and stretched to 1.39 times its width in the TD direction at 140°C to produce a film. The film was then led back to the tenter again, preheated to 140°C, and stretched to 1.39 times its width in the TD direction at 170°C to produce a film. Finally, the film was led back to the tenter, preheated to 170°C, and stretched to 1.39 times its width in the TD direction at 250°C to produce a film. After this, the film was led into a heat treatment zone adjusted to 100°C, and the film was passed through this zone for 12 seconds to perform a heat setting treatment, obtaining a biaxially oriented polyethylene terephthalate film with a thickness of 21 μm. Here, the amount of UV absorber added is 2% by mass relative to 100% by mass of the biaxially oriented polyethylene terephthalate film.

[0097] (Example 2) A sheet was prepared in the same manner as in Example 1. The sheet was then stretched 3.4 times in the longitudinal direction using a heated roll (105°C). A uniaxially oriented film having an easy-adhesion layer on one side was obtained in the same manner as in Example 1. Next, this uniaxially oriented film was led to a tenter, preheated to 90°C, and stretched 1.68 times in the width direction at 100°C to produce a film. The film was again led to the tenter, preheated to 120°C, and stretched 1.68 times in the width direction at 140°C to produce a film. Furthermore, the film was again led to the tenter, preheated to 140°C, and stretched 1.68 times in the width direction at 170°C to produce a film. Furthermore, the film was again guided into a tenter, preheated to 170°C, stretched to 1.58 times its original width at 250°C, and then subjected to the same heat-setting treatment as in Example 1 to obtain a biaxially oriented polyethylene terephthalate film with a thickness of 23 μm.

[0098] (Example 3) A uniaxially oriented film having an easy-adhesion layer on one side was obtained in the same manner as in Example 1. Next, this uniaxially oriented film was guided to a tenter, preheated to 90°C, and stretched to 3.0 times its width in the width direction at 100°C to produce a film. The film was again guided to the tenter, preheated to 120°C, and stretched to 1.39 times its width in the width direction at 140°C to produce a film. Furthermore, the film was again guided to the tenter, preheated to 140°C, and stretched to 1.39 times its width in the width direction at 170°C to produce a film. Furthermore, the film was again guided to the tenter, preheated to 170°C, and stretched to 1.47 times its width in the width direction at 250°C, after which a heat-setting treatment similar to that in Example 1 was performed to obtain a biaxially oriented polyethylene terephthalate film with a thickness of 21 μm.

[0099] (Example 4) A sheet was prepared in the same manner as in Example 1. Next, a heating furnace capable of simultaneously stretching in the longitudinal and width directions was prepared, and the sheet was stretched to 4.0 times its length in the longitudinal direction and 2.0 times its width in the first zone of the heating furnace (90°C). The coating liquid for forming the easy-adhesion layer was applied to one side of the biaxially oriented film by the roll-coating method, and then dried at 80°C for 20 seconds to obtain a biaxially oriented film having an easy-adhesion layer on one side. The final coating amount after drying (after biaxial stretching) was 0.06 g / m². ２The amount of coating solution used for easy adhesion layer formation was adjusted accordingly. Next, the biaxially oriented film was stretched 1.50 times in the longitudinal direction in two zones (240°C) of a heating furnace. This resulted in a biaxially oriented polyethylene terephthalate film with a thickness of 25 μm.

[0100] (Example 5) A biaxially oriented polyethylene terephthalate film was obtained by manufacturing in the same manner as in Example 1, except that the mixing ratio of ultraviolet-absorbing pellets to PET pellets was changed as shown in Table 1, and the thickness was changed to 50 μm. All of the films in the above examples had a total light transmittance of 85% or more and a haze of 3% or less. Therefore, all of the films in the above examples are suitable for display applications.

[0101] (Comparative Example 1) The mixing ratio of ultraviolet-absorbing pellets to PET pellets was changed to 0%, and a sheet was prepared in the same manner as in Example 1. The sheet was then stretched 1.4 times in the longitudinal direction using a heated roll (105°C). A uniaxially oriented film having an easy-adhesion layer on one side was obtained in the same manner as in Example 1. Next, this uniaxially oriented film was guided into a tenter, preheated to 90°C, stretched 4.20 times in the width direction at 100°C, and then subjected to the same heat setting treatment as in Example 1, except that the temperature of the heat treatment zone was set to 210°C, to obtain a biaxially oriented polyethylene terephthalate film with a thickness of 25 μm.

[0102] (Comparative Example 2) A sheet was prepared in the same manner as in Comparative Example 1. Next, an easy-adhesion layer was formed on one side of this sheet in the same manner as in Example 1. The sheet having the easy-adhesion layer on one side was stretched in the width direction and heat-set in the same manner as in Comparative Example 1 to obtain a uniaxially oriented polyethylene terephthalate film with a thickness of 25 μm.

[0103] (Comparative Example 3) A film was manufactured in the same manner as in Comparative Example 1, except that the mixing ratio of UV-absorbing pellets to PET pellets was changed to 2%, the MD stretching conditions were changed as shown in Table 1, and TD stretching was not performed. This resulted in a uniaxially oriented polyethylene terephthalate film with a thickness of 25 μm.

[0104]

[0105]

[0106] The polyethylene terephthalate film of the present invention possesses high elongation at break as well as high bending rigidity, allowing for sufficient stiffness even when thinned and suppressing scratches and deformation caused by foreign matter. The polyethylene terephthalate film of the present invention can be used in a wide range of industrial film fields, including surface protection applications for foldable displays, where high quality requirements are demanded.

[0107] 1: Foldable display 11: Bending radius 2: Polyethylene terephthalate film constituting the surface protective film of the foldable display 21: First folding section 22: First bending direction (direction perpendicular to the first folding section) 31: Second bending direction (direction perpendicular to the second folding section) 32: Second folding section

Claims

1. The sum of the longitudinal bending stiffness Fa and the widthwise bending stiffness Fb, converted to a thickness of 15 μm, is 0.05 gf·cm. ２ A polyethylene terephthalate film for foldable displays that can be folded in both a first bending direction and a second bending direction perpendicular to the first bending direction, having a length of 10% or more, and both the longitudinal and widthwise elongations at break being 10% or more.

2. The polyethylene terephthalate film according to claim 1, wherein the size of the crystals formed by the polymer molecules constituting the film is 3.0 nm or larger, as determined by small-angle X-ray scattering.

3. The polyethylene terephthalate film according to claim 1, wherein the transmittance of light with a wavelength of 380 nm is 20% or less and the b value is 3.5 or less.

4. The polyethylene terephthalate film according to claim 1, wherein the total light transmittance is 85% or more and the haze is 3% or less.

5. The polyethylene terephthalate film according to claim 1, characterized in that no cracks or breaks occur when a test of folding it 180° so that the distance between opposing sides is 0.5 mm is repeated 200,000 times.

6. A laminated film for a foldable display that can be folded in both directions: a first bending direction and a second bending direction perpendicular to the first bending direction, comprising a base layer and an easy-adhesion layer, wherein the base layer is a polyethylene terephthalate film according to any one of claims 1 to 5, and the easy-adhesion layer is laminated on at least one surface of the base layer.

7. The laminated film according to claim 6, wherein a hard coat layer is laminated on the easy-adhesion layer.

8. A foldable display that can be folded in both directions: a first bending direction and a second bending direction perpendicular to the first bending direction, comprising a polyethylene terephthalate film as described in any one of claims 1 to 5 as a surface protective film.

9. A foldable display that can be folded in both directions: a first bending direction and a second bending direction perpendicular to the first bending direction, comprising the laminated film described in claim 6 as a surface protective film.

10. A portable terminal device that can be folded in both directions: a first bending direction and a second bending direction perpendicular to the first bending direction, comprising the foldable display described in claim 8.

11. A portable terminal device that can be folded in both directions: a first bending direction and a second bending direction perpendicular to the first bending direction, comprising the foldable display described in claim 9.

Images