JP7815738B2 - Polyester film and method for producing polyester film - Google Patents

Description

The present invention relates to a polyester film and a method for producing a polyester film.

Polyester films, such as polyethylene terephthalate film and polyethylene naphthalate film, are highly versatile and used in a variety of applications due to their excellent heat resistance, weather resistance, mechanical strength, transparency, and chemical resistance, as well as their affordable availability.

Thermoplastic polyesters such as polyethylene terephthalate and polyethylene naphthalate, which are resins constituting polyester films, have excellent heat resistance and can therefore be used continuously under high temperature and high humidity conditions.
However, materials undergo plastic deformation at high temperatures, and molded products such as films are easily deformed by bending, so their use is currently limited in applications requiring bending resistance.

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

For example, Patent Document 1 discloses a foldable display that is easy to mass-produce and does not cause distortion of the image displayed at the fold after folding, as well as a polyester film for foldable displays that does not cause creases or cracks at the fold, making it possible to provide mobile terminal devices equipped with such a foldable display.

International Publication No. 2021/182191

However, although the polyester film described in Patent Document 1 has excellent flex resistance at room temperature, it does not have sufficient flex resistance at high temperatures, making it difficult to apply to flexible displays.

The present invention has been made in consideration of the above-mentioned circumstances, and its objective is to provide a polyester film that has excellent flex resistance under high-temperature conditions and is suitable for use in flexible displays, as well as a method for producing the polyester film.

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 that has been subjected to heat treatment, wherein the ratio (Xa/Ya) of the average curl angle (Xa) in each of the longitudinal direction (MD) and transverse direction (TD) of the film after the heat treatment to the average curl angle (Ya) in each of the longitudinal direction (MD) and transverse direction (TD) of the film before the heat treatment is 0.95 or less.
(Here, the curl angle refers to the value obtained by subtracting the angle of the crease formed after the measurement conditions below from 180°.
Measurement conditions: The film is folded 180° with a gap of 2.0 mm, kept bent, and left in an oven at 90°C for 6 hours. Then, it is taken out to room temperature, unfolded, and left as is for 24 hours.
[2] The polyester film according to the above [1], wherein the ratio (Xb/Yb) of the average stress (Xb) at 5% strain in each of the machine direction (MD) and the width direction (TD) of the film after the heat treatment to the average stress (Yb) at 5% strain in each of the machine direction (MD) and the width direction (TD) of the film before the heat treatment is 1.01 or more.
[3] The polyester film according to the above [1] or [2], which has a thickness of 9 to 125 μm.
[4] The polyester film according to any one of [1] to [3] above, which is a biaxially stretched film.
[5] The polyester film according to any one of [1] to [4] above, which is for a flexible display.
[6] A flexible display comprising the polyester film according to any one of [1] to [5] above.
[7] A method for producing a polyester film, comprising a step of performing a heat treatment at a temperature of 100 to 160°C for a time of 15 to 90 minutes, the heat treatment being performed after a heat setting treatment step.
[8] The method for producing a polyester film according to the above [7], wherein the heat treatment is an annealing treatment.
[9] The method for producing a polyester film according to the above [7] or [8], wherein the heat treatment is carried out offline.

The present invention provides a polyester film that has excellent flex resistance under high-temperature conditions and is suitable for use in flexible displays, as well as a method for producing the polyester film.

FIG. 1 is a diagram showing a method for measuring a curl angle.

The present invention is described in detail below. However, the present invention is not limited to the embodiments described below.

<<Polyester film>>
The polyester film of the present invention (hereinafter also referred to as "the film") is heat-treated, and the ratio (Xa/Ya) of the average curl angle (Xa) in each of the longitudinal direction (MD) and transverse direction (TD) of the film after the heat treatment to the average curl angle (Ya) in each of the longitudinal direction (MD) and transverse direction (TD) of the film before the heat treatment is 0.95 or less.
(Here, the curl angle refers to the value obtained by subtracting the angle of the crease formed after the measurement conditions below from 180°.
Measurement conditions: The film is folded 180° with a gap of 2.0 mm, kept bent, and left in an oven at 90°C for 6 hours. Then, it is taken out to room temperature, unfolded, and left as is for 24 hours.

More specific embodiments of the present film include the following first to third embodiments.
The film of the first embodiment contains polyethylene terephthalate (hereinafter also referred to as "PET") and has an average curl angle of 151° or less in both the machine direction (MD) and the cross direction (TD).
The film of the second embodiment contains homopolyethylene naphthalate (hereinafter also referred to as "homoPEN") and has an average curl angle of 121° or less in both the machine direction (MD) and the cross direction (TD).
The film of the third embodiment contains a polyethylene naphthalate copolymer (hereinafter also referred to as "PEN copolymer") and has an average curl angle of 128° or less in both the machine direction (MD) and the cross direction (TD).

In the present invention, the longitudinal direction (MD) of the film refers to the direction in which the film advances during the film manufacturing process, i.e., the winding direction of the film roll. The width 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.

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 a four-layer structure or more, provided that the number of layers is within the scope of the present invention, and 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.

The film may be an unstretched film (sheet) or a stretched film. Of these, a uniaxially or biaxially stretched film is preferred, and a biaxially stretched film is even more preferred from the standpoints of balance of mechanical properties, flatness, and thinning.

<Polyester>
The polyester used as the raw material for the present film is not particularly limited, and may be a homopolyester or a copolymer polyester. Specific examples include polyesters obtained by polycondensation of dicarboxylic acid and diol.

The present film preferably contains polyester as a main component, and when the present film has a multi-layer structure, the main component resin of each layer is preferably polyester.
The term "main component resin" refers to the resin that is contained in the largest proportion among the resins constituting each layer, and is, for example, a resin that accounts for 50% by mass or more, particularly 70% by mass or more, and even more particularly 80% by mass or more (including 100% by mass) of the resins constituting each layer.

Examples of the dicarboxylic acids include aromatic dicarboxylic acids such as terephthalic acid, 2,6-naphthalenedicarboxylic 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.

Examples of the diols include ethylene glycol, 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 their derivatives, or ethylene oxide adducts thereof).

Typical polyesters include polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, and polybutylene naphthalate.

Examples of copolymer polyesters include copolymer polyesters that contain a third component as a copolymerization component other than the compound that is the main component of the dicarboxylic acid that constitutes the polyester and the compound that is the main component of the diol. For example, in the case of polyethylene terephthalate, the third component is a component other than terephthalic acid and ethylene glycol.

From the standpoints of mechanical properties, heat resistance, transparency, and cost, the polyester used in this film is preferably polyethylene terephthalate, polyethylene naphthalate, or a copolymer polyester whose main component is one of these polyesters.

Usually, when polyester 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-product 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, 5 mol% or less of diethylene glycol is defined as by-product diethylene glycol, and this by-product diethylene glycol is also considered to be included in ethylene glycol and is distinguished from a copolymerization component. On the other hand, depending on the diethylene glycol content, more specifically, when diethylene glycol is contained in excess of 5 mol%, diethylene glycol is treated as a copolymerization component rather than as a by-product diethylene glycol.

Next, we will explain the polyester contained in the films of the first to third embodiments.

(First Aspect)
The film of the first embodiment comprises PET.
The content of PET 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 PET and the total PET content in the entire film is the same as above. In particular, it is preferable that each layer contains PET and the PET content in each layer is the same as above.

Specifically, the PET contains a dicarboxylic acid (a-1) and a diol (b-1), and more specifically, the dicarboxylic acid (a-1) is terephthalic acid and the diol (b-1) is ethylene glycol.

The PET may be a homopolyester or a copolymer polyester, and among these, a homopolyester is preferred.
In the case of a homopolyester, all (100 mol %) of the dicarboxylic acid (a-1) is terephthalic acid, and all (100 mol %) of the diol (b-1) is ethylene glycol.
In the case of a copolymer polyester, the dicarboxylic acid (a-1) contains terephthalic acid and the diol (b-1) contains ethylene glycol, and at least one of the dicarboxylic acid (a-1) and the diol (b-1) contains a copolymerization component. As the copolymerization component, the above-mentioned dicarboxylic acid other than terephthalic acid and/or the above-mentioned diol other than ethylene glycol can be selected. More specifically, the dicarboxylic acid may contain a dicarboxylic acid other than terephthalic acid in an amount of about 30 mol % or less, and the diol may contain a diol other than ethylene glycol in an amount of about 30 mol % or less. When the amount is 30 mol % or less, the film strength, transparency, and heat resistance are sufficiently maintained.

(Second Aspect)
The film of the second embodiment comprises homo-PEN.
The content of homo-PEN 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 homo PEN and the total homo PEN content in the entire film is the same as above. It is particularly preferable that each layer contains homo PEN and the homo PEN content in each layer is the same as above.

The homo-PEN specifically contains a dicarboxylic acid (a-2) and a diol (b-2), and more specifically, all (100 mol%) of the dicarboxylic acid (a-2) is 2,6-naphthalenedicarboxylic acid, and all (100 mol%) of the diol (b-2) is ethylene glycol.

(Third Aspect)
The film of the third embodiment 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 a PEN copolymer and the total content of the PEN copolymer in the entire film is the same as above, and it is particularly preferable that each layer contains a PEN copolymer and the content of the PEN copolymer in each layer is the same as above.

The PEN copolymer specifically contains a dicarboxylic acid (a-3) and a diol (b-3). More specifically, the dicarboxylic acid (a-3) is 2,6-naphthalenedicarboxylic acid, and the diol (b-3) is ethylene glycol, and at least one of the dicarboxylic acid (a-3) and the diol (b-3) contains a copolymerization component.

The PEN copolymer preferably contains 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid (a-3) in an amount of 80 mol % or more, more preferably 90 mol % or more, and even more preferably all (100 mol %) of the dicarboxylic acid (a-3) is 2,6-naphthalenedicarboxylic acid.
By adjusting the content of 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid (a-3) to 80 mol % or more, the film strength, transparency, and heat resistance are sufficiently maintained.

The PEN copolymer preferably contains 30 mol % or more of ethylene glycol in the diol (b-3), 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. On the other hand, the upper limit of the ethylene glycol content in the diol (b-3) 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 (b-3) within this range, the film strength, transparency, and heat resistance are sufficiently maintained.

The PEN copolymer preferably contains 20 mol % or less, more preferably 10 mol % or less of a copolymerization component in the dicarboxylic acid (a-3), and even more preferably, all of the dicarboxylic acid (a-3) is 2,6-naphthalenedicarboxylic acid, i.e., the copolymerization component is 0 mol %.
The PEN copolymer preferably contains 70 mol % or less of the copolymerization component in the diol (b-3), 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. On the other hand, the lower limit of the copolymerization component in the diol (b-3) 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 (a-3) and/or the diol (b-3) within this range, it is possible to improve the flex resistance while sufficiently maintaining the film strength, transparency, and heat resistance.

Examples of copolymerization components added to the dicarboxylic acid (a-3) include the above-mentioned dicarboxylic acids other than 2,6-naphthalenedicarboxylic acid. From the viewpoint of moldability, isophthalic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, and 3,4-furandicarboxylic acid are preferred. These copolymerization components can be used alone or in combination of two or more.

On the other hand, examples of copolymerization components added to the diol (b-3) include the above-mentioned diols other than ethylene glycol. From the viewpoint of flex resistance, 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, and bisphenols are preferred. Among these, from the viewpoint of maintaining the strength of the film, bisphenols are more preferred, and as the bisphenol, bisphenol A-ethylene oxide adduct is preferably used. These copolymerization components can be used alone or in combination of two or more.

The PEN copolymer most preferably contains 2,6-naphthalenedicarboxylic acid as the dicarboxylic acid (a-3), ethylene glycol as the diol (b-3), 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% or more and 70 mol% or less, more preferably 2 mol% or more and 50 mol% or less, even more preferably 3 mol% or more and 30 mol% or less, particularly preferably 4 mol% or more and 20 mol% or less, and especially preferably 5 mol% or more and 10 mol% or less, based on the total amount of the diol (b-3).

When the 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.

From here on, we will explain this film, not just the films of the first to third embodiments.

Polyester polymerization catalysts are not particularly limited, and conventionally known compounds can be used, such as titanium compounds, germanium compounds, antimony compounds, manganese compounds, aluminum compounds, magnesium compounds, and calcium compounds. Among these, at least one selected from titanium compounds and antimony compounds is preferred.

In order to suppress the amount of precipitation of oligomer components, the film may be produced using a polyester having a low content of oligomer components. As a method for producing a polyester having a low content of oligomer components, various known methods can be used, such as a method in which solid-state polymerization is carried out after the polyester is produced.
The amount of oligomer component precipitation may be suppressed by configuring the present film to have three or more layers and using a polyester having a low content of oligomer components as the outermost layer of the present film.
The polyester may be obtained by carrying out the esterification or transesterification reaction, followed by further increasing the reaction temperature and carrying out melt polycondensation under reduced pressure.

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

The present film may contain resins other than polyester as long as the effects of the present invention are not impaired.
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.

In the films of the first to third embodiments, the resin constituting the film may contain other polyesters as long as it contains the specific polyester.

<Particle>
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 thereof 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 polyester can also be used.

There are no particular limitations on the shape of the particles used, and they may be spherical, blocky, rod-like, flat, or the like.
There are also no particular limitations on the hardness, specific gravity, color, etc. These particles may be used alone or in combination of two or more types, as required.

The average particle size of the particles used is preferably 5 μm or less, more preferably 4 μm or less, and even more preferably 3 μm or less. On the other hand, 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 achieve both transparency and ease of handling.
When the particles are in the form of a powder, the average particle size of the particles can be determined by measuring the powder using a centrifugal sedimentation particle size distribution analyzer (for example, the "SA-CP3" manufactured by Shimadzu Corporation) and determining the particle size at an integrated volume fraction of 50% (d50) in the equivalent spherical distribution. The average particle size of the particles in the film, layer, or polyester can be determined by measuring the diameters of 10 or more particles using a scanning electron microscope (SEM) and calculating the average value. In this case, in the case of non-spherical particles, the average value of the longest diameter and the shortest diameter can be measured as the diameter of each particle. The same applies to the particles described below.

When the present film has a multilayer structure, the particles may be contained in at least one layer, but are preferably contained in the surface layer, which effectively imparts lubricity while reducing the particle content in the entire film.
The particles may be contained in one or both of the surface layers.

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 total mass of the film, while the lower limit is preferably 0.0003% by mass or more, more preferably 0.01% by mass or more.
When 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 when the particle content is 5% by mass or less, transparency is improved.
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.

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

<Others>
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.

<Thickness>
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.

<<Method of manufacturing polyester film>>
According to another aspect of the present invention, there is provided a method for producing a polyester film (hereinafter also referred to as "the method for producing the present film"), which comprises a step of performing a heat treatment at a temperature of 100 to 160°C for 15 to 90 minutes, the heat treatment being performed after the heat setting treatment step.

The method for producing the present film will be described below taking as an example a case where the present film is 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.

The unstretched sheet is preferably obtained by feeding raw materials such as the polyester mentioned above, and optional additives, into an extruder, mixing them appropriately, extruding the resulting molten sheet from a die using the extruder, and then cooling and solidifying it on a rotating cooling drum (cooling roll). In this case, to improve the flatness of the sheet, it is preferable to increase the adhesion between the sheet and the rotating cooling drum, and electrostatic application adhesion and/or liquid application adhesion methods are preferably used.

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 polyester 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.

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 70 to 140°C, preferably 80 to 130°C, and the stretching ratio is usually 2.5 to 7.0 times, preferably 3.0 to 6.0 times.
Next, the film is stretched in a direction perpendicular to the first-stage stretching direction. In this case, the stretching temperature is usually 70 to 170°C, preferably 110 to 140°C, and the stretching ratio is usually 3.0 to 7.0 times, preferably 3.5 to 6.0 times.

The film is then subsequently heat-treated under tension or relaxation of 30% or less at a temperature of 180 to 270°C to obtain a biaxially stretched film. The stretching may be performed in one direction in two or more stages. In this case, it is preferable to perform the stretching so that the final stretch ratios in both directions are within the above-mentioned ranges.
In the present invention, the above heat treatment is referred to as heat setting treatment, and is to be distinguished from the heat treatment carried out after the heat setting treatment step described below.

Examples of heat treatments that may be carried out after the heat setting step in the present film manufacturing method include annealing and aging, with annealing being preferred.
These heat treatments may be carried out offline after the formed film has been wound into a roll, or in-line during film formation, but are preferably carried out offline in consideration of the heat treatment temperature and time described below.

The heat treatment temperature after the heat setting step is 100 to 160°C, preferably 105 to 140°C, and more preferably 110 to 120°C.
The heat treatment time after the heat setting step is not particularly limited, but is 15 to 90 minutes, preferably 30 to 80 minutes, and more preferably 45 to 75 minutes.
By setting the heat treatment temperature and time after the heat setting step within the above ranges, the polymer chains constituting the film will have a stable conformation, and the film will have good flex resistance under high temperature conditions.

In the present invention, by carrying out the heat treatment after the heat setting step, the ratio (Xa/Ya) of the average curl angle (Xa) in each of the longitudinal direction (MD) and transverse direction (TD) of the film after the heat treatment to the average curl angle (Ya) in each of the longitudinal direction (MD) and transverse direction (TD) of the film before the heat treatment can be adjusted within a desired range.
Furthermore, by carrying out the heat treatment after the heat setting step, the average curl angle in each of the longitudinal direction (MD) and transverse direction (TD) of the films of the first to third embodiments can be adjusted to within the desired range.

<<Physical properties of this film>>
<Curl angle>
The bending resistance of the present film at high temperatures (90°C) can be evaluated by folding the film 180° with a gap of 2.0 mm, keeping the folded state, placing it in an oven at 90°C for 6 hours, then removing it to room temperature, immediately unfolding it, and leaving it as is for 24 hours, and measuring the curl angle. More specifically, it can be evaluated by the method described in the Examples.
The smaller the curl angle, the greater the restoring force of the film to return to its original state after bending, i.e., the better the bending resistance can be evaluated. The curl angle is evaluated as an average value, and 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).
The lower limit of the curl angle is preferably as small as possible, and is 0° or more.

The curl angle can be adjusted by adjusting the film-forming conditions, such as the polyester used, the stretching ratio, and the stretching temperature, but the reduction in the curl angle can be controlled by subjecting the polyester film to a heat treatment after the heat-setting process, or by adjusting the heat-treatment conditions after the heat-setting process.

The film has a ratio (Xa/Ya) of 0.95 or less between the average curl angle (Xa) in each of the longitudinal direction (MD) and transverse direction (TD) of the film after heat treatment and the average curl angle (Ya) in each of the longitudinal direction (MD) and transverse direction (TD) of the film before heat treatment.
The ratio (Xa/Ya) of the average values of the curl angles represents the degree of improvement in the bending resistance of the film after the heat treatment relative to the film before the heat treatment, and it can be said that the smaller the ratio (Xa/Ya) of the average values of the curl angles, the greater the effect of improving bending resistance due to the heat treatment after the heat setting treatment step.
Therefore, if the ratio (Xa/Ya) of the average values of the curl angles exceeds 0.95, the effect of improving bending resistance by the heat treatment after the heat setting step is insufficient, and bending resistance under high-temperature conditions becomes insufficient. If bending resistance is insufficient, for example, deformation of the film occurs when the display is opened, which may adversely affect the function of the display, such as reducing the visibility of the display. On the other hand, if the ratio (Xa/Ya) of the average values of the curl angles is 0.95 or less, it can be said that bending resistance under high-temperature conditions is good, and even under high-temperature conditions, deformation of the film is small and visibility of the display can be maintained good.
From this viewpoint, the ratio (Xa/Ya) of the average values of the curl angles is preferably 0.90 or less, and more preferably 0.85 or less. The lower limit of the ratio (Xa/Ya) of the average values of the curl angles is about 0.40.

Next, the physical properties of the films of the first to third embodiments will be described.
As described above, the absolute value of the curl angle can be adjusted by the film-forming conditions such as the stretching ratio and the stretching temperature, but it also depends greatly on the polyester used.
In the films of the first to third aspects, by containing a specific polyester and setting the average curl angle in each of the longitudinal direction (MD) and the transverse direction (TD) to a specific range, the ratio of the average curl angles before and after heat treatment (Xa/Ya) also falls within a desired range.
Therefore, in the films of the first to third aspects, by containing a specific polyester and setting the average curl angle to a specific value, specifically, the effect of improving the bending resistance under high temperature conditions by the heat treatment after the heat treatment step is sufficient, and as a result, a polyester film having excellent bending resistance under high temperature conditions is obtained that is suitable for flexible displays.

(First Aspect)
The film of the first embodiment has an average curl angle of 151° or less after the bending test in both the machine direction (MD) and the cross direction (TD).
If the average curl angle exceeds 151°, the film will not have enough restoring force to return to its original state after bending, resulting in insufficient bending resistance under high-temperature conditions. Insufficient bending resistance may adversely affect the function of the display, for example, by causing deformation of the film when the display is opened, reducing the visibility of the display. On the other hand, if the average curl angle is 151° or less, the bending resistance under high-temperature conditions can be said to be good, and even under high-temperature conditions, the film will deform little, allowing the display visibility to be maintained at a good level.
From this viewpoint, the average value of the curl angle is preferably 147° or less, and more preferably 143° or less.

(Second Aspect)
The film of the second embodiment has an average curl angle of 121° or less after the bending test in both the machine direction (MD) and the transverse direction (TD).
If the average value of the curl angle exceeds 121°, the film will not have enough restoring force to return to its original state after bending, resulting in insufficient bending resistance under high-temperature conditions. If the bending resistance is insufficient, the film may deform when the display is opened, potentially adversely affecting the function of the display, such as reducing the visibility of the display. On the other hand, if the average value of the curl angle is 121° or less, it can be said that the bending resistance under high-temperature conditions is good, and even under high-temperature conditions, the film will deform little and the visibility of the display can be maintained good.
From this viewpoint, the average value of the curl angle is preferably 114° or less, and more preferably 108° or less.

(Third Aspect)
The film of the third embodiment has an average curl angle of 128° or less after the bending test in both the machine direction (MD) and the transverse direction (TD).
If the average value of the curl angle exceeds 128°, the film will not have enough restoring force to return to its original state after bending, resulting in insufficient bending resistance under high-temperature conditions. If the bending resistance is insufficient, the film may deform when the display is opened, potentially adversely affecting the function of the display, such as reducing the visibility of the display. On the other hand, if the average value of the curl angle is 128° or less, it can be said that the bending resistance under high-temperature conditions is good, and even under high-temperature conditions, the film will deform little and the visibility of the display can be maintained good.
From this viewpoint, the average value of the curl angle is preferably 121° or less, and more preferably 115° or less.

<Stress at 5% strain>
The stress at 5% strain of the present film can be measured by the method described in the Examples.
At a strain of 5%, the polymer chains constituting the film undergo plastic deformation, and even when unloaded, they do not return to their original shape, leaving a permanent strain. The increase in stress at a strain of 5% due to heat treatment indicates that the conformation of the polymer chains has stabilized. In other words, it can be evaluated that the film has been altered to have increased resistance to deformation, less traces of deformation, and excellent deformation resistance. The stress at a strain of 5% is evaluated as an average value, and the average value here refers to the average value of the stress at a strain of 5% in the machine direction (MD) and the stress at a strain of 5% in the transverse direction (TD).

The stress at 5% strain can be adjusted by adjusting the film-forming conditions, such as the polyester used, the stretching ratio, and the stretching temperature, but the increase in stress at 5% strain can be controlled by subjecting the polyester film to a heat treatment after the heat-setting process, or by adjusting the heat-treatment conditions after the heat-setting process.

The present film preferably has a ratio (Xb/Yb) of 1.01 or more between the average stress (Xb) at 5% strain in each of the longitudinal direction (MD) and transverse direction (TD) of the film after heat treatment and the average stress (Yb) at 5% strain in each of the longitudinal direction (MD) and transverse direction (TD) of the film before heat treatment.
The ratio (Xb/Yb) of the average stress values at a strain of 5% represents the degree of improvement in deformation resistance of the film after heat treatment compared to the film before heat treatment, and it can be said that the larger the ratio (Xb/Yb) of the average stress values at a strain of 5%, the greater the effect of improving deformation resistance due to the heat treatment after the heat setting treatment step.
Therefore, if the ratio of the average stresses at a strain of 5% (Xb/Yb) is 1.01 or more, the effect of improving the deformation resistance by the heat treatment after the heat setting step can be said to be sufficient. Furthermore, the resistance to stresses applied by deformation becomes good, and for example, the deformation of the film when the display is opened is small, making it possible to maintain good visibility of the display.
From this viewpoint, the ratio (Xb/Yb) of the average stress values at a strain of 5% is preferably 1.02 or more, and the upper limit of the ratio (Xb/Yb) of the average stress values at a strain of 5% is about 1.70.

Next, the physical properties of the films of the first to third embodiments will be described.
The absolute value of the stress at a strain of 5% can also be adjusted by the film-forming conditions such as the stretching ratio and the stretching temperature, as described above, but it largely depends on the polyester used.
In the films of the first to third aspects, by containing a specific polyester and setting the average stress values at 5% strain in each of the longitudinal direction (MD) and the transverse direction (TD) to be within specific ranges, the ratio of the average stress values at 5% strain before and after heat treatment (Xb/Yb) also falls within a desired range.
Therefore, in the films of the first to third embodiments, by containing a specific polyester and setting the average stress at a strain of 5% to a specific value, specifically, by heat treatment after the heat treatment step, the conformation of the polymer chains is stabilized, and as a result, deformation resistance is improved.

(First Aspect)
The film of the first embodiment preferably has an average stress of 110 MPa or more at a strain of 5% in both the machine direction (MD) and the transverse direction (TD).
If the average stress value at a strain of 5% is 110 MPa or more, the film has good resistance to stress applied by deformation, and for example, when the display is opened, the film deforms less, making it possible to maintain good visibility of the display.
From this viewpoint, the average value of the stress at a strain of 5% is more preferably 111 MPa or more, and even more preferably 112 MPa or more.
The upper limit of the average value of the stress at a strain of 5% is not particularly limited, but is about 140 MPa.

(Second Aspect)
The film of the second embodiment preferably has an average stress of 147 MPa or more at a strain of 5% in both the machine direction (MD) and the transverse direction (TD).
If the average stress at a strain of 5% is 147 MPa or more, the film will have good resistance to stress applied by deformation, and for example, the film will deform less when the display is opened, making it possible to maintain good visibility of the display.
From this viewpoint, the average value of the stress at a strain of 5% is more preferably 148 MPa or more, and even more preferably 149 MPa or more.
The upper limit of the average value of the stress at a strain of 5% is not particularly limited, but is about 180 MPa.

(Third Aspect)
The film of the third embodiment preferably has an average stress of 140 MPa or more at a strain of 5% in both the machine direction (MD) and the transverse direction (TD).
If the average stress at a strain of 5% is 140 MPa or more, the film has good resistance to stress applied by deformation, and for example, when the display is opened, the film deforms less, making it possible to maintain good visibility of the display.
From this viewpoint, the average value of the stress at a strain of 5% is more preferably 141 MPa or more, and even more preferably 142 MPa or more.
The upper limit of the average value of the stress at a strain of 5% is not particularly limited, but is about 170 MPa.

<<Applications>>
Because the present film has excellent flex resistance under high-temperature conditions, it can be suitably used for displays, particularly flexible displays. Flexible displays include foldable displays, bendable displays, rollable displays, and stretchable displays. The present film is particularly suitable for foldable displays.
The foldable display may be folded into three or four.

The display may be used in mobile phones, smartphones, digital cameras, personal computers, etc. There are no particular limitations on the type of display, but examples include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs, with bendable LCDs, organic EL displays, and inorganic EL displays being preferred. Of these, organic EL displays and inorganic EL displays, which can reduce the number of layers, are more preferred, and organic EL displays, which have a wide color gamut, are even more preferred.

In the present invention, "for flexible displays" refers to any component of a flexible display, including films that protect the front side of a display device (surface protection film), substrate films for touch sensors, and films that protect the back side of a display device (rear protection film).

<<Term explanation>>
In the present invention, the term "film" includes the term "sheet", and the term "sheet" includes the term "film".

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.

The present invention will be explained in more detail below using examples. However, the present invention is not limited to the following examples in any way.

<Evaluation method>
(1) Average particle size of particles The average particle size of particles added to a polyester film was measured by the following method: The particle size at 50% of the cumulative volume distribution (d50) in the equivalent spherical distribution measured using a centrifugal sedimentation particle size distribution analyzer (SA-CP3 model) manufactured by Shimadzu Corporation was taken as the average particle size.

(2) Flexibility (curl angle) under high temperature (90°C) conditions
A test piece measuring 100 mm in the longitudinal direction (MD) x 30 mm in the transverse direction (TD) was cut out from any location of the polyester film, and the film was folded in half in the longitudinal direction (MD) with a gap of 2.0 mm, and then placed in an unhumidified 90 ° C. oven for 6 hours. The test piece was then removed to room temperature, immediately released, and left for 24 hours, after which the curl angle (MD) was measured. The curl angle (MD) was measured at three locations on the same film, and the average value was taken as the curl angle (MD). The curl angle (TD) was measured by cutting a test piece measuring 30 mm in the longitudinal direction (MD) x 100 mm in the transverse direction (TD) from any location of the polyester film, and the film was folded in half in the transverse direction (TD) with a gap of 2.0 mm, and the curl angle (TD) was then measured in the same manner.

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 so that the fold line was oriented vertically, and the angle of the fold line (fold line angle) when viewed from directly above was measured as shown in Figure 1. The curl angle was calculated by subtracting the fold line angle from 180°.
In addition, the average of the measured curl angle in the machine direction (MD) (the curl angle (MD)) and the curl angle in the width direction (TD) (the curl angle (TD)) was calculated and used as the average curl angle.

(3) Stress at 5% Strain A test piece measuring 150 mm in the longitudinal direction (MD) × 15 mm in the transverse direction (TD) was cut out from any position of the polyester film, and a tensile test was performed using a tensile tester Model 2005 manufactured by Intesco Co., Ltd., with a chuck distance of 50 mm and a test speed of 200 mm/min, to measure the stress at 5% strain (MD). Similarly, a test piece cut out so that the longitudinal direction was TD and the transverse direction was MD was used to measure the stress at 5% strain (TD).
In addition, the average of the measured stress at 5% strain in the longitudinal direction (MD) (stress (MD) at 5% strain) and the stress at 5% strain in the transverse direction (TD) (stress (TD) at 5% strain) was calculated, and this was used as the average stress at 5% strain.

<Materials used>
[Polyethylene terephthalate (PET)]
Dicarboxylic acid (a-1): terephthalic acid = 100 mol %, diol (b-1): ethylene glycol = 100 mol % were used.
A masterbatch (PET-particle masterbatch) containing 0.7% by mass of silica particles having an average particle size of 2.3 μm in PET was also used.

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

[Polyethylene naphthalate copolymer (PEN copolymer)]
Dicarboxylic acid (a-3): 2,6-naphthalenedicarboxylic acid = 100 mol %, diol (b-3): ethylene glycol = 95 mol %, bisphenol A-ethylene oxide adduct = 5 mol % were used.

Example 1
A raw material obtained by dry-blending PET (70% by mass) and a PET-particle masterbatch (30% by mass) for the surface layer and PET (100% by mass) for the middle layer were fed into separate twin-screw extruders, extruded together at 280°C, and cooled and solidified on a cooling roll set at 25°C using an electrostatic adhesion method, to obtain a two-kind, 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 width direction (TD) at 115°C. Finally, it was heat-set at 235°C to obtain a polyester film having a thickness of 50 μm (thickness ratio: surface layer/intermediate layer/surface layer = 1/8/1).
The polyester film thus obtained was subjected to offline annealing at 110° C. for 1 hour to obtain a polyester film that had been subjected to heat treatment after the heat setting step. The evaluation results are shown in Table 1.

Example 2
The homo-PEN was fed into a twin-screw extruder and extruded at 300°C, and 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 3.4 times in the machine direction (MD) at 128°C using a roll stretching machine. After preheating at 125°C in a tenter, the sheet was stretched 4.1 times in the transverse direction (TD) at 135°C. Finally, the sheet was heat-set at 230°C to obtain a polyester film having a thickness of 50 µm.
The polyester film thus obtained was subjected to offline annealing at 100° C. for 1 hour to obtain a polyester film that had been subjected to heat treatment after the heat setting step. The evaluation results are shown in Table 1.

(Examples 3 to 6)
A polyester film that had been subjected to heat treatment after the heat setting step was obtained in the same manner as in Example 2, except that offline annealing was performed under the heat treatment conditions shown in Table 1. The evaluation results are shown in Table 1.

Example 7
The PEN copolymer was fed into a twin-screw extruder, extruded at 300°C, and 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 3.4 times in the machine direction (MD) at 128°C using a roll stretching machine. After preheating at 125°C in a tenter, the sheet was stretched 4.1 times in the transverse direction (TD) at 135°C. Finally, the sheet was heat-set at 210°C to obtain a polyester film having a thickness of 50 µm.
The polyester film thus obtained was subjected to offline annealing at 100° C. for 1 hour to obtain a polyester film that had been subjected to heat treatment after the heat setting step. The evaluation results are shown in Table 1.

(Examples 8 to 10)
A polyester film that had been subjected to heat treatment after the heat setting step was obtained in the same manner as in Example 7, except that offline annealing was performed under the heat treatment conditions shown in Table 1. The evaluation results are shown in Table 1.

(Comparative Examples 1 and 2)
A polyester film that had been subjected to heat treatment after the heat setting step was obtained in the same manner as in Example 1, except that offline annealing was performed under the heat treatment conditions shown in Table 1. The evaluation results are shown in Table 1.

(Comparative Example 3)
A polyester film that had been subjected to heat treatment after the heat setting step was obtained in the same manner as in Example 2, except that offline annealing was performed under the heat treatment conditions shown in Table 1. The evaluation results are shown in Table 1.

(Comparative Example 4)
A polyester film that had been subjected to heat treatment after the heat setting step was obtained in the same manner as in Example 7, except that offline annealing was performed under the heat treatment conditions shown in Table 1. The evaluation results are shown in Table 1.

(Reference example 1)
A polyester film was obtained in the same manner as in Example 1, except that no heat treatment was carried out after the heat setting step. The evaluation results are shown in Table 2.
If Example 1 and Comparative Examples 1 and 2 are films after heat treatment, Reference Example 1 corresponds to the film before heat treatment.

(Reference example 2)
A polyester film was obtained in the same manner as in Example 2, except that no heat treatment was carried out after the heat setting step. The evaluation results are shown in Table 2.
If Examples 2 to 6 and Comparative Example 3 are films after heat treatment, Reference Example 2 corresponds to the film before heat treatment.

(Reference example 3)
A polyester film was obtained in the same manner as in Example 7, except that no heat treatment was carried out after the heat setting step. The evaluation results are shown in Table 2.
If Examples 7 to 10 and Comparative Example 4 are films after heat treatment, then Reference Example 3 corresponds to the film before heat treatment.

As shown in the above examples, by subjecting a polyester film to heat treatment after the heat setting process and setting the ratio (Xa/Ya) of the average curl angle (Xa) in each of the longitudinal direction (MD) and transverse direction (TD) of the film after the heat treatment to the average curl angle (Ya) in each of the longitudinal direction (MD) and transverse direction (TD) of the film before the heat treatment to a specific value or less, the heat treatment after the heat setting process has a sufficient effect of improving bending resistance, and bending resistance under high temperature conditions is excellent.
In contrast, the polyester film of the comparative example, although having the same composition as that of the example, had the ratio (Xa/Ya) of the average curl angles above a specific value, and therefore did not have sufficient flex resistance under high temperature conditions.

Furthermore, as shown in the above examples, by setting the ratio (Xb/Yb) of the average stress (Xb) at 5% strain in each of the longitudinal direction (MD) and transverse direction (TD) of the film after heat treatment to the average stress (Yb) at 5% strain in each of the longitudinal direction (MD) and transverse direction (TD) of the film before heat treatment to a specific value or more, the effect of improving deformation resistance by heat treatment after the heat setting treatment step becomes sufficient.
In contrast, the polyester film of the comparative example, although having the same composition as that of the example, did not have sufficient deformation resistance because the ratio of the average stresses at a strain of 5% (Xb/Yb) did not satisfy the specific value.

The polyester film of the example therefore has good bending resistance under high temperature conditions and resistance to stress caused by deformation, so that, for example, when the display is opened, the film undergoes little deformation, and the display's visibility can be maintained at a good level.

Next, examples of films according to the first to third aspects are shown.

<First Aspect>
(Example 1-1)
A raw material obtained by dry-blending PET (70% by mass) and a PET-particle masterbatch (30% by mass) for the surface layer and PET (100% by mass) for the middle layer were fed into separate twin-screw extruders, extruded together at 280°C, and cooled and solidified on a cooling roll set at 25°C using an electrostatic adhesion method, to obtain a two-kind, 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 width direction (TD) at 115°C. Finally, it was heat-set at 235°C to obtain a polyester film having a thickness of 50 μm (thickness ratio: surface layer/intermediate layer/surface layer = 1/8/1).
The polyester film thus obtained was subjected to offline annealing treatment at 110° C. for 1 hour to obtain a polyester film that had been subjected to heat treatment after the heat setting treatment step. The evaluation results are shown in Table 3.

(Comparative Examples 1-1 to 1-2)
A polyester film that had been subjected to heat treatment after the heat setting step was obtained in the same manner as in Example 1-1, except that offline annealing was performed under the heat treatment conditions shown in Table 3. The evaluation results are shown in Table 3.

(Comparative Examples 1-3)
A polyester film was obtained in the same manner as in Example 1-1, except that no heat treatment was performed after the heat setting treatment step. The evaluation results are shown in Table 3.

In the first aspect, as shown in the above examples, the film contains polyethylene terephthalate and has an average curl angle of 151° or less in both the longitudinal direction (MD) and the transverse direction (TD), thereby achieving excellent bending resistance under high temperature conditions.
In contrast, the polyester film of the comparative example, although having the same composition as that of the example, had an average curl angle exceeding 151°, and therefore did not have sufficient flex resistance under high temperature conditions.

<Second Aspect>
(Example 2-1)
The homo-PEN was fed into a twin-screw extruder and extruded at 300°C, and 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 3.4 times in the machine direction (MD) at 128°C using a roll stretching machine. After preheating at 125°C in a tenter, the sheet was stretched 4.1 times in the transverse direction (TD) at 135°C. Finally, the sheet was heat-set at 230°C to obtain a polyester film having a thickness of 50 µm.
The polyester film thus obtained was subjected to offline annealing treatment at 100° C. for 1 hour to obtain a polyester film that had been subjected to heat treatment after the heat setting treatment step. The evaluation results are shown in Table 4.

(Examples 2-2 to 2-5, Comparative Example 2-1)
A polyester film that had been subjected to heat treatment after the heat setting step was obtained in the same manner as in Example 2-1, except that offline annealing was performed under the heat treatment conditions shown in Table 4. The evaluation results are shown in Table 4.

(Comparative Example 2-2)
A polyester film was obtained in the same manner as in Example 2-1, except that no heat treatment was performed after the heat setting step. The evaluation results are shown in Table 4.

In the second aspect, as shown in the above examples, by containing homopolyethylene naphthalate and setting the average curl angle in each of the longitudinal direction (MD) and the transverse direction (TD) to 121° or less, excellent bending resistance under high temperature conditions is achieved.
In contrast, the polyester film of the comparative example, although having the same composition as that of the example, had an average curl angle exceeding 121°, and therefore did not have sufficient flex resistance under high temperature conditions.

<Third Aspect>
(Example 3-1)
The PEN copolymer was fed into a twin-screw extruder, extruded at 300°C, and 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 3.4 times in the machine direction (MD) at 128°C using a roll stretching machine. After preheating at 125°C in a tenter, the sheet was stretched 4.1 times in the transverse direction (TD) at 135°C. Finally, the sheet was heat-set at 210°C to obtain a polyester film having a thickness of 50 µm.
The polyester film thus obtained was subjected to offline annealing at 100° C. for 1 hour to obtain a polyester film that had been subjected to heat treatment after the heat setting step. The evaluation results are shown in Table 5.

(Examples 3-2 to 3-4, Comparative Example 3-1)
A polyester film that had been subjected to heat treatment after the heat setting step was obtained in the same manner as in Example 3-1, except that offline annealing was performed under the heat treatment conditions shown in Table 5. The evaluation results are shown in Table 5.

(Comparative Example 3-2)
A polyester film was obtained in the same manner as in Example 3-1, except that no heat treatment was performed after the heat setting step. The evaluation results are shown in Table 5.

In the third aspect, as shown in the above examples, the film contains a polyethylene naphthalate copolymer, and the average curl angle in each of the longitudinal direction (MD) and the transverse direction (TD) is set to 128° or less, thereby achieving excellent bending resistance under high temperature conditions.
In contrast, the polyester film of the comparative example, although having the same composition as that of the example, had an average curl angle exceeding 128°, and therefore did not have sufficient flex resistance under high temperature conditions.

The polyester film of the present invention has excellent flex resistance under high temperature conditions, and therefore, even under high temperature conditions, the film undergoes little deformation when, for example, a display is opened, and good visibility of the display can be maintained.
Thus, the polyester film of the present invention has achieved an improvement in flex resistance at high temperatures, which has been a problem in the past, and can be suitably used particularly for 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 (17)

Contains 50% by mass or more of polyethylene terephthalate,
A polyester film for flexible displays, having an average curl angle (Xa) of 151° or less in both the machine direction (MD) and the width direction (TD) of the film.
(Here, the curl angle refers to the value obtained by subtracting the angle of the crease formed after the measurement conditions below from 180°.
Measurement conditions: The film is folded 180° with a gap of 2.0 mm, kept bent, and left in an oven at 90°C for 6 hours. Then, it is taken out to room temperature, unfolded, and left as is for 24 hours. Contains 50% by mass or more of homopolyethylene naphthalate,
A polyester film for flexible displays, having an average curl angle (Xa) of 121° or less in both the machine direction (MD) and the width direction (TD) of the film.
(Here, the curl angle refers to the value obtained by subtracting the angle of the crease formed after the measurement conditions below from 180°.
Measurement conditions: The film is folded 180° with a gap of 2.0 mm, kept bent, and left in an oven at 90°C for 6 hours. Then, it is taken out to room temperature, unfolded, and left as is for 24 hours. Contains 50% by mass or more of a polyethylene naphthalate copolymer containing a structural unit derived from a dicarboxylic acid and a structural unit derived from a diol,
the polyethylene naphthalate copolymer is one in which all of the dicarboxylic acids are 2,6-naphthalenedicarboxylic acid, or the diol contains 4 mol % or more and 70 mol % or less of at least one selected from 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, and bisphenols;
A polyester film for flexible displays, having an average curl angle (Xa) of 128° or less in both the machine direction (MD) and the width direction (TD) of the film.
(Here, the curl angle refers to the value obtained by subtracting the angle of the crease formed after the measurement conditions below from 180°.
Measurement conditions: The film is folded 180° with a gap of 2.0 mm, kept bent, and left in an oven at 90°C for 6 hours. Then, it is taken out to room temperature, unfolded, and left as is for 24 hours. The polyester film for flexible displays according to claim 3, wherein all of the dicarboxylic acids are 2,6-naphthalenedicarboxylic acid, and the diol contains at least one selected from 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, and bisphenols in an amount of 4 mol % to 70 mol %. The polyester film for flexible displays according to claim 1, wherein the average stress (Xb) at a strain of 5% in both the machine direction (MD) and the width direction (TD) of the film is 110 MPa or more. The polyester film for flexible displays according to claim 2, wherein the average stress (Xb) at a strain of 5% in both the machine direction (MD) and the transverse direction (TD) of the film is 147 MPa or more. The polyester film for flexible displays according to claim 3 or 4, wherein the average stress (Xb) at a strain of 5% in both the machine direction (MD) and the transverse direction (TD) of the film is 140 MPa or more. The polyester film for flexible displays according to any one of claims 1 to 7, having a thickness of 9 to 125 μm. The polyester film for flexible displays according to any one of claims 1 to 8, which is a biaxially stretched film. The polyester film for flexible displays according to any one of claims 1 to 9, wherein the flexible display is a foldable display or a bendable display. The polyester film for flexible displays according to any one of claims 1 to 10, used as a protective film for a display device or a substrate film for a touch sensor. A flexible display comprising the polyester film for flexible displays described in any one of claims 1 to 11. A foldable display comprising the polyester film for flexible displays described in any one of claims 1 to 11. A bendable display comprising the polyester film for flexible displays described in any one of claims 1 to 11. A method for producing a polyester film for flexible displays, comprising:
The method includes a step of subjecting the polyester film to a heat treatment at a temperature of 100 to 160°C for a time of 15 to 90 minutes,
The heat treatment is carried out after the heat setting treatment step.
A manufacturing method for polyester films for flexible displays. The method for producing a polyester film for flexible displays according to claim 15, wherein the heat treatment is an annealing treatment. The method for producing a polyester film for flexible displays according to claim 15 or 16, wherein the heat treatment is performed offline.