JP2023014732A - polyester film - Google Patents

Abstract

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

Description

The present invention relates to polyester films.

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 polybutylene terephthalate, 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, crystallization and thermal degradation reduce toughness, resulting in a significant drop in flex resistance (hinge characteristics), and flexing can easily destroy a molded product such as a film. As a result, the current situation is that the use thereof is restricted in applications where bending resistance is required.

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).

In addition, in Patent Document 1, it is mentioned as a problem that repeated folding and unfolding at low temperatures imposes a large load on the base member made of a resin material, resulting in a decrease in reliability. It is in a situation where it is necessary to improve the quality of life.

Further, for example, Patent Document 2 discloses a flexible plastic film having high hardness and excellent flexibility, and exemplifies a polyethylene naphthalate film as such a film.

In Patent Document 3, the dicarboxylic acid component consists mainly of structural units derived from naphthalene dicarboxylic acid, and the diol component consists of structural units derived from ethylene glycol and structural units derived from alkylene oxide adducts of bisphenols. A polyethylene naphthalate copolymer characterized by

Japanese Patent Application Laid-Open No. 2020-46608 JP 2020-114673 A JP-A-10-204165

However, the polyethylene naphthalate film exemplified in Patent Document 2 may not have sufficient flex resistance under low-temperature conditions.

Moreover, the polyethylene naphthalate copolymer disclosed in Patent Document 3 is not expected to be used for displays, particularly for flexible displays.
In addition, even if a film is formed using the polyethylene naphthalate copolymer described in Patent Document 3, depending on film forming conditions such as stretching conditions, flex resistance, especially flex resistance under high temperature conditions, is desired. in some cases did not reach the required level.

The present invention has been made in view of the above circumstances, and the problem to be solved is to provide a polyester film that is particularly suitable for flexible displays and has excellent flex resistance regardless of the usage environment.

The present inventors 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 containing a polyethylene naphthalate copolymer, having an average refractive index of 1.661 or more and a plane orientation coefficient ΔP of 0.170 or more.
[2] The polyethylene naphthalate copolymer contains a dicarboxylic acid component (a-1) and a diol component (a-2), and the dicarboxylic acid component (a-1) contains 2,6-naphthalenedicarboxylic acid. The polyester film according to [1] above, containing 80 mol % or more.
[3] The polyethylene naphthalate copolymer contains a dicarboxylic acid component (a-1) and a diol component (a-2), and the diol component (a-2) contains ethylene glycol in an amount of 30 mol% or more and 99 mol. % or less, the polyester film according to the above [1] or [2].
[4] The polyester film according to [3] above, further comprising a bisphenol A-ethylene oxide adduct as the diol component (a-2).
[5] The polyester film according to [4] above, wherein the bisphenol A-ethylene oxide adduct is contained in the diol component (a-2) in an amount of 1 mol % or more and 70 mol % or less.
[6] Under conditions of 23 ° C., the average value of the hysteresis loss rate when performing a tensile cycle test up to 5% tensile strain in each of the longitudinal direction (MD) and the transverse direction (TD) is 59.0% or less. The polyester film according to any one of [1] to [5] above.
[7] The polyester film of any one of [1] to [6] above, which has a haze of 3.0% or less.
[8] The polyester film according to any one of [1] to [7] above, which has a thickness of 9 μm or more and 125 μm or less.
[9] The polyester film according to any one of [1] to [8], which is a biaxially stretched film.
[10] The polyester film according to any one of [1] to [9] above, which is used for displays.
[11] The polyester film according to [10] above, which is for a flexible display.
[12] A flexible display comprising the polyester film according to any one of [1] to [11] above.

ADVANTAGE OF THE INVENTION According to this invention, the polyester film which has the outstanding bending resistance regardless of a use environment is provided.

FIG. 4 shows a profile of a stress-strain curve; It is a figure which shows the bending-resistant evaluation method.

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") contains a polyethylene naphthalate (hereinafter also referred to as "PEN") copolymer, has an average refractive index of 1.661 or more, and has a plane orientation coefficient ΔP is 0.170 or more.

In the present invention, since the polyester film contains the PEN copolymer, it has excellent flex resistance under normal temperature, high temperature and low temperature conditions, especially low temperature conditions. It is believed that the reason why the film has excellent flex resistance even under low-temperature conditions is that the inclusion of a copolymer component can suppress an increase in the hardness of the film under low-temperature conditions.

Moreover, in the present invention, by setting the average refractive index and the plane orientation coefficient ΔP to a specific value or more, excellent bending resistance is obtained under high-temperature conditions.
In order to exhibit flex resistance, it is necessary to achieve both bending and returning to the original state. In particular, under high-temperature conditions, when the amorphous region is dominant, the film becomes flexible and has poor restoring force (restoring force). Therefore, it is believed that the restoring force can be maintained even under high temperature conditions by controlling the crystal region predominantly, that is, by moderately promoting the crystal orientation.

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.

In addition, the present film is preferably a biaxially stretched film from the viewpoint of making the average refractive index and plane orientation coefficient ΔP equal to or higher than a specific value, and after stretching in the longitudinal direction (MD), Or, after stretching in the width direction (TD), by a sequential biaxial stretching method of stretching in the longitudinal direction (MD), or by stretching the film in the longitudinal direction (MD) and the width direction (TD) almost simultaneously. It can be obtained by stretching by a simultaneous biaxial stretching method or the like. Among them, sequential biaxial stretching is preferable, and sequential biaxial stretching in which stretching is performed in the longitudinal direction (MD) and then in the width direction (TD) is more preferable.

In the present invention, the longitudinal direction (MD) of the film refers to the direction in which the film advances in the manufacturing process of the film, 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), ie, a direction parallel to the central axis of the roll when the film is shaped like a roll.

<Polyethylene naphthalate copolymer>
The film contains 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. .
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 component (a-1) and a diol component (a-2). More specifically, the dicarboxylic acid component (a-1) is 2, 6-naphthalenedicarboxylic acid and ethylene glycol as the diol component (a-2) are included as essential components, and at least one of the dicarboxylic acid component (a-1) and the diol component (a-2) contains a copolymer component. .
By including the PEN copolymer in the present film, the bending resistance, especially the bending resistance under low temperature conditions, is improved.

The PEN copolymer preferably contains 80 mol% or more of 2,6-naphthalene dicarboxylic acid in the dicarboxylic acid component (a-1), more preferably 90 mol% or more, and still more preferably the dicarboxylic acid component ( All (100 mol %) of a-1) is 2,6-naphthalenedicarboxylic acid.
By setting the content of 2,6-naphthalene dicarboxylic acid in the dicarboxylic acid component (a-1) to 80 mol % or more, the bending resistance, especially under normal temperature and high temperature conditions, is improved.

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

The PEN copolymer preferably contains 20 mol % or less, more preferably 10 mol % or less of the copolymer component in the dicarboxylic acid component (a-1), and more preferably contains the dicarboxylic acid component (a-1). All are 2,6-naphthalenedicarboxylic acids, ie 0 mol % of the copolymer component.
In addition, the PEN copolymer preferably contains 70 mol% or less of the copolymer component in the diol component (a-2), more preferably 50 mol% or less, even more preferably 30 mol% or less, and particularly preferably contains 20 mol % or less, particularly preferably 10 mol % or less. On the other hand, the lower limit of the copolymer component in the diol component (a-2) 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 component (a-1) and/or the diol component (a-2) within such a range, in particular, the plane orientation coefficient ΔP and average refractive index of the polyester film can be adjusted to desired values. It becomes easy to adjust the range, and it becomes excellent in the balance of flex resistance under normal temperature, high temperature and low temperature conditions.

Copolymerization components added to the dicarboxylic acid component (a-1) include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, and 2,5-furandicarboxylic acid. , 2,4-furandicarboxylic acid, 3,4-furandicarboxylic acid, benzophenonedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, etc. group dicarboxylic acids; aliphatic dicarboxylic acids such as cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid and dimer acid. Among them, isophthalic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, and 3,4-furandicarboxylic acid are preferable from the viewpoint of moldability. These copolymerization components can be used individually by 1 type or in combination of 2 or more types.

On the other hand, the copolymer component added to the diol component (a-2) includes diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer diols, bisphenols (bisphenol compounds such as bisphenol A, bisphenol F or bisphenol S, derivatives thereof, or ethylene oxide adducts thereof); Methylene 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.
As will be described later, the PEN copolymer and the polyester resin other than the PEN copolymer, which can be included as a resin constituting the present film, are produced using ethylene glycol as one of the raw materials. It may contain diethylene glycol by-produced from ethylene glycol. In the present invention, this by-product diethylene glycol shall be included in ethylene glycol. 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.

The PEN copolymer contains 2,6-naphthalene dicarboxylic acid as the dicarboxylic acid component (a-1), ethylene glycol as the diol component (a-2), and a bisphenol A-ethylene oxide adduct as the copolymer component. is 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 component (a-2). % or more and 50 mol % or less, 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.

Polymerization catalysts such as PEN copolymers are not particularly limited, and conventionally known compounds can be used. Examples include titanium compounds, germanium compounds, antimony compounds, manganese compounds, aluminum compounds, magnesium compounds and calcium compounds. mentioned. 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 raw material having a low content of the oligomer component. Various known methods can be used as a method for producing a raw material having a low content of oligomer components.

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

The present film can contain other resins than the PEN copolymer and the polyester resin other than the PEN copolymer as long as the effects of the present invention are not impaired.

Moreover, as long as the resin constituting the present film contains a PEN copolymer, it may contain a polyester resin other than the PEN copolymer and other resins.
As the polyester resin other than the PEN copolymer, homo-PEN is preferable. The homo-PEN is a PEN polymer composed of 100 mol % of 2,6-naphthalenedicarboxylic acid as a dicarboxylic acid component and 100 mol % of ethylene glycol as a diol component.
Generally, when a polyester resin (including a PEN copolymer) is produced (polycondensed) using ethylene glycol as one of 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 the by-product diethylene glycol is also included in ethylene glycol, and is distinguished from the copolymer component.
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.

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 a PEN copolymer can also be used.

The shape of the particles is not particularly limited, and may be spherical, massive, rod-like, or flat.
Further, the hardness, specific gravity, color, etc. of the particles are not particularly limited.

The average particle size of the particles is preferably 5 μm or less, more preferably 4 μm or less, and even more preferably 3 μm or less. 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 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 a PEN copolymer. Since the present film is a polyester film, it is preferably added after completion of the esterification or transesterification reaction.

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.

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.

<Manufacturing method of this film>
Next, a method for producing the present film will be described, taking as an example the 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 prepare an unstretched sheet and then stretch it in two directions to obtain a biaxially stretched film.

The unstretched sheet is obtained by appropriately mixing the raw material such as the PEN copolymer described above and additives added as necessary to an extruder and extruding from a die as a molten sheet using an extruder. It is preferably obtained by extruding and cooling and solidifying with a rotating cooling drum (cooling roll). In this case, in order to improve the flatness of the sheet, it is preferable to increase the adhesion between the sheet and the rotating cooling drum, and 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.
In addition, the raw material such as the PEN copolymer may be suitably dried as pellets and then supplied to the extruder, and additives such as particles and ultraviolet absorbers may be appropriately blended into the pellets. .

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 110 to 150° C., preferably 120 to 140° C., more preferably 125 to 133° C., still more preferably 127 to 130° C. The stretching ratio is usually 2.0 to 7.0 times, preferably 2.5 to 5.0 times, more preferably 2.7 to 4.5 times, still more preferably 3.0 to 4.0 times.
Next, the film is stretched in a direction perpendicular to the stretching direction of the first stage. , and the draw ratio is usually 3.0 to 7.0 times, preferably 3.3 to 6.0 times, more preferably 3.5 to 5.0 times, still more preferably 3.7 to 4.5 times is.

Subsequently, heat treatment is performed at a temperature of usually 150 to 270° C., preferably 170 to 250° C., more preferably 180 to 240° C., still more preferably 200 to 220° C. under tension or under relaxation of 30% or less, biaxially. A stretched film is obtained. 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 ranges described above.

<Physical properties of this film>
The film should have an average refractive index of 1.661 or more. When the average refractive index of the present film is less than 1.661, the bending resistance of the film, especially under high temperature conditions, is poor. The average refractive index can be adjusted by film forming conditions and the like.
The average refractive index of this film is preferably 1.662 or higher, more preferably 1.663 or higher.
The average refractive index is the average value of the refractive indices in the longitudinal direction (MD), width direction (MD), and thickness direction of the film.

The plane orientation coefficient ΔP of this film is required to be 0.170 or more. When the plane orientation coefficient ΔP of the present film is less than 0.170, the bending resistance of the film, especially under high temperature conditions, is poor. The plane orientation coefficient ΔP can be adjusted by film forming conditions and the like.
The plane orientation coefficient ΔP of this film is preferably 0.190 or more, more preferably 0.210 or more.
The plane orientation coefficient ΔP is obtained by measuring the refractive index (nβ) in the longitudinal direction (MD) of the film, the refractive index (nγ) in the width direction (TD), and the refractive index (nα) in the thickness direction. It is a value calculated by (1).
ΔP=(nβ+nγ)/2−nα (1)
The plane orientation coefficient .DELTA.P is measured on both sides of the film and may be within a range that applies to at least one side, but is preferably within a range that applies to both sides.

The bending resistance evaluation of this film at normal temperature (23 ° C.) was performed under conditions of 23 ° C. under conditions of up to 5% tensile strain in each of the longitudinal direction (MD) and the transverse direction (TD) in accordance with JIS K 7312: 1996. It can be evaluated by the average value of the hysteresis loss rate when a tensile cycle test is performed. The average value of the hysteresis loss rate is preferably 59.0% or less, more preferably 54.5% or less, and even more preferably 50.0% or less. The lower limit is not particularly limited, but is 0.1% or more, generally 10.0% or more.
When the average value of the hysteresis loss ratio at 23° C. of the present film is 59.0% or less, the restoring force of the present film increases and the bending resistance of the film becomes good. The hysteresis loss rate can be adjusted by the type and content of the resin constituting the film, stretching conditions, and the like.
The average value here means the average value of the hysteresis loss rate in the longitudinal direction (MD) and the hysteresis loss rate in the width direction (TD).

To evaluate the bending resistance of this film at high temperature (90°C), the film was left in an oven at 90°C for 6 hours while being bent with a bending radius of 1.0 mm. , and the curl angle after being left as it is for 24 hours. More specifically, it can be evaluated by the method described in Examples. The average curl angle after bending tests in the longitudinal direction (MD) and the transverse direction (TD) is preferably 145° or less, more preferably 142° or less, and still more preferably 139° or less. be.
If the average value of the curl angles is 145° or less, the film has a large restoring force to return to its original state after bending, that is, the bending resistance of the present film is good. The lower limit value is preferably 0° or more, the smaller the better.
The average value here means the average value of the curl angle in the longitudinal direction (MD) and the curl angle in the width direction (TD).

The bending resistance evaluation of this film at a low temperature (-20°C) was performed by performing a bending test under the conditions of -20°C, a bending radius of 1.0 mm, a bending speed of 70 rpm, and a number of bending times of 200,000. It can be evaluated by checking. More specifically, it can be evaluated by the method described in the Examples, and the more the crease cannot be visually confirmed, the greater the restoring force of the film after bending to return to its original state, that is, the durability of the film. It can be said that the flexibility is good.

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

<Application>
Since the film has excellent flex resistance regardless of the usage environment, it can be suitably used for displays, particularly for flexible displays. Examples of the flexible display include a foldable display, a bendable display, a rollable display, and a stretchable display.
In addition, the flexible display means a structural member for a display such as a front plate, a base film for a touch sensor, a film for protecting the back side of a display device, and the like.

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 examples thereof include a liquid crystal display, a plasma display, an organic EL display, and the like, and may be a touch panel type display.

<<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 refractive index A refractive index (nβ) in the longitudinal direction (MD) of the film and a refractive index in the transverse direction (TD) ( nγ) and the refractive index (nα) in the thickness direction were measured, and the average value of the respective refractive indices was taken as the average refractive index.

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

(3) Bending resistance evaluation under normal temperature (23°C) conditions (hysteresis loss rate)
Based on JIS K 7312:1996, the hysteresis loss rate at 23°C was determined by the following method. A tensile tester (Autograph AG-I manufactured by Shimadzu Corporation) was used as a measuring device.
As a test piece, a rectangle having a length of 100 mm and a width of 10 mm was cut from an arbitrary position of the polyester film. Both ends of the test piece in the length direction are chucked at a distance between chucks of 50 mm, and the strain is increased to 5% at a crosshead speed of 0.5 mm / min, and then lowered to the initial position at the same speed One cycle of tension A stress-strain curve obtained from the cycle test was obtained. The stress-strain curve takes a profile as shown in FIG. It was calculated by the following formula (2) using the area A2 (abcef), which is the difference between the areas of the obtained curves. The test was measured 3 times and the average value was calculated. The tensile cycle test was carried out in the longitudinal direction (MD) and transverse direction (TD) of the film, respectively.
Hysteresis loss rate=A2/A1×100 (2)

In addition, the average value of the measured hysteresis loss rate in the longitudinal direction (MD) and the measured hysteresis loss rate in the width direction (TD) was calculated and used as the average value of the hysteresis loss rate.
In addition, the evaluation criteria are as follows.
○ (Excellent): The average value is 54.5% or less △ (Average): The average value is greater than 54.5% and 59.0% or less × (poor): The average value is greater than 59.0%

(4) Bending resistance evaluation (curl angle) under high temperature (90°C) conditions
A test piece is cut out from an arbitrary place of the polyester film in the longitudinal direction (MD) 100 mm × width direction (TD) 30 mm, and the test piece is bent and held in a bending radius of 1.0 mm in the direction perpendicular to the longitudinal direction (MD), It was placed in an oven at 90°C for 6 hours. 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. For the curl angle (TD), a test piece was cut out from an arbitrary location of the polyester film in the longitudinal direction (MD) 30 mm × width direction (TD) 100 mm, and the bending radius was 1.0 mm in the direction perpendicular to the width direction (TD). The test piece was bent and held, and the curl angle (TD) was measured in the same manner thereafter.

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. 2, 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.
In addition, the evaluation criteria are as follows.
○ (Excellent): The average value is 139° or less △ (Average): The average value is greater than 139° and 145° or less × (poor): The average value is greater than 145°

(5) Flex resistance evaluation under low temperature (-20°C) conditions (visual observation)
Using a bending tester (CL09-type D01-FSC90, manufactured by Yuasa System Equipment Co., Ltd.), a test piece was cut out from an arbitrary location of the polyester film in the longitudinal direction (MD) 100 mm × width direction (TD) 30 mm, and the test piece was measured in the longitudinal direction ( The bending resistance (MD) was evaluated after bending the test piece 200,000 times with a bending radius of 1.0 mm in the direction perpendicular to MD). Bending resistance (TD) is 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, and bending the radius 1.0 mm in the direction perpendicular to the width direction (TD). After bending the test piece 200,000 times, the bending resistance (TD) was evaluated. The test was conducted under an environment of −20° C. and a bending speed of 70 rpm.

Evaluation was made by visually confirming the crease portion of the test piece after the bending test was completed.
In addition, the evaluation criteria are as follows.
〇 (Excellent): Almost no crease traces could be confirmed △ (Average): Faint crease traces could be confirmed × (poor): Crease traces could be clearly confirmed

(6) Haze Measured using a haze meter NDH-2000 manufactured by Nippon Denshoku Industries Co., Ltd. in accordance with JIS K 7136:2000.

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

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

[Polyethylene terephthalate (PET)]
Dicarboxylic acid component: terephthalic acid = 100 mol%, diol component: 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.

(Example 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 3.0 times at 128° C. in the longitudinal direction (MD) with a roll stretching machine. Furthermore, after preheating at 115° C. in a tenter, the film was stretched in the width direction (TD) at 130° C. to 4.1 times. Finally, heat treatment was performed at 210° C. to obtain a polyester film having a thickness of 50 μm.
The properties of the resulting polyester film were evaluated by the methods described above. Table 2 shows the evaluation results.

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

(Comparative example 2)
A polyester film was obtained in the same manner as in Example 1, except that the homo-PEN was used instead of the PEN copolymer and the film-forming conditions shown in Table 1 were used. Table 2 shows the evaluation results.

(Comparative Example 3)
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 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).

As shown in the above examples, by including a polyethylene naphthalate copolymer and setting the average refractive index and plane orientation coefficient ΔP of the polyester film to a specific value or more, bending resistance under normal temperature, high temperature and low temperature conditions It has a good gender balance. From this, it can be said that the polyester films of Examples have excellent flex resistance regardless of the usage environment.
On the other hand, the polyester film of Comparative Example 1 has an average refractive index of less than 1.661 despite having the same composition as that of Examples, so that it does not have sufficient flex resistance under high temperature conditions. I didn't. Comparative Example 2 did not have sufficient flex resistance under low-temperature conditions because it did not contain a copolymer component. Furthermore, in Comparative Example 3, excellent flex resistance could not be exhibited under any conditions.

Moreover, all the polyester films of Examples had low haze and excellent transparency.
Therefore, the present film can be suitably used for displays, particularly for flexible displays.

INDUSTRIAL APPLICABILITY The polyester film of the present invention can be suitably used for displays, particularly flexible displays, because it has excellent flex resistance regardless of the usage environment.
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 (12)

including a polyethylene naphthalate copolymer,
an average refractive index of 1.661 or more,
A polyester film having a plane orientation coefficient ΔP of 0.170 or more. The polyethylene naphthalate copolymer contains a dicarboxylic acid component (a-1) and a diol component (a-2), and 80 mol% of 2,6-naphthalenedicarboxylic acid is contained in the dicarboxylic acid component (a-1). The polyester film according to claim 1, containing the above. The polyethylene naphthalate copolymer contains a dicarboxylic acid component (a-1) and a diol component (a-2), and the diol component (a-2) contains 30 mol% or more and 99 mol% or less of ethylene glycol. The polyester film according to claim 1 or 2. 4. The polyester film according to claim 3, further comprising a bisphenol A-ethylene oxide adduct as the diol component (a-2). 5. The polyester film according to claim 4, wherein the bisphenol A-ethylene oxide adduct is contained in the diol component (a-2) at 1 mol % or more and 70 mol % or less. Claim that the average value of the hysteresis loss rate is 59.0% or less when a tensile cycle test is performed up to 5% tensile strain in each of the longitudinal direction (MD) and the transverse direction (TD) at 23 ° C. Item 6. The polyester film according to any one of Items 1 to 5. The polyester film according to any one of claims 1 to 6, which has a haze of 3.0% or less. The polyester film according to any one of claims 1 to 7, which has a thickness of 9 µm or more and 125 µm or less. The polyester film according to any one of claims 1 to 8, which is a biaxially stretched film. The polyester film according to any one of claims 1 to 9, which is used for displays. The polyester film according to claim 10, which is used for flexible displays. A flexible display comprising the polyester film according to any one of claims 1 to 11.

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