JP2023090081A - Polyester film and method for producing polyester film - Google Patents

Abstract

To provide a polyester film which is suitable for a flexible display and has excellent bending resistance under a high temperature condition, and to provide a method for producing the polyester film.SOLUTION: There is provided a heat-treated polyester film, wherein a ratio (Xa/Ya) of the average value (Xa) of curl angles in each of the machine direction (MD) and the transverse direction (TD) of the film after heat treatment to the average value (Ya) of curl angles in each of the machine direction (MD) and the transverse direction (TD) of the film before heat treatment is 0.95 or less. (Wherein, the curl angle refers to a value obtained by subtracting an angle formed by a crease formed after the following measurement conditions from 180°. The film is folded 180° in a gap of 2.0 mm, held in a bent state, and allowed to stand in an oven at 90°C for 6 hours. After that, it is taken out to room temperature, the bend is released, and left as it is for 24 hours.)SELECTED DRAWING: None

Description

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

Polyester films represented by polyethylene terephthalate film and polyethylene naphthalate film are excellent in heat resistance, weather resistance, mechanical strength, transparency, chemical resistance, etc. It is highly versatile and used for various purposes.

Thermoplastic polyesters such as polyethylene terephthalate and polyethylene naphthalate, which are resins constituting polyester films, have excellent heat resistance and can be used continuously under high temperature and high humidity conditions.
However, at high temperatures, the material undergoes plastic deformation, and in particular, molded articles such as films are easily deformed by bending.

On the other hand, in recent years, flexible substrates and flexible printed circuits have tended to be used along with the miniaturization and weight reduction of electronic devices and the like. Along with this trend, there is an increasing demand for flexibility in display applications, and there is a strong demand for films that are excellent in resilience and resistance to repeated bending (bending resistance).

For example, Patent Literature 1 provides a foldable display that is excellent in mass productivity and does not cause disturbance in the image displayed on the folded portion after being folded, and a mobile terminal equipped with such a foldable display. To enable this, a polyester film for folding displays is disclosed that does not crease or crack at the folds.

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 under high temperature conditions, and is sometimes difficult to apply to flexible displays. .

The present invention has been made in view of the above circumstances, and the problem to be solved is to provide a polyester film suitable for use in flexible displays and having excellent flex resistance under high temperature conditions, and a method for producing the polyester film. It is in.

The inventors of the present invention have completed the present invention as a result of earnest studies to achieve the above-mentioned problems. That is, the present invention has the following aspects.
[1] A polyester film subjected to heat treatment, the average value (Xa) of the curl angles in the longitudinal direction (MD) and the transverse direction (TD) of the film after heat treatment, and the longitudinal direction of the film before heat treatment ( A polyester film having a ratio (Xa/Ya) of 0.95 or less to the average value (Ya) of curl angles in the MD) and the width direction (TD).
(Here, the curl angle refers to the value obtained by subtracting the angle formed by the crease formed after the following measurement conditions from 180°.
Measurement conditions: The film is folded 180° in a gap of 2.0 mm, held bent, and allowed to stand in an oven at 90°C for 6 hours. After that, it is taken out to room temperature, the bend is released, and left as it is for 24 hours. )
[2] The average value (Xb) of the stress at 5% strain in each of the longitudinal direction (MD) and transverse direction (TD) of the film after heat treatment, and the longitudinal direction (MD) and transverse direction (TD) of the film before heat treatment ) The polyester film according to [1] above, wherein the ratio (Xb/Yb) to the average value (Yb) of the stress at 5% strain is 1.01 or more.
[3] The polyester film according to [1] or [2] above, 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 used for flexible displays.
[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 heat-treating at a temperature of 100-160° C. for 15-90 minutes, wherein the heat-treating is performed after the heat setting step.
[8] The method for producing a polyester film according to [7] above, wherein the heat treatment is an annealing treatment.
[9] The method for producing a polyester film according to [7] or [8] above, wherein the heat treatment is performed off-line.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the polyester film suitable for flexible displays and the polyester film which has the outstanding bending resistance under high temperature conditions is provided.

It is a figure which shows the measuring method of a curl angle.

The present invention will be 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 "this film") is subjected to heat treatment, and the average curl angle (Xa) in the longitudinal direction (MD) and width direction (TD) of the heat-treated film, The ratio (Xa/Ya) to the average curl angle (Ya) in each of the longitudinal direction (MD) and transverse direction (TD) of the film before heat treatment is 0.95 or less.
(Here, the curl angle refers to the value obtained by subtracting the angle formed by the crease formed after the following measurement conditions from 180°.
Measurement conditions: The film is folded 180° in a gap of 2.0 mm, held bent, and allowed to stand in an oven at 90°C for 6 hours. After that, it is taken out to room temperature, the bend is released, and left as it is for 24 hours. )

More specific embodiments of the present film include films of the following first to third embodiments.
The film of the first mode contains polyethylene terephthalate (hereinafter also referred to as “PET”), and has an average curl angle of 151° or less in each of the longitudinal direction (MD) and transverse 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 the longitudinal direction (MD) and the transverse direction (TD).
The film of the third aspect contains a polyethylene naphthalate copolymer (hereinafter also referred to as "PEN copolymer"), and has an average curl angle of 128° in each of the longitudinal direction (MD) and the transverse direction (TD). It is below.

Here, in the present invention, the longitudinal direction (MD) of the film refers to the direction in which the film advances in the film manufacturing process, that is, the winding direction of the film roll. The width direction (TD) of the film refers to a direction parallel to the film surface and perpendicular to the longitudinal direction (MD), that is, a direction parallel to the central axis of the film roll.

The film may have a single layer structure or a multilayer structure (ie, laminated film). When the present film has a multi-layer structure, it may have a two-layer structure, a three-layer structure, or a multi-layer structure of four or more layers without departing from the gist of the present invention, and the number of layers is not particularly limited.
Among them, from the viewpoint of suppressing the manufacturing cost of the present film, a single layer structure or a multilayer structure of two to three layers is preferable.

Further, the present film may be an unstretched film (sheet) or a stretched film. Among them, a stretched film uniaxially or biaxially stretched is preferable, and a biaxially stretched film is more preferable from the viewpoint of balance of mechanical properties, flatness and thinning.

<Polyester>
Polyester, which is the raw material of the present film, is not particularly limited, and may be homopolyester or copolyester. Specific examples include polyesters obtained by polycondensation of dicarboxylic acids and diols.

The present film preferably contains polyester as a main component. Moreover, when the present film has a multilayer structure, it is preferable that the main component resin of each layer is polyester.
In addition, the “main component resin” means a resin having the highest content rate among the resins constituting each layer, for example, 50% by mass or more, particularly 70% by mass or more, and especially 80% by mass of the resins constituting each layer. above (including 100% by mass).

Examples of the dicarboxylic acid include terephthalic acid, 2,6-naphthalenedicarboxylic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2, Aromatic dicarboxylic acids such as 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 diol include ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer diol, bisphenols (bisphenol A, bisphenol F or bisphenol compounds such as bisphenol S, derivatives thereof, and ethylene oxide adducts thereof) and the like.

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

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

As the polyester used in this film, polyethylene terephthalate, polyethylene naphthalate, or a copolymer polyester containing these polyester constituents as a main component is preferable from the viewpoint of mechanical properties, heat resistance, transparency, and cost. .

Generally, when polyester is produced (polycondensation) using ethylene glycol as one of raw materials, diethylene glycol is produced as a by-product from ethylene glycol. This diethylene glycol is referred to herein as by-product diethylene glycol. The amount of diethylene glycol by-produced from ethylene glycol varies depending on the mode of polycondensation and the like, but is about 5 mol % or less of ethylene glycol. In the present invention, 5 mol % or less of diethylene glycol is defined as by-product diethylene glycol, and the above-mentioned by-product diethylene glycol is also included in ethylene glycol, and is distinguished from the copolymer component. On the other hand, depending on the content of diethylene glycol, more specifically, when the content of diethylene glycol exceeds 5 mol%, diethylene glycol is treated as a copolymerization component rather than as a by-product diethylene glycol.

Next, the polyester contained in the films of the first to third embodiments will be described.

(First aspect)
The film of the first aspect comprises PET.
The content of PET in the film is preferably 50% by mass or more, more preferably 70% by mass or more, and still more preferably 90% by mass or more (including 100% by mass) in the resin constituting the film.
When the present film has a multilayer structure, any layer may contain PET, and the total content of PET in the entire film may be the same as above. Above all, it is preferable that each layer contains PET and that the content of PET contained in each layer is the same as above.

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

The PET may be a homopolyester or a copolymer polyester, and is preferably a homopolyester.
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.
When it is a copolymer polyester, it contains terephthalic acid as the dicarboxylic acid (a-1) and ethylene glycol as the diol (b-1), and the dicarboxylic acid (a-1) and the diol (b-1) At least one of them contains a copolymer component. As the copolymerization component, the above-described dicarboxylic acids other than terephthalic acid and/or the above-described diols other than ethylene glycol can be selected. More specifically, about 30 mol % or less of the dicarboxylic acid may contain a dicarboxylic acid other than terephthalic acid, and about 30 mol % or less of the diol may contain a diol other than ethylene glycol. If it is 30 mol % or less, film strength, transparency and heat resistance are sufficiently maintained.

(Second aspect)
The film of the second embodiment comprises homo-PEN.
The homo-PEN content in the film is preferably 50% by mass or more, more preferably 70% by mass or more, and still more preferably 90% by mass or more (including 100% by mass) in the resin constituting the film.
When the present film has a multilayer structure, any layer may contain homo-PEN, and the total content of homo-PEN in the entire film may be the same as above. Above all, it is preferable that each layer contains homo-PEN and that the content of homo-PEN contained in each layer is the same as above.

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

(Third aspect)
A film of the third aspect comprises a PEN copolymer.
The content of the PEN copolymer in the film is preferably 50% by mass or more, more preferably 70% by mass or more, and still more preferably 90% by mass or more (including 100% by mass) in the resin constituting the film. be.
When the present film has a multilayer structure, any layer may contain a PEN copolymer, and the total content of the PEN copolymer in the entire film may be the same as above. Above all, it is preferable that each layer contains a PEN copolymer and the content of the PEN copolymer contained in each layer is the same as above.

The PEN copolymer specifically includes a dicarboxylic acid (a-3) and a diol (b-3), and more specifically, 2,6-naphthalene as the dicarboxylic acid (a-3) It contains ethylene glycol as the dicarboxylic acid and the diol (b-3), and at least one of the dicarboxylic acid (a-3) and the diol (b-3) contains a copolymer component.

The PEN copolymer preferably contains 80 mol% or more of 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid (a-3), more preferably 90 mol% or more, still more preferably dicarboxylic acid (a- All (100 mol %) of 3) is 2,6-naphthalenedicarboxylic acid.
By setting the content of 2,6-naphthalene dicarboxylic acid in the dicarboxylic acid (a-3) to 80 mol % or more, 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, still more preferably 70 mol% or more, and particularly preferably 80 mol%. above, particularly preferably 90 mol % or more. On the other hand, the upper limit of ethylene glycol 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, particularly preferably is 95 mol % or less.
By setting the content of ethylene glycol in the diol (b-3) within this range, 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 the copolymerization component in the dicarboxylic acid (a-3), and more preferably all of the dicarboxylic acid (a-3) It is 2,6-naphthalenedicarboxylic acid, ie, the copolymer content is 0 mol %.
The PEN copolymer preferably contains 70 mol% or less, more preferably 50 mol% or less, still more preferably 30 mol% or less, particularly preferably 70 mol% or less of the copolymer component in the diol (b-3). The content is 20 mol % or less, particularly 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, still more preferably 3 mol% or more, particularly preferably 4 mol% or more, especially Preferably, it is 5 mol % or more.
By setting the content of the copolymerization component in the dicarboxylic acid (a-3) and/or the diol (b-3) to such a range, the film strength, transparency, and heat resistance are sufficiently maintained while the bending resistance is improved. can be made good.

Examples of the copolymerization component added to the dicarboxylic acid (a-3) include the dicarboxylic acids described above 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 individually by 1 type or in combination of 2 or more types.

On the other hand, examples of the copolymerization component added to the diol (b-3) include the above diols other than ethylene glycol. From the viewpoint of bending resistance, 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, and bisphenols are preferred. Among them, bisphenols are more preferable from the viewpoint of maintaining the strength of the film, and bisphenol A-ethylene oxide adducts are preferably used as the bisphenols. These copolymerization components can be used individually by 1 type or in combination of 2 or more types.

The PEN copolymer may contain 2,6-naphthalene dicarboxylic acid as a dicarboxylic acid (a-3), ethylene glycol as a diol (b-3), and a bisphenol A-ethylene oxide adduct as a copolymer component. Most preferred. 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%, based on the total amount of the diol (b-3). 50 mol % or more, more preferably 3 mol % or more and 30 mol % or less, particularly preferably 4 mol % or more and 20 mol % or less, particularly preferably 5 mol % or more and 10 mol % or less.

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

Hereinafter, the present film will be described without being limited to the films of the first to third embodiments.

The polyester polymerization catalyst is 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 them, at least one selected from titanium compounds and antimony compounds is preferable.

Moreover, in order to suppress the precipitation amount of the oligomer component, the film may be produced using a polyester having a low content of the oligomer component. As a method for producing a polyester having a low content of oligomer components, various known methods can be used.
The film may have a structure of three or more layers, and the outermost layer of the film may be a layer using polyester having a low content of the oligomer component, thereby suppressing the precipitation amount of the oligomer component.
The polyester may be obtained by further raising the reaction temperature and subjecting it to melt polycondensation under reduced pressure after the esterification or transesterification reaction.

Furthermore, for the purpose of suppressing manufacturing costs, the film may be manufactured using recycled raw materials.

The present film may contain resins other than polyester as long as the effects of the present invention are not impaired.
Other resins include, but are not limited to, epoxy resins, polystyrene resins, polyvinyl chloride resins, polycarbonate resins, polyamide resins, polyacetal resins, polyethersulfone resins, polyetherketone resins, polysulfone resins, and polycycloolefins. Resin etc. can be illustrated.

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

<Particle>
The present film may contain particles for the main purpose of imparting slipperiness and preventing the occurrence of scratches in each step. The type of particles is not particularly limited as long as it can impart lubricity. Examples include silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, Examples include inorganic particles such as aluminum oxide and titanium oxide, and organic particles such as acrylic resins, styrene resins, urea resins, phenol resins, epoxy resins, and benzoguanamine resins. These can be used individually by 1 type or in combination of 2 or more types.
Furthermore, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst during the production process of a raw material such as polyester can also be used.

The shape of the particles to be used is also not particularly limited, and may be spherical, massive, rod-like, flat, or the like.
Moreover, there are no particular restrictions on its hardness, specific gravity, color, and the like. One of these series of particles may be used alone, or two or more of them may be used in combination, if necessary.

The average particle diameter 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, still more preferably 0.3 μm or more. If the average particle size of the particles is within this range, it is possible to achieve both transparency and handleability of the present film.
When the particles are powder, the average particle diameter of the particles is measured using a centrifugal sedimentation particle size distribution analyzer (for example, "SA-CP3 type" manufactured by Shimadzu Corporation) in an equivalent spherical distribution. The particle size (d50) at the cumulative volume fraction of 50% can be taken as the average particle size. The average particle size of the particles in the film, layer or polyester can be obtained by observing 10 or more particles with a scanning electron microscope (SEM), measuring the diameter of the particles, and calculating the average value. At that time, 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 particles to be described later.

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. By including particles in the surface layer, it is possible to effectively impart lubricity while reducing the content of particles in the entire film.
The particles may be contained in one surface layer, or may be contained in both surface layers.

The content of the particles is preferably 5% by mass or less, more preferably 3% by mass or less, still more preferably 2% by mass or less, relative to the film. On the other hand, the lower limit is preferably 0.0003% by mass or more, more preferably 0.01% by mass or more.
If the content of the particles is 0.0003% by mass or more, it is possible to impart lubricity to the film surface and prevent the occurrence of scratches in each step. On the other hand, when the content is 5% by mass or less, good transparency is obtained.
The content of particles as used herein refers to the entire film when the film has a single layer structure, and refers to the content in a layer containing particles when the film has a multilayer structure.

The method for adding particles to the present film is not particularly limited, and conventionally known methods can be employed. For example, it can be added at any stage of manufacturing a raw material such as polyester. Since the present film is a polyester film, it is preferably added after completion of the esterification or transesterification reaction.

<Others>
In addition to the particles described above, conventionally known additives such as ultraviolet absorbers, antioxidants, antistatic agents, heat stabilizers, lubricants, dyes, and pigments can be added to the present film as necessary. .
When the present film has a multilayer structure, each additive need not be contained in all layers, but may be contained 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 still more preferably 20 μm or more. On the other hand, 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, from the viewpoint of improving bending resistance.

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

As a method for producing the present film, an example in which the present film is a biaxially stretched film obtained by a sequential biaxial stretching method will be described.
When the present film is a biaxially stretched film, it is preferable to first prepare an unstretched sheet and then stretch it in two directions to obtain a biaxially stretched film.

The unstretched sheet is obtained by supplying the raw materials such as the polyester described above and additives added as necessary to an extruder and appropriately mixing them, extruding them as a molten sheet from a die using an extruder, and rotating It is preferably obtained by cooling and solidifying with a 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 the electrostatic application adhesion method and/or the liquid coating adhesion method are preferably employed.

Moreover, when the present film has a multilayer structure, it is preferable to co-extrude a plurality of layers by a co-extrusion method to obtain an unstretched sheet having a multilayer structure.
The raw material such as polyester may be supplied to the extruder after being appropriately dried as pellets or the like, 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 even biaxially.
Specifically, first, the unstretched sheet is stretched in one direction by 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 stretching direction of the first stage. It is preferably 3.5 to 6.0 times.

Subsequently, heat treatment is performed at a temperature of 180 to 270° C. under tension or under relaxation of 30% or less to obtain a biaxially stretched film. In the above stretching, a method of stretching in one direction in two or more stages can also be employed. In that case, it is preferable that the stretching ratios in the two directions finally fall within the above ranges.
In the present invention, the heat treatment is referred to as heat setting treatment, and is distinguished from the heat treatment performed after the heat setting treatment step described below.

Examples of the heat treatment after the heat setting treatment step in the production method of the present film include annealing treatment and aging treatment, among which annealing treatment is preferable.
In addition, these heat treatments may be performed off-line after the formed film is once wound into a roll, or may be performed in-line during film formation. Considering the above, it is preferable to perform offline.

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

In the present invention, by performing the heat treatment after the heat setting treatment step, the average value (Xa) of the curl angles in the longitudinal direction (MD) and the width direction (TD) of the film after heat treatment, and the longitudinal direction of the film before heat treatment The ratio (Xa/Ya) of the curl angles in the direction (MD) and the width direction (TD) to the average value (Ya) can be adjusted within a desired value range.
In addition, by performing the heat treatment after the heat setting treatment step, the average values of the curl angles in the longitudinal direction (MD) and the transverse direction (TD) of the films of the first to third embodiments are adjusted to the respective desired value ranges. be able to.

<<Physical properties of this film>>
<Curl angle>
The bending resistance evaluation of this film at high temperature (90 ° C.) is performed by folding the film 180 ° in a gap of 2.0 mm and keeping it folded in a 90 ° C. oven for 6 hours. It can be evaluated by the curl angle after being unfolded and left as it is for 24 hours. More specifically, it can be evaluated by the method described in Examples.
It can be evaluated that the smaller the curl angle, the greater the restoring force of the film to return to its original state after bending, that is, the better the bending resistance. The curl angle is evaluated as an average value, and the average value here means the average value of the curl angle in the longitudinal direction (MD) and the curl angle in the transverse direction (TD).
The lower limit of the curl angle is preferably 0° or more, the smaller the better.

The curl angle can be adjusted by film forming conditions such as the polyester to be used, the draw ratio, and the draw temperature. It can be controlled by adjusting the heat treatment conditions after the treatment step.

For this film, the average value (Xa) of the curl angles in the longitudinal direction (MD) and the transverse direction (TD) of the film after heat treatment, and the average value (Xa) of the curl angles in the longitudinal direction (MD) and the transverse direction (TD) of the film before heat treatment The ratio (Xa/Ya) to the average curl angle (Ya) is 0.95 or less.
The ratio of the average curl angles (Xa/Ya) represents the degree of improvement in bending resistance of the film after the heat treatment compared to the film before the heat treatment. It can be said that the smaller the value, the greater the effect of improving the bending resistance by the heat treatment after the heat setting treatment step.
Therefore, when the ratio (Xa/Ya) of the average values of the curl angles exceeds 0.95, the effect of improving the bending resistance by the heat treatment after the heat setting treatment step is insufficient, and the bending resistance under high temperature conditions is insufficient. be enough. If the flex resistance is insufficient, for example, the film may be deformed when the display is opened, which may adversely affect the functions of the display, such as lowering 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 the bending resistance under high temperature conditions is good, and the film deformation is small even under high temperature conditions. Good visibility of the display can be maintained.
From this point of view, the ratio (Xa/Ya) of the average values of the curl angles is preferably 0.90 or less, 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, 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 adjusting the film-forming conditions such as the draw ratio and the drawing temperature, but it greatly depends on the polyester used.
In the films of the first to third aspects, the film contains a specific polyester, and the average curl angle in the longitudinal direction (MD) and the transverse direction (TD) is set to a specific value range, so that the curl before and after the heat treatment The ratio of the average values of the angles (Xa/Ya) also falls within the desired value range.
Therefore, in the films of the first to third embodiments, by containing a specific polyester and setting the average value of the curl angle to a specific value, specifically, the bending resistance under high temperature conditions due to the heat treatment after the heat treatment step The improvement effect is sufficient. As a result, a polyester film having excellent flex resistance under high-temperature conditions, which is suitable for use in flexible displays, is obtained.

(First aspect)
The film of the first embodiment has an average curl angle of 151° or less after the bending test in the longitudinal direction (MD) and the transverse direction (TD).
When the average curl angle exceeds 151°, the restoring force of the film to return to its original state after bending is insufficient, resulting in insufficient bending resistance under high-temperature conditions. If the flex resistance is insufficient, for example, the film may be deformed when the display is opened, which may adversely affect the functions of the display, such as lowering the visibility of the display. On the other hand, if the average value of the curl angle is 151° or less, it can be said that the bending resistance under high temperature conditions is good, and even under high temperature conditions, the film is less deformed and the visibility of the display is kept good. be able to.
From this point of view, the average curl angle is preferably 147° or less, 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 the longitudinal direction (MD) and the transverse direction (TD).
When the average value of the curl angles exceeds 121°, the restoring force of the film to return to its original state after bending is insufficient, resulting in insufficient bending resistance under high-temperature conditions. If the flex resistance is insufficient, for example, the film may be deformed when the display is opened, which may adversely affect the functions of the display, such as lowering 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 deformation of the film is small and the visibility of the display is kept good. be able to.
From this point of view, the average curl angle is preferably 114° or less, 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 the longitudinal direction (MD) and in the transverse direction (TD).
When the average curl angle exceeds 128°, the restoring force of the film to return to its original state after bending is insufficient, resulting in insufficient bending resistance under high-temperature conditions. If the flex resistance is insufficient, for example, the film may be deformed when the display is opened, which may adversely affect the functions of the display, such as lowering 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 is less deformed and the visibility of the display is kept good. be able to.
From this point of view, the average curl angle is preferably 121° or less, more preferably 115° or less.

<Stress at 5% strain>
The stress of this film at 5% strain can be measured by the method described in Examples.
At a strain of 5%, the polymer chains forming the film are plastically deformed and remain permanently strained without returning to their original shape even after unloading. The increase in stress at 5% strain due to heat treatment indicates that the conformation of polymer chains is stabilized. In other words, it can be evaluated that the resistance to deformation is increased, that deformation marks are less likely to be left, and that the property has changed so as to have excellent deformation resistance. The stress at 5% strain is evaluated as an average value, and the average value here is the average value of the stress at 5% strain in the longitudinal direction (MD) and the stress at 5% strain in the transverse direction (TD). means

The stress at 5% strain can be adjusted by film forming conditions such as the polyester to be used, the draw ratio, and the stretching temperature. or by adjusting the heat treatment conditions after the heat setting treatment step.

In this film, the average value (Xb) of the stress at 5% strain in each of the longitudinal direction (MD) and transverse direction (TD) of the film after heat treatment, and the longitudinal direction (MD) and transverse direction ( TD) The ratio (Xb/Yb) to the average stress value (Yb) at 5% strain is preferably 1.01 or more.
The ratio of the average stress values at 5% strain (Xb/Yb) represents the degree of improvement in deformation resistance of the film after heat treatment relative to the film before heat treatment, and the ratio of the average stress values at 5% strain. It can be said that the larger the ratio (Xb/Yb), the greater the effect of improving the deformation resistance by the heat treatment after the heat setting treatment step.
Therefore, if the ratio (Xb/Yb) of the average value of the stress at 5% strain is 1.01 or more, the effect of improving the deformation resistance by the heat treatment after the heat setting treatment process is sufficient. In addition, resistance to stress applied by deformation is improved, and for example, when the display is opened, deformation of the film is small, and visibility of the display can be kept good.
From this point of view, the ratio (Xb/Yb) of the average values of the stress at 5% strain is preferably 1.02 or more. The upper limit of the ratio (Xb/Yb) of the stress average values at 5% strain is about 1.70.

Next, 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 draw ratio and the draw temperature, as described above, but it largely depends on the polyester used.
In the films of aspects 1 to 3, the specific polyester is included, and the average value of the stress at 5% strain in each of the longitudinal direction (MD) and the transverse direction (TD) is set to a specific value range, The ratio (Xb/Yb) of the average values of the stress at 5% strain before and after the heat treatment also falls within the desired value range.
Therefore, in the films of the first to third aspects, by containing a specific polyester and setting the average value of the stress at 5% strain to a specific value, specifically, the heat treatment after the heat treatment step will cause the polymer chains to The conformation is stabilized, resulting in improved deformation resistance.

(First aspect)
The film of the first aspect preferably has an average stress of 110 MPa or more at 5% strain in each of the longitudinal direction (MD) and the transverse direction (TD).
If the average value of the stress at 5% strain is 110 MPa or more, the resistance to the stress applied by deformation is good, for example, the film is less deformed when the display is opened, and the visibility of the display is kept good. becomes possible.
From this point of view, the average value of the stress at 5% strain is more preferably 111 MPa or more, and still more preferably 112 MPa or more.
The upper limit of the average stress at 5% strain is not particularly limited, but is about 140 MPa.

(Second aspect)
The film of the second aspect preferably has an average stress of 147 MPa or more at 5% strain in each of the longitudinal direction (MD) and transverse direction (TD).
If the average value of the stress at 5% strain is 147 MPa or more, the resistance to the stress applied by deformation is good, for example, the film is less deformed when the display is opened, and the visibility of the display is kept good. becomes possible.
From this point of view, the average value of the stress at 5% strain is more preferably 148 MPa or more, and still more preferably 149 MPa or more.
The upper limit of the average stress at 5% strain is not particularly limited, but is about 180 MPa.

(Third aspect)
The film of the third aspect preferably has an average stress of 140 MPa or more at 5% strain in each of the longitudinal direction (MD) and the transverse direction (TD).
If the average value of the stress at 5% strain is 140 MPa or more, the resistance to the stress applied by deformation is good, for example, the film is less deformed when the display is opened, and the visibility of the display is kept good. becomes possible.
From this point of view, the average value of the stress at 5% strain is more preferably 141 MPa or more, and still more preferably 142 MPa or more.
The upper limit of the average stress at 5% strain is not particularly limited, but is about 170 MPa.

<<Usage>>
Since the film has excellent flex resistance under high-temperature conditions, it can be suitably used for displays, particularly for flexible displays. Examples of flexible displays include foldable displays that can be folded, bendable displays that can be folded back, rollable displays that can be rolled up, and stretchable displays that can be expanded and contracted. Among others, the film is preferably used for foldable displays.
The foldable display may be folded in three or four.

Also, the display may be used in mobile phones, smart phones, digital cameras, personal computers, and the like. The type of display is not particularly limited, but includes LCD, organic EL display, inorganic EL display, LED, FED and the like, preferably bendable LCD, organic EL and inorganic EL. Among them, organic EL and inorganic EL which can reduce the layer structure are more preferable, and organic EL which has a wide color gamut is more preferable.

In the present invention, the flexible display may be used in any part as long as it is a constituent member of the flexible display. , a film for protecting the back surface of the display device (back surface protective film), and the like.

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

In the present invention, when described as "X to Y" (X and Y are arbitrary numbers), unless otherwise specified, "X or more and Y or less" and "preferably larger than X" or "preferably Y It also includes the meaning of "less than".
In addition, when described as "X or more" (X is an arbitrary number), it includes the meaning of "preferably greater than X" unless otherwise specified, and is described as "Y or less" (Y is an arbitrary number). If not otherwise specified, it also includes the meaning of "preferably smaller than Y".

The present invention will be described in more detail below with reference to examples. However, the present invention is by no means limited by the following examples.

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

(2) Bending resistance (curl angle) under high temperature (90°C) conditions
A test piece of 100 mm in the longitudinal direction (MD) × 30 mm in the width direction (TD) was cut out from an arbitrary location of the polyester film, and the film was held in a gap of 2.0 mm in the form of being folded in half in the longitudinal direction (MD). It was left for 6 hours in an oven at 90° C. without drying. After that, the sheet was taken out to room temperature, the bending was immediately released, and the curl angle (MD) was measured after the sheet was left as it was for 24 hours. The curl angle (MD) was measured at three points on the same film, and the average value was taken as the curl angle (MD). The curl angle (TD) was measured by cutting out a test piece of 30 mm in the longitudinal direction (MD) × 100 mm in the width direction (TD) from an arbitrary location of the polyester film, folding the width direction (TD) in half, and leaving a gap of 2.0 mm. After that, the curl angle (TD) was measured in the same manner.

The curl angle was measured in more detail as follows. After standing for 24 hours, the test piece is placed on a table so that the direction of the crease is in the vertical direction, and as shown in Fig. 1, the angle formed by the crease when viewed from directly above (the crease angle) was measured. The curl angle was a value obtained by subtracting the crease angle from 180°.
In addition, the average value of the measured curl angle in the longitudinal direction (MD) (the curl angle (MD)) and the curl angle in the width direction (TD) (the curl angle (TD)) was calculated, and the average value of the curl angles was calculated. bottom.

(3) Stress at strain of 5% A test piece is cut out from an arbitrary place of the polyester film in the longitudinal direction (MD) 150 mm x the width direction (TD) 15 mm, and a tensile tester Model 2005 manufactured by Intesco Co., Ltd. 50 mm between chucks, A tensile test was performed at a test speed of 200 mm/min, and the stress (MD) at 5% strain was measured. Similarly, using a test piece cut out so that the longitudinal direction is TD and the width direction is MD, the stress (TD) at 5% strain was measured.
In addition, the measured stress at 5% strain in the longitudinal direction (MD) (the stress at 5% strain (MD)) and the stress at 5% strain in the transverse direction (TD) (the stress at 5% strain ( The average value of TD)) was calculated and used as the average value of the stress at a strain of 5%.

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

[Homo polyethylene naphthalate (homo PEN)]
Dicarboxylic acid (a-2) 2,6-naphthalenedicarboxylic acid = 100 mol% and diol (b-2): ethylene glycol = 100 mol% were used.

[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%.

(Example 1)
A raw material obtained by dry blending PET (70% by mass) and PET-particle masterbatch (30% by mass) as a surface layer, and using PET (100% by mass) as an intermediate layer, respectively, and charged into separate twin-screw extruders. Then, both of them were extruded at 280° C. and cooled and solidified on a cooling roll set at 25° C. using an electrostatic contact method to obtain a two-kind, three-layer unstretched sheet.
Then, the obtained unstretched sheet was stretched 3.2 times at 86° C. in the longitudinal direction (MD) with a roll stretching machine. Furthermore, after preheating at 100° C. in a tenter, the film was stretched in the width direction (TD) at 115° C. to 4.2 times. Finally, heat setting treatment was performed 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).
Then, the obtained polyester film was subjected to offline annealing treatment at 110° C. for 1 hour to obtain a polyester film subjected to heat treatment after the heat setting treatment step. Table 1 shows the evaluation results.

(Example 2)
The homo-PEN was put into a twin-screw extruder, extruded at 300° C., and solidified by cooling on a cooling roll set at 50° C. using an electrostatic contact method to obtain an unstretched sheet.
Then, the obtained unstretched sheet was stretched 3.4 times in the longitudinal direction (MD) at 128° C. with a roll stretching machine. Furthermore, after preheating at 125° C. in a tenter, the film was stretched in the width direction (TD) at 135° C. to 4.1 times. Finally, heat setting treatment was performed at 230° C. to obtain a polyester film having a thickness of 50 μm.
Then, the obtained polyester film was subjected to offline annealing treatment at 100° C. for 1 hour to obtain a polyester film subjected to heat treatment after the heat setting treatment step. Table 1 shows the evaluation results.

(Examples 3-6)
A polyester film heat-treated after the heat-setting treatment step was obtained in the same manner as in Example 2, except that the off-line annealing treatment was performed under the heat treatment conditions shown in Table 1. Table 1 shows the evaluation results.

(Example 7)
A PEN copolymer is put into a twin-screw extruder, extruded at 300° C., and solidified by cooling on a cooling roll set at 50° C. using an electrostatic contact method to obtain an unstretched sheet. rice field.
Then, the obtained unstretched sheet was stretched 3.4 times in the longitudinal direction (MD) at 128° C. with a roll stretching machine. Furthermore, after preheating at 125° C. in a tenter, the film was stretched in the width direction (TD) at 135° C. to 4.1 times. Finally, heat setting treatment was performed at 210° C. to obtain a polyester film having a thickness of 50 μm.
Then, the obtained polyester film was subjected to offline annealing treatment at 100° C. for 1 hour to obtain a polyester film subjected to heat treatment after the heat setting treatment step. Table 1 shows the evaluation results.

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

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

(Comparative Example 3)
A polyester film heat-treated after the heat-setting treatment step was obtained in the same manner as in Example 2, except that the off-line annealing treatment was performed under the heat treatment conditions shown in Table 1. Table 1 shows the evaluation results.

(Comparative Example 4)
A polyester film heat-treated after the heat-setting treatment step was obtained in the same manner as in Example 7, except that the offline annealing treatment was performed under the heat treatment conditions shown in Table 1. Table 1 shows the evaluation results.

(Reference example 1)
A polyester film was obtained in the same manner as in Example 1, except that heat treatment was not performed after the heat setting treatment step. Table 2 shows the evaluation results.
If Example 1 and Comparative Examples 1 and 2 are films after heat treatment, Reference Example 1 corresponds to the films before heat treatment.

(Reference example 2)
A polyester film was obtained in the same manner as in Example 2, except that heat treatment was not performed after the heat setting treatment step. Table 2 shows the evaluation results.
If Examples 2 to 6 and Comparative Example 3 are films after heat treatment, Reference Example 2 corresponds to the films before heat treatment.

(Reference example 3)
A polyester film was obtained in the same manner as in Example 7, except that heat treatment was not performed after the heat setting treatment step. Table 2 shows the evaluation results.
If Examples 7 to 10 and Comparative Example 4 are films after heat treatment, Reference Example 3 corresponds to the films before heat treatment.

As shown in the above examples, the polyester film is subjected to heat treatment after the heat setting treatment step, and the average value (Xa) of the curl angle in each of the longitudinal direction (MD) and width direction (TD) of the film after heat treatment, and the heat treatment By setting the ratio (Xa/Ya) to the average value (Ya) of the curl angles in the longitudinal direction (MD) and the transverse direction (TD) of the previous film to a specific value or less, the heat treatment after the heat setting treatment step The effect of improving the bending resistance by is sufficient, and the bending resistance under high temperature conditions is excellent.
On the other hand, in the polyester film of the comparative example, although the composition was the same as that of the example, the ratio of the average values of the curl angles (Xa/Ya) exceeded a specific value. The bending resistance of was not sufficient.

Further, as shown in the above examples, the average value (Xb) of the stress at 5% strain in each of the longitudinal direction (MD) and transverse direction (TD) of the film after heat treatment, and the longitudinal direction of the film before heat treatment By setting the ratio (Xb/Yb) to the average value (Yb) of the stress at 5% strain in each of the (MD) and the width direction (TD) at a specific value or more, deformation due to heat treatment after the heat setting treatment step The effect of improving resistance is sufficient.
On the other hand, although the polyester film of the comparative example has the same composition as that of the example, the ratio of the average stress values (Xb/Yb) at 5% strain did not satisfy the specific value. Deformation resistance was not sufficient.

Therefore, the polyester films of Examples have good resistance to stress applied by deformation and bending resistance under high temperature conditions, so that, for example, when the display is opened, the film deformation is small, and the visibility of the display is improved. can be kept in good condition.

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

<First aspect>
(Example 1-1)
A raw material obtained by dry blending PET (70% by mass) and PET-particle masterbatch (30% by mass) as a surface layer, and using PET (100% by mass) as an intermediate layer, respectively, and charged into separate twin-screw extruders. Then, both of them were extruded at 280° C. and cooled and solidified on a cooling roll set at 25° C. using an electrostatic contact method to obtain a two-kind, three-layer unstretched sheet.
Then, the obtained unstretched sheet was stretched 3.2 times at 86° C. in the longitudinal direction (MD) with a roll stretching machine. Furthermore, after preheating at 100° C. in a tenter, the film was stretched in the width direction (TD) at 115° C. to 4.2 times. Finally, heat setting treatment was performed 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).
Then, the obtained polyester film was subjected to offline annealing treatment at 110° C. for 1 hour to obtain a polyester film subjected to heat treatment after the heat setting treatment step. Table 3 shows the evaluation results.

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

(Comparative Example 1-3)
A polyester film was obtained in the same manner as in Example 1-1, except that heat treatment was not performed after the heat setting treatment step. Table 3 shows the evaluation results.

In the first aspect, as shown in the above examples, polyethylene terephthalate is included, and the average curl angle in each of the longitudinal direction (MD) and the transverse direction (TD) is 151 ° or less, so that high temperature conditions The flex resistance was excellent under the condition.
On the other hand, the average curl angle of the polyester film of the comparative example exceeded 151° despite having the same composition as that of the example. I didn't.

<Second aspect>
(Example 2-1)
The homo-PEN was put into a twin-screw extruder, extruded at 300° C., and solidified by cooling on a cooling roll set at 50° C. using an electrostatic contact method to obtain an unstretched sheet.
Then, the obtained unstretched sheet was stretched 3.4 times in the longitudinal direction (MD) at 128° C. with a roll stretching machine. Furthermore, after preheating at 125° C. in a tenter, the film was stretched in the width direction (TD) at 135° C. to 4.1 times. Finally, heat setting treatment was performed at 230° C. to obtain a polyester film having a thickness of 50 μm.
Then, the obtained polyester film was subjected to offline annealing treatment at 100° C. for 1 hour to obtain a polyester film subjected to heat treatment after the heat setting treatment step. Table 4 shows the evaluation results.

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

(Comparative Example 2-2)
A polyester film was obtained in the same manner as in Example 2-1, except that heat treatment was not performed after the heat setting treatment step. Table 4 shows the evaluation results.

In the second aspect, as shown in the above examples, homopolyethylene naphthalate is included, and the average value of the curl angles in the longitudinal direction (MD) and the transverse direction (TD) is 121 ° or less. The flex resistance was excellent under high temperature conditions.
On the other hand, the average curl angle of the polyester film of the comparative example exceeded 121° despite having the same composition as that of the example. I didn't.

<Third aspect>
(Example 3-1)
A PEN copolymer is put into a twin-screw extruder, extruded at 300° C., and solidified by cooling on a cooling roll set at 50° C. using an electrostatic contact method to obtain an unstretched sheet. rice field.
Then, the obtained unstretched sheet was stretched 3.4 times in the longitudinal direction (MD) at 128° C. with a roll stretching machine. Furthermore, after preheating at 125° C. in a tenter, the film was stretched in the width direction (TD) at 135° C. to 4.1 times. Finally, heat setting treatment was performed at 210° C. to obtain a polyester film having a thickness of 50 μm.
Then, the obtained polyester film was subjected to offline annealing treatment at 100° C. for 1 hour to obtain a polyester film subjected to heat treatment after the heat setting treatment step. Table 5 shows the evaluation results.

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

(Comparative Example 3-2)
A polyester film was obtained in the same manner as in Example 3-1, except that heat treatment was not performed after the heat setting treatment step. Table 5 shows the evaluation results.

In the third aspect, as shown in the above examples, the polyethylene naphthalate copolymer is included, and the average curl angle in the longitudinal direction (MD) and the transverse direction (TD) is 128° or less. , the flex resistance was excellent under high temperature conditions.
On the other hand, the average curl angle of the polyester film of the comparative example exceeded 128° despite having the same composition as that of the example. I didn't.

The polyester film of the present invention has excellent flex resistance under high temperature conditions, so even under high temperature conditions, for example, when the display is opened, the film is less deformed and the visibility of the display is kept good. be able to.
As described above, the polyester film of the present invention achieves improvement in flex resistance at high temperatures, which has been a problem in the past, and can be particularly suitably used 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, folded, rolled, and stretched. is.

Claims (9)

A heat-treated polyester film,
The average value (Xa) of the curl angles in the longitudinal direction (MD) and the transverse direction (TD) of the film after heat treatment, and the average curl angle in the longitudinal direction (MD) and the transverse direction (TD) of the film before heat treatment A polyester film having a ratio (Xa/Ya) to a value (Ya) of 0.95 or less.
(Here, the curl angle refers to the value obtained by subtracting the angle formed by the crease formed after the following measurement conditions from 180°.
Measurement conditions: The film is folded 180° in a gap of 2.0 mm, held bent, and allowed to stand in an oven at 90°C for 6 hours. After that, it is taken out to room temperature, the bend is released, and left as it is for 24 hours. ) The average value (Xb) of the stress at 5% strain in each of the longitudinal direction (MD) and transverse direction (TD) of the film after heat treatment, and the longitudinal direction (MD) and transverse direction (TD) of the film before heat treatment The polyester film according to claim 1, wherein the ratio (Xb/Yb) to the average value (Yb) of stress at strain of 5% is 1.01 or more. 3. The polyester film according to claim 1, which has a thickness of 9 to 125 μm. The polyester film according to any one of claims 1 to 3, which is a biaxially stretched film. The polyester film according to any one of claims 1 to 4, which is used for flexible displays. A flexible display comprising the polyester film according to any one of claims 1 to 5. A method for producing a polyester film, comprising:
A step of heat-treating at a temperature of 100 to 160 ° C. for a time of 15 to 90 minutes,
Applying the heat treatment after the heat setting treatment step,
A method for producing a polyester film. The method for producing a polyester film according to claim 7, wherein the heat treatment is annealing treatment. The method for producing a polyester film according to claim 7 or 8, wherein the heat treatment is performed off-line.

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