Polyester film

Abstract

To provide a polyester film which has excellent bending resistance irrespective of use environment.SOLUTION: A polyester film comprises a polyethylene naphthalate copolymer, has an average refractive index of 1.661 or more, and has a plane orientation coefficient ΔP of 0.170 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyester film. [Background technology]

[0002] Polyester films, typified by polyethylene terephthalate films and polyethylene naphthalate films, are excellent in heat resistance, weather resistance, mechanical strength, transparency, chemical resistance, etc., and are also readily available at reasonable prices. As a result, they are highly versatile and are used in a variety of applications.

[0003] Thermoplastic polyesters such as polyethylene terephthalate and polybutylene terephthalate, which are resins constituting polyester films, have excellent heat resistance and can therefore be used continuously under high temperature and high humidity conditions. However, at high temperatures, crystallization and thermal degradation cause a decrease in toughness, significantly reducing bending resistance (hinge properties) in particular, and molded products such as films can easily break when bent. For this reason, their use is currently limited in applications requiring bending resistance.

[0004] On the other hand, in recent years, there has been a trend toward the use of flexible substrates and flexible printed circuits in line with the trend toward smaller and lighter electronic devices. This trend has led to an increased demand for flexibility in display applications, and there is a strong demand for films with excellent recovery and resistance to repeated bending (flexibility).

[0005] Furthermore, Patent Document 1 cites as an issue that repeated folding and unfolding at low temperatures places a large load on the base member made of a resin material, reducing reliability, and there is a need to improve the bending resistance of the film under low-temperature conditions.

[0006] Furthermore, for example, Patent Document 2 discloses a flexible plastic film that exhibits high hardness and excellent flexibility, and cites a polyethylene naphthalate film as an example of such a film.

[0007] Patent Document 3 proposes a polyethylene naphthalate copolymer characterized in that the dicarboxylic acid component is mainly composed of structural units derived from naphthalenedicarboxylic acid, and the diol component is composed of structural units derived from ethylene glycol and structural units derived from an alkylene oxide adduct of a bisphenol. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2020-46608 [Patent Document 2] Japanese Patent Publication No. 2020-114673 [Patent Document 3] Japanese Patent Application Publication No. 10-204165 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the polyethylene naphthalate film exemplified in Patent Document 2 sometimes does not have sufficient bending resistance under low temperature conditions.

[0010] Moreover, the polyethylene naphthalate copolymer disclosed in Patent Document 3 is not intended to be used for displays, particularly flexible displays. In addition, even if a film is formed using the polyethylene naphthalate copolymer described in Patent Document 3, depending on the film formation conditions such as the stretching conditions, the bending resistance, particularly the bending resistance under high temperature conditions, may not reach a desired level.

[0011] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a polyester film that is particularly suitable for flexible displays and has excellent flex resistance regardless of the use environment. [Means for solving the problem]

[0012] The present inventors have conducted extensive research to achieve the above object and have completed the present invention, which has the following aspects. [1] A polyester film containing a polyethylene naphthalate copolymer, having an average refractive index of 1.661 or more and a plane orientation coefficient ΔP of 0.170 or more. [2] The polyester film according to [1] above, wherein the polyethylene naphthalate copolymer contains a dicarboxylic acid component (a-1) and a diol component (a-2), and the dicarboxylic acid component (a-1) contains 80 mol % or more of 2,6-naphthalenedicarboxylic acid. [3] The polyester film according to [1] or [2] above, wherein the polyethylene naphthalate copolymer contains a dicarboxylic acid component (a-1) and a diol component (a-2), and the diol component (a-2) contains 30 mol % or more and 99 mol % or less of ethylene glycol. [4] The polyester film according to [3] above, further comprising a bisphenol A-ethylene oxide adduct as the diol component (a-2). [5] The polyester film according to [4], wherein the bisphenol A-ethylene oxide adduct is contained in the diol component (a-2) in an amount of 1 mol % to 70 mol %. [6] The polyester film according to any one of the above [1] to [5], which has an average hysteresis loss rate of 59.0% or less when subjected to a tensile cycle test up to a tensile strain of 5% in each of the machine direction (MD) and the transverse direction (TD) at 23°C. [7] The polyester film according to any one of the above [1] to [6], which has a haze of 3.0% or less. [8] The polyester film according to any one of the above [1] to [7], which has a thickness of 9 μm or more and 125 μm or less. [9] The polyester film according to any one of the above [1] to [8], which is a biaxially stretched film.

[10] The polyester film according to any one of the above [1] to [9], which is used for a display.

[11] The polyester film according to

[10] above, which is for a flexible display.

[12] A flexible display comprising the polyester film according to any one of [1] to

[11] above. [Effects of the Invention]

[0013] According to the present invention, a polyester film having excellent flex resistance regardless of the use environment is provided. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram showing the profile of a stress-strain curve. [Figure 2] FIG. 1 is a diagram showing a method for evaluating bending resistance. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described in detail below, but the present invention is not limited to the embodiments described below.

[0016] <<Polyester film>> The polyester film of the present invention (hereinafter also referred to as "the film") contains a polyethylene naphthalate (hereinafter also referred to as "PEN") copolymer, and has an average refractive index of 1.661 or more and a plane orientation coefficient ΔP of 0.170 or more.

[0017] In the present invention, the polyester film contains a PEN copolymer, and thus has excellent flex resistance under normal temperature, high temperature, and low temperature conditions, particularly under low temperature conditions. The reason for the excellent flex resistance even under low temperature conditions is believed to be that the inclusion of the copolymer component prevents the film from becoming hard under low temperature conditions.

[0018] Furthermore, in the present invention, by setting the average refractive index and the plane orientation coefficient ΔP to specific values ​​or more, the film has excellent flex resistance under high temperature conditions. To achieve flex resistance, it is necessary to achieve both bending and returning to the original shape. In particular, under high temperature conditions, if the amorphous region is dominant, the film becomes flexible and its ability to return to its original shape (restoring force) is poor. Therefore, we believe that by making the crystalline region dominant, i.e., by promoting crystal orientation to an appropriate degree, the restoring force can be maintained even under high temperature conditions.

[0019] The present film may have a single layer structure or a multilayer structure (i.e., a laminated film). When the present film has a multilayer structure, it may have a two-layer structure, a three-layer structure, or may have four or more layers without departing from the gist of the present invention, and the number of layers is not particularly limited. Among these, from the viewpoint of reducing the production cost of the present film, a single layer structure or a multilayer structure of 2 to 3 layers is preferred.

[0020] Furthermore, from the viewpoint of setting the average refractive index and the plane orientation coefficient ΔP to a specific value or more, the present film is preferably a biaxially stretched film, and can be obtained by stretching in the machine direction (MD) and then in the width direction (TD), or by a sequential biaxial stretching method in which the film is stretched in the width direction (TD) and then in the machine direction (MD), or by a simultaneous biaxial stretching method in which the film is stretched in the machine direction (MD) and the width direction (TD) almost simultaneously. Of these, sequential biaxial stretching is preferred, and sequential biaxial stretching in which the film is stretched in the machine direction (MD) and then in the width direction (TD) is more preferred.

[0021] In the present invention, the longitudinal direction (MD) of the film refers to the direction in which the film advances during the film production process, i.e., the winding direction of the film roll, and the transverse direction (TD) of the film refers to the direction parallel to the film surface and perpendicular to the longitudinal direction (MD), i.e., the direction parallel to the central axis of the roll when the film is rolled.

[0022] <Polyethylene naphthalate copolymer> The film comprises a PEN copolymer. The content of the PEN copolymer in the present film is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more (including 100% by mass) of the resin constituting the present film. When the present film has a multilayer structure, it is sufficient that any one of the layers contains the PEN copolymer and the total content of the PEN copolymer in the entire film is the same as above. It is particularly preferable that each layer contains the PEN copolymer and the content of the PEN copolymer in each layer is the same as above.

[0023] Specifically, the PEN copolymer contains a dicarboxylic acid component (a-1) and a diol component (a-2), and more specifically, contains 2,6-naphthalenedicarboxylic acid as the dicarboxylic acid component (a-1) and ethylene glycol as the diol component (a-2) as essential components, and contains a copolymerization component in at least one of the dicarboxylic acid component (a-1) and the diol component (a-2). The inclusion of the PEN copolymer in the film improves flex resistance, particularly flex resistance under low temperature conditions.

[0024] The PEN copolymer preferably contains 80 mol % or more of 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid component (a-1), more preferably 90 mol % or more, and even more preferably all (100 mol %) of the dicarboxylic acid component (a-1) is 2,6-naphthalenedicarboxylic acid. By adjusting the content of 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid component (a-1) to 80 mol % or more, the flex resistance, particularly the flex resistance under normal temperature and high temperature conditions, becomes good.

[0025] The PEN copolymer preferably contains 30 mol% or more, more preferably 50 mol% or more, even more preferably 70 mol% or more, particularly preferably 80 mol% or more, and especially preferably 90 mol% or more of ethylene glycol in the diol component (a-2). On the other hand, the upper limit of the ethylene glycol content in the diol component (a-2) is preferably 99 mol% or less, more preferably 98 mol% or less, even more preferably 97 mol% or less, particularly preferably 96 mol% or less, and especially preferably 95 mol% or less. By setting the content of ethylene glycol in the diol component (a-2) within this range, an excellent balance of flex resistance, particularly flex resistance under normal temperature, high temperature and low temperature conditions, is achieved.

[0026] The PEN copolymer preferably contains 20 mol % or less, more preferably 10 mol % or less of a copolymerization component in the dicarboxylic acid component (a-1), and even more preferably, all of the dicarboxylic acid component (a-1) is 2,6-naphthalenedicarboxylic acid, i.e., the copolymerization component is 0 mol %. The PEN copolymer preferably contains 70 mol% or less of copolymerization components in the diol component (a-2), more preferably 50 mol% or less, even more preferably 30 mol% or less, particularly preferably 20 mol% or less, and especially preferably 10 mol% or less. Meanwhile, the lower limit of the copolymerization components in the diol component (a-2) is preferably 1 mol% or more, more preferably 2 mol% or more, even more preferably 3 mol% or more, particularly preferably 4 mol% or more, and especially preferably 5 mol% or more. By setting the content of the copolymerization component in the dicarboxylic acid component (a-1) and / or the diol component (a-2) within this range, it becomes easier to adjust the plane orientation coefficient ΔP and the average refractive index of the polyester film to the desired range, and the film will have an excellent balance of bending resistance under normal temperature, high temperature, and low temperature conditions.

[0027] Examples of copolymerizable components that can be added to the dicarboxylic acid component (a-1) include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, 3,4-furandicarboxylic acid, benzophenonedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, and 4,4'-diphenyletherdicarboxylic acid; and aliphatic dicarboxylic acids such as cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and dimer acid. Among these, isophthalic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, and 3,4-furandicarboxylic acid are preferred from the viewpoint of moldability. These copolymerizable components can be used alone or in combination of two or more.

[0028] On the other hand, examples of copolymerization components added to the diol component (a-2) include diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer diol, and bisphenols (bisphenol compounds such as bisphenol A, bisphenol F, or bisphenol S, or derivatives thereof, or ethylene oxide adducts thereof). From the viewpoint of flex resistance, 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, and bisphenols are preferred. Of these, from the viewpoint of maintaining film strength, bisphenols are more preferred, and as the bisphenol, bisphenol A-ethylene oxide adducts are preferably used. These copolymerization components can be used alone or in combination of two or more. As will be described later, the PEN copolymer and polyester resins other than the PEN copolymer that may be included as resins constituting the present film may contain diethylene glycol as a by-product of ethylene glycol when produced using ethylene glycol as one of the raw materials. In the present invention, this by-product diethylene glycol is considered to be included in ethylene glycol. On the other hand, depending on the diethylene glycol content, more specifically, when diethylene glycol is contained in an amount exceeding 5 mol%, diethylene glycol is treated as a copolymerization component rather than as a by-product diethylene glycol.

[0029] The PEN copolymer most preferably contains 2,6-naphthalenedicarboxylic acid as the dicarboxylic acid component (a-1), ethylene glycol as the diol component (a-2), and a bisphenol A-ethylene oxide adduct as a copolymerization component. In this PEN copolymer, the content of the bisphenol A-ethylene oxide adduct is preferably 1 mol% to 70 mol%, more preferably 2 mol% to 50 mol%, even more preferably 3 mol% to 30 mol%, particularly preferably 4 mol% to 20 mol%, and especially preferably 5 mol% to 10 mol%, based on the total amount of the diol component (a-2).

[0030] When the present film has a multilayer structure, the type and content of the copolymer component constituting the PEN copolymer contained in any layer, preferably in each layer, may be the same as described above, and the resin constituting each layer and the PEN copolymer in each layer may be the same or different.

[0031] The polymerization catalyst for PEN copolymers and the like is not particularly limited, and any conventionally known compound can be used, such as titanium compounds, germanium compounds, antimony compounds, manganese compounds, aluminum compounds, magnesium compounds, calcium compounds, etc. Among these, at least one selected from titanium compounds and antimony compounds is preferred.

[0032] In order to suppress the amount of precipitation of oligomer components, the film may be produced using a raw material having a low content of oligomer components. As a method for producing a raw material having a low content of oligomer components, various known methods can be used.

[0033] Furthermore, in order to reduce manufacturing costs, the film may be manufactured using recycled materials.

[0034] The present film may contain other resins in addition to the PEN copolymer and polyester resins other than the PEN copolymer, as long as the effects of the present invention are not impaired.

[0035] Furthermore, the resin constituting the present film may contain a polyester resin other than the PEN copolymer or other resins, so long as it contains a PEN copolymer. As the polyester resin other than the PEN copolymer, homo-PEN is preferred, which is a PEN polymer composed of 100 mol % of 2,6-naphthalenedicarboxylic acid as a dicarboxylic acid component and 100 mol % of ethylene glycol as a diol component. Generally, when a polyester resin (including a PEN copolymer) is produced (polycondensed) using ethylene glycol as one of the raw materials, diethylene glycol is by-produced from the ethylene glycol. In this specification, this diethylene glycol is referred to as by-produced diethylene glycol. The amount of diethylene glycol by-produced from ethylene glycol varies depending on the type of polycondensation, but is approximately 5 mol % or less of the ethylene glycol. In the present invention, this by-produced diethylene glycol of 5 mol % or less is also included in ethylene glycol and is distinguished from the copolymerization component. The other resins are not particularly limited, but examples thereof include epoxy resins, polystyrene resins, polyvinyl chloride resins, polycarbonate resins, polyamide resins, polyacetal resins, polyethersulfone resins, polyetherketone resins, polysulfone resins, and polycycloolefin resins.

[0036] The present film may contain particles primarily for the purposes of imparting lubricity and preventing scratches during each process. The type of particles is not particularly limited as long as they are capable of imparting lubricity, and examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, and titanium oxide, and organic particles such as acrylic resin, styrene resin, urea resin, phenolic resin, epoxy resin, and benzoguanamine resin. These may be used alone or in combination of two or more. Furthermore, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst during the manufacturing process of raw materials such as a PEN copolymer can also be used.

[0037] The shape of the particles is not particularly limited, and may be any of spherical, blocky, rod-like, flat, and the like. Furthermore, there are no particular limitations on the hardness, specific gravity, color, etc. of the particles.

[0038] The average particle size of the particles is preferably 5 μm or less, more preferably 4 μm or less, and even more preferably 3 μm or less, while the lower limit is preferably 0.01 μm or more, more preferably 0.1 μm or more, and even more preferably 0.3 μm or more. If the average particle size of the particles is within this range, the present film can have both transparency and ease of handling.

[0039] When the present film has a multilayer structure, particles may be contained in at least one layer, but it is preferable to contain particles in the surface layer. By containing particles in the surface layer, it is possible to effectively impart slipperiness while reducing the particle content in the entire film. The particles may be contained in one or both of the surface layers.

[0040] The content of the particles is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less, based on the film, while the lower limit is preferably 0.0003% by mass or more, more preferably 0.01% by mass or more. If the particle content is 0.0003% by mass or more, the film surface can be made slippery and scratches can be prevented from occurring during each process, while if the content is 5% by mass or less, transparency is good. The particle content referred to here means the content of the entire film when the film has a single layer structure, and means the content of the layer containing the particles when the film has a multilayer structure.

[0041] The method for adding particles to the present film is not particularly limited, and any conventionally known method can be used. For example, particles can be added at any stage during the production of raw materials such as PEN copolymer. Since the present film is a polyester film, it is preferable to add particles after the completion of the esterification or transesterification reaction.

[0042] In addition to the above-mentioned particles, the present film may contain conventionally known additives such as ultraviolet absorbers, antioxidants, antistatic agents, heat stabilizers, lubricants, dyes, pigments, etc., if necessary. When the present film has a multi-layer structure, it is not necessary to contain each additive in all layers, but it is sufficient to contain each additive in at least one layer.

[0043] From the viewpoint of film strength, the thickness of the present film is preferably 9 μm or more, more preferably 12 μm or more, and even more preferably 20 μm or more. On the other hand, from the viewpoint of improving flex resistance, the upper limit of the thickness of the present film is preferably 125 μm or less, more preferably 100 μm or less, and even more preferably 75 μm or less.

[0044] <Manufacturing method of this film> Next, the method for producing the present film will be described using as an example a biaxially stretched film obtained by a sequential biaxial stretching method. When the present film is a biaxially stretched film, it is preferable to first produce an unstretched sheet, which is then stretched in two directions to obtain a biaxially stretched film.

[0045] The unstretched sheet is preferably obtained by feeding raw materials such as the PEN copolymer described above and additives added as needed into an extruder, mixing them appropriately, extruding the mixture as a molten sheet from a die using the extruder, and cooling and solidifying it on a rotating cooling drum (cooling roll). In this case, in order to improve the flatness of the sheet, it is preferable to increase the adhesion between the sheet and the rotating cooling drum, and an electrostatic application adhesion method and / or a liquid application adhesion method are preferably used.

[0046] When the present film has a multilayer structure, it is preferable to co-extrude the layers by a co-extrusion method to form an unstretched sheet having a multilayer structure. The raw materials such as the PEN copolymer may be fed to the extruder as pellets or the like after being appropriately dried, and additives such as particles and ultraviolet absorbers may be appropriately blended into the pellets or the like.

[0047] The resulting unstretched sheet is then stretched uniaxially and then biaxially. Specifically, an unstretched sheet is first stretched in one direction using a roll or tenter type stretching machine. The stretching temperature is usually 110 to 150°C, preferably 120 to 140°C, more preferably 125 to 133°C, and even more preferably 127 to 130°C, and the stretching ratio is usually 2.0 to 7.0 times, preferably 2.5 to 5.0 times, more preferably 2.7 to 4.5 times, and even more preferably 3.0 to 4.0 times. Next, the film is stretched in a direction perpendicular to the first-stage stretching direction. In this case, the stretching temperature is usually 110 to 200°C, preferably 120 to 160°C, more preferably 122 to 140°C, and even more preferably 125 to 135°C, and the stretching ratio is usually 3.0 to 7.0 times, preferably 3.3 to 6.0 times, more preferably 3.5 to 5.0 times, and even more preferably 3.7 to 4.5 times.

[0048] Subsequently, the film is heat-treated under tension or relaxation of 30% or less at a temperature of typically 150 to 270°C, preferably 170 to 250°C, more preferably 180 to 240°C, and even more preferably 200 to 220°C to obtain a biaxially stretched film. In the stretching, a method of stretching in one direction in two or more stages can also be used. In such a case, it is preferable to perform the stretching so that the final stretch ratios in both directions are each within the above ranges.

[0049] <Physical properties of this film> The average refractive index of the present film must be 1.661 or more. If the average refractive index of the present film is less than 1.661, the film will have poor flex resistance, especially under high temperature conditions. The average refractive index can be adjusted by adjusting the film formation conditions, etc. The average refractive index of the present film is preferably 1.662 or more, more preferably 1.663 or more. The average refractive index refers to the average value of the refractive index in the machine direction (MD), width direction (MD), and thickness direction of the film.

[0050] The plane orientation coefficient ΔP of the present film must be 0.170 or more. If the plane orientation coefficient ΔP of the present film is less than 0.170, the film will have poor flex resistance, especially under high temperature conditions. The plane orientation coefficient ΔP can be adjusted by film formation conditions, etc. The planar orientation coefficient ΔP of the present film is preferably 0.190 or more, more preferably 0.210 or more. The plane orientation coefficient ΔP is a value calculated by measuring the refractive index (nβ) in the longitudinal direction (MD), the refractive index (nγ) in the width direction (TD), and the refractive index (nα) in the thickness direction of the film using the following general formula (1): ΔP=(nβ+nγ) / 2-nα (1) The planar orientation coefficient ΔP is measured on both sides of the film, and it is sufficient that at least one of them falls within the range, but it is preferable that both sides fall within the range.

[0051] The bending resistance of the present film at room temperature (23°C) can be evaluated in accordance with JIS K 7312:1996 by measuring the average value of the hysteresis loss rate when a tensile cycle test is performed at 23°C up to a tensile strain of 5% in each of the machine direction (MD) and the transverse direction (TD). The average value of the hysteresis loss rate is preferably 59.0% or less, more preferably 54.5% or less, and even more preferably 50.0% or less. The lower limit is not particularly limited, but is 0.1% or more, and generally 10.0% or more. If the average hysteresis loss rate of the present film at 23°C is 59.0% or less, the restoring force of the present film is large and the bending resistance of the film is good. The hysteresis loss rate can be adjusted by the type and content of the resin that constitutes the present film, the stretching conditions, etc. The average value here means the average value of the hysteresis loss rate in the machine direction (MD) and the hysteresis loss rate in the transverse direction (TD).

[0052] The bending resistance of the present film at high temperatures (90°C) can be evaluated by placing the film in a 90°C oven for 6 hours while bending it at a bending radius of 1.0 mm, then removing it to room temperature, immediately unfolding it, and leaving it for 24 hours, and measuring the curl angle. More specifically, it can be evaluated by the method described in the Examples. After bending tests in both the machine direction (MD) and the cross direction (TD), the average curl angle is preferably 145° or less, more preferably 142° or less, and even more preferably 139° or less. If the average curl angle is 145° or less, the film has a large restoring force to return to its original state after bending, i.e., the film has good bending resistance. The lower limit is preferably 0° or more. The average value here means the average value of the curl angle in the machine direction (MD) and the curl angle in the transverse direction (TD).

[0053] The flex resistance of the present film at low temperature (-20°C) can be evaluated by visually inspecting the fold marks after a flex test under the following conditions: -20°C, flex radius 1.0 mm, flex speed 70 rpm, and flex count 200,000 times. More specifically, the evaluation can be performed using the method described in the Examples. The less fold marks can be visually inspected, the greater the restoring force of the film to return to its original state after flexion, i.e., the better the flex resistance of the present film.

[0054] The haze of the present film is preferably 3.0% or less, more preferably 2.0% or less, even more preferably 1.0% or less, and particularly preferably less than 1.0%. The lower limit is not particularly limited, and is 0.01% or more. If the haze of the present film is 3.0% or less, it can be said to have excellent transparency and can be suitably used for display applications.

[0055] <Application> The present film has excellent flex resistance regardless of the usage environment, and therefore can be suitably used for displays, particularly flexible displays. Flexible displays include foldable displays, bendable displays, rollable displays, and stretchable displays, and is particularly suitable for foldable displays. The term "for flexible displays" refers to display components such as front panels, substrate films for touch sensors, and films for protecting the back side of display devices.

[0056] The display may be used in mobile phones, smartphones, digital cameras, personal computers, etc. The type of display is not particularly limited, but examples include liquid crystal displays, plasma displays, and organic EL displays, and may also be touch panel displays.

[0057] <<Explanation of terms>> In the present invention, the term "film" includes the term "sheet", and the term "sheet" includes the term "film".

[0058] In the present invention, when it is stated that "X to Y" (X and Y are any numbers), unless otherwise specified, it means "X or more and Y or less", and also means "preferably larger than X" or "preferably smaller than Y". Furthermore, when it is stated that the amount is "X or more" (X is any number), it also means that the amount is "preferably greater than X" unless otherwise specified, and when it is stated that the amount is "Y or less" (Y is any number), it also means that the amount is "preferably smaller than Y" unless otherwise specified. [Example]

[0059] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0060] <Evaluation method> (1) Average refractive index The refractive index (nβ) in the longitudinal direction (MD), the refractive index (nγ) in the transverse direction (TD), and the refractive index (nα) in the thickness direction of the film were measured using an Abbe refractometer (NAR-4T) manufactured by Atago Co., Ltd. using sodium D-line as a light source, and the average value of each refractive index was taken as the average refractive index.

[0061] (2) Plane orientation coefficient ΔP The refractive index (nβ) in the longitudinal direction (MD), the refractive index (nγ) in the transverse direction (TD), and the refractive index (nα) in the thickness direction of the film were measured using an Abbe refractometer (NAR-4T) manufactured by Atago Co., Ltd. using sodium D-line as a light source, and the plane orientation coefficient ΔP was calculated using the following formula (1). ΔP=(nβ+nγ) / 2-nα (1)

[0062] (3) Bending resistance evaluation (hysteresis loss rate) at room temperature (23°C) The hysteresis loss rate at 23° C. was determined by the following method in accordance with JIS K 7312:1996. The measuring device used was a tensile tester (Autograph AG-I, manufactured by Shimadzu Corporation). Test specimens were cut from polyester film at any location into rectangular shapes measuring 100 mm in length and 10 mm in width. Both longitudinal ends of the specimen were clamped with a chuck distance of 50 mm. The specimen was raised to a strain of 5% at a crosshead speed of 0.5 mm / min and then lowered to the initial position at the same speed. A stress-strain curve was obtained from one cycle of tensile cyclic testing. The stress-strain curve had a profile as shown in Figure 1. The hysteresis loss rate was calculated from the stress-strain curve using the area A1 (abcda) of the curve obtained during the ascending stroke and the area A2 (abcef), which is the difference between the area A1 and the area obtained during the descending stroke, according to the following formula (2). The test was performed three times, and the average value was calculated. The tensile cyclic testing was performed in both the machine direction (MD) and the cross direction (TD) of the film. Hysteresis loss rate = A2 / A1 × 100 (2)

[0063] Further, the average value of the measured hysteresis loss rate in the machine direction (MD) and the hysteresis loss rate in the transverse direction (TD) was calculated, and this was taken as the average value of the hysteresis loss rate. The evaluation criteria are as follows: 〇(Excellent): Average value 54.5% or less △(Average): The average value is greater than 54.5% and less than 59.0% × (poor): The average value is greater than 59.0%

[0064] (4) Evaluation of bending resistance (curl angle) under high temperature (90°C) conditions A 100mm long x 30mm wide test piece was cut from a random location on the polyester film. The test piece was bent perpendicular to the MD with a 1.0mm radius and then placed in a 90°C oven for 6 hours. The test piece was then removed to room temperature and immediately released. The curl angle (MD) was then measured after 24 hours. The TD curl angle (TD) was measured by cutting a 30mm long x 100mm wide test piece from a random location on the polyester film. The test piece was bent perpendicular to the TD with a 1.0mm radius and then placed in a 90°C oven for 6 hours. The MD curl angle (TD) was then measured by cutting a 30mm long x 100mm wide test piece from a random location on the polyester film. The test piece was bent perpendicular to the TD with a 1.0mm radius and then placed in a 90°C oven for 6 hours. The TD curl angle (TD) was then measured by repeating the same procedure.

[0065] More specifically, the curl angle was measured as follows: After leaving the test piece for 24 hours, the test piece was placed on a table with the fold line oriented vertically, and the angle of the fold line (fold line angle) when viewed from directly above was measured as shown in Figure 2. The curl angle was calculated by subtracting the fold line angle from 180°. The average of the measured curl angle in the machine direction (MD) (the curl angle (MD)) and the curl angle in the transverse direction (TD) (the curl angle (TD)) was calculated and used as the average curl angle. The evaluation criteria are as follows: 〇(Excellent): Average value 139° or less △(Average): The average value is greater than 139° and less than 145° × (poor): The average value is greater than 145°

[0066] (5) Evaluation of bending resistance under low temperature (-20°C) conditions (visual observation) Using a bending tester (Yuasa System Co., Ltd., CL09-type D01-FSC90), a 100 mm long x 30 mm wide test piece was cut from a random location on the polyester film. The test piece was bent 200,000 times in the direction perpendicular to the MD with a 1.0 mm radius, and then the flex resistance (MD) was evaluated. The flex resistance (TD) was evaluated by cutting a 30 mm long x 100 mm wide test piece from a random location on the polyester film and bending the test piece 200,000 times in the direction perpendicular to the TD with a 1.0 mm radius. The test was performed at -20°C and a flex speed of 70 rpm.

[0067] After the bending test, the bending traces on the test specimen were visually inspected for evaluation. The evaluation criteria are as follows: 〇(Excellent): Almost no creases were observed △(Average): Fold marks were faintly visible × (poor): The crease was clearly visible.

[0068] (6) Haze Measurement was carried out in accordance with JIS K 7136:2000 using a haze meter NDH-2000 manufactured by Nippon Denshoku Industries Co., Ltd.

[0069] <Materials used> [Polyethylene naphthalate copolymer (PEN copolymer)] Dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol %, diol component (a-2): ethylene glycol = 95 mol %, bisphenol A-ethylene oxide adduct = 5 mol % were used.

[0070] [Homopolyethylene naphthalate (HomoPEN)] The dicarboxylic acid component used was 2,6-naphthalenedicarboxylic acid (100 mol %), and the diol component used was ethylene glycol (100 mol %).

[0071] [Polyethylene terephthalate (PET)] The dicarboxylic acid component used was 100 mol % of terephthalic acid, and the diol component used was 100 mol % of ethylene glycol. A masterbatch (PET-particle masterbatch) containing 0.7 mass % of silica particles with an average particle size of 2.3 μm in PET was also used.

[0072] Example 1 The PEN copolymer was fed into a twin-screw extruder and extruded at 300°C. The extrusion was then cooled and solidified on a cooling roll set at 50°C using an electrostatic adhesion method to obtain an unstretched sheet. The unstretched sheet was then stretched 3.0 times in the machine direction (MD) at 128°C using a roll stretching machine. After preheating at 115°C in a tenter, it was stretched 4.1 times in the transverse direction (TD) at 130°C. Finally, it was heat-treated at 210°C to obtain a polyester film with a thickness of 50 μm. The properties of the resulting polyester film were evaluated by the methods described above, and the evaluation results are shown in Table 2.

[0073] (Examples 2 to 4, Comparative Example 1) A polyester film was obtained in the same manner as in Example 1, except that the film-forming conditions were as shown in Table 1. The evaluation results are shown in Table 2.

[0074] (Comparative Example 2) A polyester film was obtained in the same manner as in Example 1, except that the above homo-PEN was used instead of the PEN copolymer and the film formation conditions were as shown in Table 1. The evaluation results are shown in Table 2.

[0075] (Comparative Example 3) The surface layer was a dry blend of PET (70% by mass) and PET-particle masterbatch (30% by mass), and the middle layer was PET (100% by mass). Each was fed into separate twin-screw extruders, extruded at 280°C, and cooled and solidified on a cooling roll set at 25°C using an electrostatic adhesion method, to obtain a two-type, three-layer unstretched sheet. The unstretched sheet was then stretched 3.2 times in the machine direction (MD) at 86°C using a roll stretching machine. After preheating at 100°C in a tenter, it was stretched 4.2 times in the transverse direction (TD) at 115°C. Finally, it was heat-treated at 235°C to obtain a polyester film with a thickness of 50 μm (thickness ratio: surface layer / intermediate layer / surface layer = 1 / 8 / 1).

[0076] [Table 1]

[0077] [Table 2]

[0078] As shown in the above examples, by including a polyethylene naphthalate copolymer and adjusting the average refractive index and plane orientation coefficient ΔP of the polyester film to a specific value or more, the polyester film exhibits an excellent balance of flex resistance under normal, high, and low temperature conditions. From this, it can be said that the polyester films of the examples have excellent flex resistance regardless of the usage environment. In contrast, the polyester film of Comparative Example 1, despite having the same composition as the Examples, had an average refractive index of less than 1.661, and therefore did not exhibit sufficient flex resistance under high-temperature conditions. Comparative Example 2 did not contain a copolymer component, and therefore did not exhibit sufficient flex resistance under low-temperature conditions. Furthermore, Comparative Example 3 failed to exhibit excellent flex resistance under any conditions.

[0079] Furthermore, all of the polyester films of the examples had low haze and excellent transparency. Therefore, the present film can be suitably used for displays, particularly flexible displays. [Industrial Applicability]

[0080] The polyester film of the present invention has excellent flex resistance regardless of the use environment, and therefore can be suitably used for displays, particularly flexible displays. Therefore, the embodiments of the present disclosure are useful for flexible displays such as foldable displays, bendable displays, rollable displays, and stretchable displays that utilize the advantages of flexible display panels that can be folded, bent, rolled, and stretched.

Claims

1. Contains polyethylene naphthalate copolymer, the polyethylene naphthalate copolymer contains a dicarboxylic acid component (a-1) and a diol component (a-2), The dicarboxylic acid component (a-1) contains 100 mol % of 2,6-naphthalenedicarboxylic acid, The diol component (a-2) contains ethylene glycol in an amount of 30 mol% or more and 99 mol% or less, The diol component (a-2) contains a bisphenol A-ethylene oxide adduct in an amount of 1 mol % to 70 mol %. The average refractive index is 1.661 or more, A polyester film having a plane orientation coefficient ΔP of 0.170 or more.

2. 2. The polyester film according to claim 1, wherein the average hysteresis loss rate when subjected to a tensile cycle test at 23°C up to a tensile strain of 5% in each of the machine direction (MD) and the transverse direction (TD) is 59.0% or less.

3. 3. The polyester film according to claim 1, having a haze of 3.0% or less.

4. The polyester film according to any one of claims 1 to 3, having a thickness of 9 µm or more and 125 µm or less.

5. The polyester film according to any one of claims 1 to 4, which is a biaxially stretched film.

6. The polyester film according to any one of claims 1 to 5, which is used for a display.

7. The polyester film according to claim 6, which is for a flexible display.

8. The polyester film according to claim 6, which is for a foldable display.

9. A flexible display comprising the polyester film according to any one of claims 1 to 7.

10. A foldable display comprising the polyester film according to any one of claims 1 to 6 and claim 8.

11. A polymeric material comprising a polyethylene naphthalate copolymer, The average refractive index is 1.661 or more, The plane orientation coefficient ΔP is 0.170 or more, After evaluating the bending resistance under conditions of 90°C, 6 hours, and a bending radius of 1.0 mm, the average curl angle in the machine direction (MD) and the average curl angle in the transverse direction (TD) are 145° or less, the polyethylene naphthalate copolymer contains a dicarboxylic acid component (a-1) and a diol component (a-2), The dicarboxylic acid component (a-1) contains 100 mol % of 2,6-naphthalenedicarboxylic acid, The diol component (a-2) contains ethylene glycol in an amount of 30 mol% or more and 99 mol% or less, The polyester film for foldable displays contains 1 mol % or more and 70 mol % or less of a bisphenol A-ethylene oxide adduct in the diol component (a-2).

Images