JP2020185789A - Hard coat film and use thereof - Google Patents

Abstract

To provide a hard coat film for a foldable display, the film having excellent mass productivity and never generating cracks on the foldable part, for a purpose of providing a foldable display which has no fear of generating disturbance in the image displayed at a folding portion after repeated folding.SOLUTION: Provided is a hard coat film for a foldable display with all light transmittance of 85% or more, having a hard coat layer on at least one surface of a polyester film of a thickness of 10 to 80 μm. The polyester film satisfies the following conditions: (1) a refractive index in the bending direction is 1.590 to 1.620; (2) a refractive index in the direction of the folding part is 1.670 to 1.700; (3) a refractive index in the thickness direction is 1.520 or less; and (4) the density is 1.380 g/cm3 or more (here, the bending direction refers to a direction orthogonal to the folding part when folding the polyester film).SELECTED DRAWING: Figure 1

Description

The present invention relates to a hard coat film for a foldable display, a foldable display, and a portable terminal device. Regarding hard coat film.

The thin film and weight of mobile terminal devices are becoming lighter, and mobile terminal devices such as smartphones are becoming widespread. While mobile terminal devices are required to have various functions, they are also required to be convenient. Therefore, popular mobile terminal devices need to have a small screen size of about 6 inches because they can be easily operated with one hand and are supposed to be stored in a pocket of clothes.

On the other hand, a tablet terminal having a screen size of 7 inches to 10 inches is expected to be used not only for video contents and music but also for business use, drawing use, reading, etc., and has high functionality. However, it cannot be operated with one hand, is inferior in portability, and has a problem in convenience.

In order to achieve these, a method of making it compact by connecting a plurality of displays has been proposed (see Patent Document 1), but since the bezel part remains, the image is cut off and the visibility is deteriorated. It has become a problem and is not widespread.

Therefore, in recent years, mobile terminals incorporating flexible displays and foldable displays have been proposed. With this method, the image can be conveniently carried as a mobile terminal device equipped with a large screen display without interruption.

Here, with respect to conventional displays and mobile terminal devices that do not have a foldable structure, the surface of the display could be protected with a non-flexible material such as glass, but in a foldable display, the foldable portion is used. In the case of a one-sided display, it is necessary to use a hard coat film or the like that is flexible and can protect the surface. However, in the foldable display, since the portion corresponding to a certain foldable portion is repeatedly folded, there is a problem that the film at the portion is deformed with time and the image displayed on the display is distorted. In addition to the surface protective film, the foldable display uses films for various parts such as polarizing plates, retardation plates, touch panel base materials, base materials for display cells such as organic EL, and protective members on the back surface. , These films were also required to have durability against repeated folding.

Therefore, a method of partially changing the film thickness has been proposed (see Patent Document 2), but there is a problem of poor mass productivity.

A method of adjusting the refractive index of the polyester film in the bending direction has also been proposed (see Patent Document 3), but as the refractive index in the bending direction is lowered, the pencil hardness at the time of applying a hard coat decreases, and the surface of the display is protected. There was a problem that the function deteriorated. Further, when the refractive index in one direction is lowered, the deformation at the time of folding is improved, but the uniaxial orientation in the folding direction is increased, and there is a problem that cracks are generated or broken in the folded portion.

Japanese Unexamined Patent Publication No. 2010-228391 Japanese Unexamined Patent Publication No. 2016-155124 International Publication No. 2018/150940

The present invention is intended to solve the problems of conventional display members as described above, is excellent in mass productivity, and does not cause distortion in the image displayed in the folded portion after being repeatedly folded. In order to be able to provide a foldable display and a mobile terminal device equipped with such a foldable display, we tried to provide a hard coat film for a foldable display without creases or cracks in the foldable part. Is what you do.

That is, the present invention has the following configuration.
1. 1. A hard coat film having a hard coat layer on at least one side of a polyester film having a thickness of 10 to 80 μm and having a total light transmittance of 85% or more, wherein the polyester film satisfies the following conditions.
(1) The refractive index in the bending direction is 1.590 to 1.620.
(2) The refractive index in the direction of the folding part is 1.670 to 1.700.
(3) Refractive index in the thickness direction is 1.520 or less (4) Density is 1.380 g / cm ³ or more
(Here, the bending direction means a direction orthogonal to the folding portion when the polyester film is folded.)
2. 2. The hard coat film for a foldable display according to the first aspect, wherein the polyester film has an elastic modulus in the bending direction of 2.7 GPa or less and an elastic modulus in the direction of the folding portion of 4.5 GPa or more.
3. 3. The hard coat film for a foldable display according to the first or second above, wherein the haze is 3% or less.
4. The hard coat film for a foldable display according to any one of the above 1st to 3rd, wherein the thickness of the hard coat layer is 1 to 50 μm.
5. The fourth hard coat film for a foldable display is a foldable display arranged as a surface protective film so that the hard coat layer is located on the surface, and is continuous through a foldable portion of the foldable display. A foldable display with a single hard coat film.
6. A mobile terminal device having the foldable display according to the fifth item.

The foldable display using the hard coat film for a foldable display of the present invention maintains mass productivity, and the hard coat film does not cause cracks in the foldable portion and causes deformation after repeated folding. However, the image is not distorted at the folded portion of the display. A mobile terminal device equipped with a foldable display using a hard coat film as described above provides a beautiful image, is rich in functionality, and is excellent in convenience such as portability.

It is a schematic diagram for showing the bending radius when the foldable display in this invention is folded. It is a schematic diagram for showing the bending direction of the polyester film constituting the hard coat film for a foldable display in this invention.

(display)
The display referred to in the present invention generally refers to a display device, and the types of displays include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs, such as LCDs having a bendable structure. , Organic EL and inorganic EL are preferable. In particular, organic EL and inorganic EL that can reduce the layer structure are particularly preferable, and organic EL having a wide color gamut is further preferable.

(Folding display)
The foldable display is a display in which one continuous display can be folded in half when carried. By folding, the size can be halved and portability can be improved. The bending radius of the foldable display is preferably 5 mm or less, more preferably 3 mm or less. If the bending radius is 5 mm or less, the thickness can be reduced in the folded state. It can be said that the smaller the bending radius is, the better, but the smaller the bending radius, the easier it is to make creases. The bending radius is preferably 0.1 mm or more, but may be 0.5 mm or more, or 1 mm or more. Even if the bending radius is 1 mm, it is possible to achieve a practically sufficient thinning when carrying. The bending radius when folded is for measuring the portion of reference numeral 11 in the schematic diagram of FIG. 1, and means the radius inside the folded portion when folded. The surface protection film, which is one of the uses of the hard coat film of the present invention described later, may be located on the folded outer side or the inner side of the foldable display.
Further, the foldable display may be folded in three, folded in four, or further, and may be a retractable type called rollable, all of which fall within the scope of the foldable display according to the present invention.

The hard coat film for a foldable display of the present invention may be used for any part as long as it is a constituent member of the foldable display. Hereinafter, a typical configuration of a foldable display and a portion where the hard coat film of the present invention can be used will be described by taking an organic EL display as an example. Hereinafter, the hard coat film for a folding display of the present invention may be simply referred to as the hard coat film of the present invention.

(Folding organic EL display)
An essential configuration of the foldable organic EL display is an organic EL module, but if necessary, a circularly polarizing plate, a touch panel module, a front surface protective film, a back surface protective film, and the like are provided.
(Organic EL module)
The general configuration of an organic EL module consists of an electrode / electron transport layer / light emitting layer / hole transport layer / transparent electrode.

(Touch panel module)
It is preferable that the mobile terminal device has a touch panel. When an organic EL display is used, it is preferable that the touch panel module is arranged on the upper part of the organic EL display or between the organic EL module / circular polarizing plate. The touch panel module has a transparent base material such as a film and a transparent electrode arranged on the transparent base material. The hard coat film of the present invention can be used as this transparent base material. When used as a transparent base material for a touch panel, it is preferable to provide a refractive index adjusting layer.

(Circular polarizing plate)
The circularly polarizing plate suppresses deterioration of image quality due to reflection of external light by a member inside the display. The circular polarizing plate has a linear polarizing plate and a retardation plate. The linear polarizing plate has a protective film on at least the surface of the polarizer on the visible side. A protective film may be provided on the surface opposite to the viewing side of the polarizer, or a retardation plate may be directly laminated on the polarizer. As the retardation plate, a resin film having a retardation such as polycarbonate or a cyclic olefin or a resin film provided with a retardation layer made of a liquid crystal compound is used. The hard coat film of the present invention can be used as a polarizer protective film. In these cases, when the base film of the hard coat film of the present invention is a polyester film, it is preferable that the slow axis direction of the polyester film is parallel or orthogonal to the absorption axis direction of the polarizer. A deviation of up to 10 degrees, preferably up to 5 degrees, is allowed with respect to this parallelism or orthogonality.

(Surface protection film)
When an impact is applied to the display from above, the circuits of the organic EL module and the touch panel module may be disconnected. Therefore, in many cases, a surface protective film is provided. The hard coat film of the present invention is used as this surface protection film. The surface protective film includes a cover window built into the outermost surface of the display and a replaceable after-sale film that can be attached and detached by the user himself. In any case, the hardware of the present invention is used. A coated film is used. It is provided on the surface of a foldable display with the hard coat layer on the visible side. The hard coat layer may be provided on both sides.

(Back side protective film)
It is also preferable that a protective film is also provided on the back surface side of the display. The hard coat film of the present invention can be used as a protective film on the back surface side.

The hard coat film of the present invention may be other than the above as long as it is used in a place where it is folded in a component of a foldable display.
Among these, the hard coat film of the present invention is preferably used for the cover window surface protective film, the after surface protective film, the base film of the touch panel module, and the back surface protective film. Furthermore, it is preferably used for the cover window surface protective film and the after surface protective film.

Further, as a foldable display, the hard coat film of the present invention does not necessarily have to be used for all the above-mentioned applications. Foldable displays are hard based on polyester film, polyimide film, polyamide film, polyamideimide film, polycarbonate film, acrylic film, triacetyl cellulose film, cycloolefin polymer film, polyphenylene sulfide film, polymethylpentene film, etc. The coat film can be appropriately used according to suitability.

When the base film constituting the hard coat film of the present invention is a polyester film, it may be a single-layer film made of one or more types of polyester resins, or when two or more types of polyesters are used, it may be a multilayer structure film. It may be a super multi-layer laminated film having a repeating structure.

Examples of the polyester resin used for the polyester film which is the base film of the hard coat film include polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or a common component containing these resins as main components. Examples thereof include a polyester film made of a polymer. Among them, a stretched polyethylene terephthalate film is particularly preferable from the viewpoints of mechanical properties, heat resistance, transparency, price and the like.

When a polyester copolymer is used for the polyester film which is the base film of the hard coat film, the dicarboxylic acid component of the polyester is, for example, an aliphatic dicarboxylic acid such as adipic acid or sebacic acid; terephthalic acid, isophthalic acid, phthalic acid Aromatic dicarboxylic acids such as acids and 2,6-naphthalenedicarboxylic acids; polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid. Examples of the glycol component include fatty acid glycols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol and neopentyl glycol; aromatic glycols such as p-xylene glycol; 1,4-cyclohexanedimethanol and the like. Alicyclic glycols; polyethylene glycols having an average molecular weight of 150 to 20,000. The mass ratio of the copolymerization component of the preferred copolymer is less than 20% by mass. When it is less than 20% by mass, the film strength, transparency and heat resistance are maintained, which is preferable.

Further, in the production of the polyester film which is the base film of the hard coat film, the ultimate viscosity of at least one kind of resin pellets is preferably in the range of 0.50 to 1.0 dl / g. When the ultimate viscosity is 0.50 dl / g or more, the impact resistance of the obtained film is improved, and it is preferable that the internal circuit of the display is less likely to be broken due to an external impact. On the other hand, when the ultimate viscosity is 1.00 dl / g or less, the filter pressure increase of the molten fluid does not become too large, and it is preferable that the film production can be operated stably.

The thickness of the polyester film, which is the base film of the hard coat film, is preferably 10 to 80 μm, and more preferably 25 to 75 μm. When the thickness is 10 μm or more, the pencil hardness improving effect and the impact resistance improving effect are observed, and when the thickness is 80 μm or less, it is advantageous for weight reduction and also excellent in flexibility, workability and handleability.

The surface of the hard coat film of the present invention may be smooth or has irregularities, but since it is used for the surface cover of a display, deterioration of optical characteristics due to the irregularities is not preferable. The haze is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. When the haze is 3% or less, the visibility of the image can be improved. The smaller the lower limit of the haze, the better, but from the viewpoint of stable production, 0.1% or more is preferable, and 0.3% or more may be used.

As described above, it is better that the unevenness of the film surface is not so large for the purpose of reducing the haze, but from the viewpoint of handling, as a method of forming the unevenness, the base material polyester film is used. It can be formed by blending particles into the film or coating a coat layer containing the particles during the film formation of the base polyester film.

As a method of blending the particles into the base polyester film, a known method can be adopted. For example, it can be added at any stage in the production of polyester, but is preferably added as a slurry dispersed in ethylene glycol or the like at the stage of esterification or at the stage after the completion of the transesterification reaction and before the start of the polycondensation reaction. Then, the polycondensation reaction may proceed. Further, a method of blending a slurry of particles dispersed in ethylene glycol or water with a polyester raw material using a kneading extruder with a vent, or a method of blending dried particles with a polyester raw material using a kneading extruder. It can be done by such as.

Among them, aggregate inorganic particles are homogeneously dispersed in a monomer solution that is a part of the polyester raw material, and then filtered to be used as the remainder of the polyester raw material before the esterification reaction, during the esterification reaction, or after the esterification reaction. The method of addition is preferable. According to this method, since the monomer solution has a low viscosity, homogeneous dispersion of particles and high-precision filtration of the slurry can be easily performed, and when added to the rest of the raw material, the dispersibility of the particles is good, which is new. Aggregates are also unlikely to occur. From this point of view, it is particularly preferable to add it to the balance of the raw material in a low temperature state before the esterification reaction.

Further, the number of protrusions on the film surface can be further reduced by a method (masterbatch method) in which a polyester containing particles is obtained in advance and then the pellets and the pellets containing no particles are kneaded and extruded.

Further, the polyester film as a base material may contain various additives within a range that maintains a preferable range of total light transmittance. Examples of the additive include an antistatic agent, a UV absorber, and a stabilizer.

The total light transmittance of the polyester film as a base material is preferably 85% or more, more preferably 87% or more. If the transmittance is 85% or more, sufficient visibility can be ensured. The total light transmittance of the polyester film as the base material is preferably 85% or more in order to increase the total light transmittance of the hard coat film described later. It can be said that the higher the total light transmittance of the polyester film, the better, but from the viewpoint of stable production, 99% or less is preferable, and 97% or less may be used.

The maximum heat shrinkage of the polyester film as a base material after heat treatment at 150 ° C. for 30 minutes is preferably 6% or less, more preferably 5% or less. If the heat shrinkage rate is 6% or less, it is possible to suppress flat surface defects such as curl and swell during HC processing. It can be said that the lower the heat shrinkage rate is, the better, but it is preferably -1% or more, and preferably 0% or more. A minus here means that it has expanded after heating, and even if it is less than -1%, a flat surface may be defective.

In order to give the hard coat film for a foldable display of the present invention sufficient pencil hardness, the polyester film as a base film preferably has the following characteristics. In the pencil hardness evaluation of the pencil hardness of the hard coat film after laminating the hard coat layer on the conventional base material polyester film, the pencil hardness was lowered due to the film being deformed in the thickness direction. Conceivable. In the present invention, it is preferable to set the pushing depth of the base polyester film after unloading the test force in the thickness direction by a dynamic ultrafine hardness tester described later to a specific range. In the pencil hardness evaluation of the hard coat film using the base polyester film, high hardness can be achieved, which is preferable. The pushing depth of the base material polyester film in the thickness direction after unloading is preferably 1.5 μm or less, more preferably 1.4 μm or less, and further preferably 1.3 μm or less. When the pushing depth (final deformation amount under load) after unloading the test force is 1.5 μm or less, the film becomes thicker in the pencil hardness evaluation of the hard coat film after laminating the hard coat layer. It is hard to deform and the pencil hardness can be increased. If the pencil hardness of the hard coat film can be increased, scratches and dents are less likely to occur on the display surface, and the visibility of the display is improved. It can be said that the lower the pushing depth after the test force is unloaded, the better, but 0.3 μm or more is preferable, and 0.5 μm or more is more preferable, from the viewpoint of saturating stable production and effects.

In order to reduce the pushing depth after unloading the test force, it is effective to adjust the refractive index in the thickness direction to 1.520 or less. As a means for reducing the refractive index to 1.520 or less, as will be described later, the stretching ratio in the bending direction and the folding direction is adjusted to be high within a range in which the refractive index in the bending direction and the folding direction can be controlled within a preferable range. It is possible to exemplify the setting of conditions such as setting the stretching temperature in the bending direction and the folding direction low, and setting the heat fixing temperature high.

The elastic modulus of the polyester film as the base material in the bending direction is preferably 2.7 GPa or less, more preferably 2.6 GPa or less, and further preferably 2.5 GPa or less. By reducing the elastic modulus in the bending direction, the stress applied to the hard coat layer when the hard coat film is bent can be reduced. It can be said that the lower the elastic modulus in the bending direction, the better the flexibility, but 1.8 GPa or more is preferable because the flatness of the display surface is impaired. The elastic modulus in the folding direction is preferably 4.5 GPa or more, more preferably 4.6 GPa or more, and further preferably 4.7 GPa or more. By increasing the elastic modulus in the folding direction, the flatness of the display surface can be maintained when the display is created. The higher the elastic modulus in the folding direction, the more preferable, but from the viewpoint of film forming property, 8.0 GPa or less is preferable.

The non-hard coated surface of the hard coat film of the present invention can be coated with an adhesive or treated to improve the adhesion to the hard coat layer.

Examples of the surface treatment method include sandblasting, unevenness treatment by solvent treatment, corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone / ultraviolet irradiation treatment, flame treatment, chromic acid treatment, hot air treatment, and the like. Oxidation treatment and the like can be mentioned and can be used without particular limitation.

In addition, the adhesiveness can be improved by an adhesiveness improving layer such as an easy adhesive layer. As the easy-adhesive layer, acrylic resin, polyester resin, polyurethane resin, polyether resin and the like can be used without particular limitation, and can be formed by a general coating method, preferably a so-called in-line coat formulation.

The above-mentioned polyester film has, for example, a polymerization step in which inorganic particles are homogeneously dispersed in a monomer solution that is a part of a polyester raw material, filtered, and then added to the rest of the polyester raw material to polymerize the polyester, and the polyester thereof. It can be produced through a film forming step of forming a base film by melt-extruding it into a sheet through a filter, cooling it, and then stretching it.

Next, the method for producing the polyester film as the base material will be described in detail with reference to an example in which polyethylene terephthalate (hereinafter, may be referred to as PET) pellets are used as the raw material for the base film, but the method is not limited thereto. Absent. Further, the number of layers is not limited, such as a single-layer structure or a multi-layer structure.

After the PET pellets are mixed and dried at a predetermined ratio, they are supplied to a known melt lamination extruder, extruded into a sheet from a slit-shaped die, and cooled and solidified on a casting roll to form an unstretched film. .. In the case of a single layer, one extruder is sufficient, but in the case of producing a multi-layer film, two or more extruders, two or more layers of manifolds or a merging block (for example, a merging having a square merging part). A block) can be used to stack a plurality of film layers constituting each outermost layer, extrude two or more sheets from a base, and cool them with a casting roll to form an unstretched film.

In this case, it is preferable to perform high-precision filtration in order to remove foreign substances contained in the resin at an arbitrary place where the molten resin is maintained at about 280 ° C. during melt extrusion. The filter medium used for high-precision filtration of the molten resin is not particularly limited, but the filter medium of the stainless sintered body is excellent in the removal performance of aggregates containing Si, Ti, Sb, Ge and Cu as main components and high melting point organic substances. Therefore, it is preferable.

Further, the filtered particle size (initial filtration efficiency 95%) of the filter medium is preferably 20 μm or less, and particularly preferably 15 μm or less. If the filtered particle size (initial filtration efficiency 95%) of the filter medium exceeds 20 μm, foreign matter having a size of 20 μm or more cannot be sufficiently removed. High-precision filtration of the molten resin using a filter medium having a filter medium size (initial filtration efficiency of 95%) of 20 μm or less may reduce productivity, but a film with few protrusions due to coarse particles can be obtained. Preferred above.

(Refractive index in the bending direction)
In the present invention, the refractive index of the polyester film as a base material in at least one of the longitudinal direction (mechanical flow direction) and the width direction is preferably 1.590 to 1.620, and more preferably 1. It is 591 to 1.600. The refractive index of the polyester film in the bending direction is preferably 1.590 to 1.620, and more preferably 1.591 to 1.600. Here, the bending direction refers to a direction orthogonal to the folding portion (reference numeral 21) assumed in the application of the foldable display, as shown by reference numeral 22 on the polyester film (reference numeral 2) of FIG. When the refractive index in at least one of the longitudinal direction and the width direction is 1.590 to 1.620, there is little deformation when repeatedly folded, and there is no risk of deteriorating the image quality of the foldable display, which is preferable. The refractive index is more preferably 1.591 to 1.600. Of course, the direction is preferably the above-mentioned bending direction. If it is 1.590 or more, there is no possibility that cracks will occur in the folding portion direction after the bending test described later, and of course, breakage will not occur, so that the visibility of the display can be kept good. The refractive index of the polyester film can be effectively adjusted by adjusting the stretching ratio and the stretching temperature. Further, a relaxation step in the stretching direction and multi-step stretching may be used to adjust the refractive index. When performing multi-stage stretching, it is preferable that the stretching ratio of the second and subsequent stages is higher than the stretching ratio of the first stage.

By controlling the refractive index in at least one of the longitudinal direction (mechanical flow direction) and the width direction of the polyester film in the above range, more preferably by controlling the refractive index in the bending direction in the above range, at the time of folding. Fatigue due to compressive stress applied to the inside of the fold can be reduced. Fatigue due to compressive stress is thought to occur mainly in the crystal part, and the smaller the number of crystals in the bending direction, the less fatigue. Therefore, it is considered that by lowering the refractive index, the amount of oriented crystals in the bending direction is reduced and compression fatigue is suppressed.

Further, the creep phenomenon caused by the tensile stress applied to the outside of the folding at the time of folding can be suppressed by reducing the refractive index. Fatigue due to tensile stress is thought to occur mainly in the amorphous part, and the molecular chains are aligned and deformed due to repeated stress. It can be inferred that the smaller the number of molecular chains aligned in the bending direction, the smaller the deformation due to alignment. Further, since fatigue due to tension can be suppressed when the number of amorphous portions is small, a high crystallinity, that is, a high density is preferable.

In the present invention, the unstretched polyester sheet preferably has a draw ratio of 1.2 to 2.0 times in at least one of the longitudinal direction (mechanical flow direction) and the width direction, 1.7 to 2. 0 times is more preferable. The stretching direction is preferably the bending direction. A draw ratio of 1.2 times or more is preferable because there is no deformation in post-processing such as during hard coat coating, and a draw ratio of 2.0 times or less is preferable because uneven film thickness does not occur. The stretching temperature is preferably 75 to 120 ° C, more preferably 75 to 105 ° C. As the heating method at the time of stretching, conventionally known means such as a hot air heating method, a roll heating method, and an infrared heating method can be adopted. By setting the stretching temperature to 75 to 120 ° C., it is possible to prevent large thickness unevenness due to stretching at the above stretching ratio. In addition, the refractive index in the thickness direction can be reduced by stretching at a low temperature as much as possible within a range that does not cause large thickness unevenness as described above.

(About the refractive index in the direction of the folding part)
The refractive index of the polyester film used as the base material is preferably 1.670 to 1.700 in the direction orthogonal to the direction in which the refractive index is 1.590 to 1.620. That is, it is preferable that the refractive index in the direction orthogonal to the bending direction (direction of the folded portion) is 1.670 to 1.700. By setting the value to 1.670 to 1.700, the deformation when folded in the bending direction can be reduced. By setting the value to 1.700 or less, it is possible to prevent cracks or breakage in the direction of the folded portion. By setting it to 1.670 or more, the flexibility in the bending direction can be improved and the surface hardness can be improved. More preferably 1.680 to 1.695. Examples of the method for adjusting the refractive index in the direction orthogonal to the bending direction include stretching ratio, stretching preheating temperature, stretching temperature, multi-stage stretching, and film relaxation. The draw ratio is preferably 4.0 to 6.0 times, more preferably 4.4 to 6.0 times. Further, the stretching preheating temperature in the direction orthogonal to the bending direction is preferably 70 to 110 ° C. In the case of multi-step stretching in the direction orthogonal to the bending direction, it is preferable to increase the stretching ratio of the second and subsequent steps rather than the first step. The film may be relaxed by 1 to 10% in either the machine flow direction (longitudinal direction) or the vertical direction (width direction).

(Regarding the refractive index in the thickness direction)
The refractive index of the polyester film as the base material in the thickness direction is preferably 1.520 or less. By setting it to 1.520 or less, even if the refractive index in the bending direction is designed to be low, it is possible to suppress a decrease in the hardness of the film surface, and it is possible to achieve both flexibility and surface hardness. .. By setting it to 1.520 or less, the pushing depth after unloading the test force in the thickness direction can be reduced, and the hardness of the film surface, particularly the pencil hardness of the hard coat film after laminating the hard coat layer can be improved. It is more preferably 1.515 or less, further preferably 1.510 or less, particularly preferably 1.505 or less, and most preferably 1.500 or less. The refractive index in the thickness direction is preferably low, but 1.3 or more is preferable in terms of stable production, and it may be 1.4 or more. Especially preferably, it is 1.410 or more. It can be said that the above range can be achieved by increasing the stretching ratio in both the bending direction and the folding direction, but the refractive index in the thickness direction is controlled after controlling the refractive index in the bending direction and the width direction within a preferable range. In order to do so, it is preferable to set the conditions while checking the balance of each process condition in the film forming process.

The method of controlling the refractive index in the thickness direction of the polyester film as the base material within the above range is the stretching preheating temperature in the bending direction, the stretching temperature, the stretching ratio, the stretching preheating temperature in the folding portion direction, the stretching temperature, the multi-stage stretching, and the high. There is a magnification stretching or heat fixing temperature setting. The stretching preheating temperature in the bending direction is preferably 70 ° C. to 110 ° C. The stretching temperature in the bending direction is preferably 75 to 120 ° C. The draw ratio in the bending direction is preferably 1.2 to 2.0 times, more preferably 1.7 to 2.0 times. By lowering the stretching temperature and stretching at a low stretching ratio, the refractive index in the thickness direction can be effectively lowered while maintaining the flexibility in the bending direction. The stretching preheating temperature in the folding portion direction is also preferably 75 ° C. to 110 ° C. The stretching temperature is preferably 75 to 120 ° C. The draw ratio of the folded portion is preferably 4.0 to 6.0 times, more preferably 4.4 to 6.0 times. The refractive index in the thickness direction can be effectively reduced while maintaining or reducing the refractive index in the bending direction. As a method of high-magnification stretching, multi-stage stretching may be used. In that case, it is preferable to make the stretching ratio of the second stage higher than the stretching ratio of the first stage because the refractive index can be effectively controlled. Further, a method of stretching again after the crystallization step may be used. Accelerated stretching that increases the stretching speed from the initial stage to the latter half of the stretching may be used.
The heat fixing temperature is preferably 180 to 240 ° C. By performing thermal fixation, orientation crystallization in the stretching direction proceeds, and the refractive index in the thickness direction can be lowered.
The reason why the hardness of the film surface is improved by lowering the refractive index in the thickness direction is not always clear, but aromatics such as benzene rings in the molecular chain are oriented in the plane direction to suppress deformation due to stress applied in the thickness direction. It is thought that it has the effect of

(About the density of polyester film)
The density of the polyester film as a base material is preferably 1.380 g / cm ³ or more. More preferably, it is 1.383 g / cm ³ or more. By setting it to 1.380 g / cm ³ or more, the flexibility can be improved, and the film surface hardness, particularly the pencil hardness of the hard coat film after laminating the hard coat layer can be improved. The higher the density, the more preferable it is, and although it depends to some extent depending on the presence or absence of particles in the film, it is preferably 1.40 g / cm ³ or less. By setting the heat fixing temperature at the time of film formation to 180 to 240 ° C., crystallization can proceed and the density can be effectively increased.

It is preferable that the bending direction of the polyester film as the base material corresponds to the longitudinal direction (machine flow direction). By doing so, it is easy to lower the refractive index in the bending direction at the biaxial stretching and improve the flexibility. That is, it is preferable to stretch the unstretched polyester sheet at a stretching ratio of 1.2 to 2.0 times, more preferably 1.7 to 2.0 times in the longitudinal direction to obtain a polyester film. Then, in the width direction, it can be said that stretching at a stretching ratio of 4.0 to 6.0 times, more preferably 4.4 to 6.0 times is preferable.

Further, in the hard coat film of the present invention, the polyester film as a base material has (1) a refractive index in the bending direction of 1.590 to 1.620.
(2) The refractive index in the direction of the folding part is 1.670 to 1.700.
(3) It can be said that it is a particularly preferable embodiment that the refractive index in the thickness direction is 1.520 or less and (4) the density is 1.380 g / cm ³ or more at the same time, but it is within the above-mentioned preferable production conditions. In the combination of, for example, the stretching ratio in the bending direction is 1.4 times or less, the stretching ratio in the folding portion direction is less than 4.4 times, and the heat fixing temperature is 220 ° C. or less. In the case of a combination of conditions that cannot be said to be the best within each preferable production condition range, it may not always be possible to obtain one that satisfies the above four characteristics at the same time. In this case, the stretching ratio in the bending direction may be increased to 1.7 times or more, the stretching ratio in the direction of the folding portion may be increased to 4.4 times or more, or the heat fixing temperature may be increased to about 230 ° C. Alternatively, the above four characteristics can be satisfied at the same time by fine-tuning any of the conditions or a combination thereof, such as lowering the stretching temperature in the bending direction and / or the folding portion direction.

In order to adjust the film-forming property, film strength, thermal dimensional stability, and poor appearance, any film-forming method such as stretching, relaxation, heat fixing, and surface treatment may be used, but the polyester film as the base material. It can be said that it is a particularly preferable embodiment in the present invention to control the refractive index and the density of the above-mentioned above-mentioned preferable range. By controlling the refractive index and density within a preferable range, it is suitable for foldable displays, which can obtain better bending resistance and surface hardness than conventional films, especially high pencil hardness of hard coat film after laminating a hard coat layer. Polyester film can be provided.

Specifically, for example, PET pellets are sufficiently vacuum-dried, then supplied to an extruder, melt-extruded into a sheet at about 280 ° C., and cooled and solidified to form an unstretched PET sheet. The obtained unstretched sheet is stretched 1.2 to 2.0 times, more preferably 1.7 to 2.0 times in the longitudinal direction with a roll heated to 75 to 120 ° C. to obtain a uniaxially oriented PET film. .. Further, the edge of the film is gripped with a clip and guided to a hot air zone heated to 75 to 120 ° C., and after drying, 4.0 to 6.0 times in the width direction, more preferably 4.4 to 6. Stretch 0 times. Subsequently, the heat treatment zone can be led to a heat treatment zone of 180 to 240 ° C., and the heat treatment can be performed for 1 to 60 seconds. In this heat treatment step, if necessary, a relaxation treatment of 0 to 10% may be performed in the width direction or the longitudinal direction.

The ultimate viscosity of the polyester film as the base material is preferably in the range of 0.50 to 1.0 dl / g. When the ultimate viscosity is 0.50 dl / g or more, the impact resistance is improved and the internal circuit of the display is less likely to be disconnected due to an external impact, which is preferable. On the other hand, when the ultimate viscosity is 1.00 dl / g or less, the film production is stable and preferable without the increase in the filter pressure of the molten fluid becoming too large.

(Easy adhesive layer)
In the present invention, in order to improve the adhesiveness between the polyester film as a base material and the hard coat layer or the like, it is also preferable to laminate an easy-adhesion layer on the polyester film. For the easy-adhesive layer, a coating liquid for forming the easy-adhesive layer is applied to one or both sides of an unstretched or longitudinally uniaxially stretched film, heat-treated and dried as necessary, and further unstretched in at least one direction. It can be obtained by stretching to. Heat treatment can be performed even after biaxial stretching. The final coating amount of the easy-adhesion layer is preferably controlled to 0.005 to 0.20 g / m ² . When the coating amount is 0.005 g / m ² or more, adhesiveness is obtained, which is preferable. On the other hand, when the coating amount is 0.20 g / m ² or less, blocking resistance is obtained, which is preferable.

As the resin contained in the coating liquid used for laminating the easy-adhesion layer, for example, polyester resin, polyether polyurethane resin, polyester polyurethane resin, polycarbonate polyurethane resin, acrylic resin and the like can be used without particular limitation. Examples of the cross-linking agent contained in the coating liquid for forming an easy-adhesion layer include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, and carbodiimide compounds. It is also possible to use a mixture of two or more of each. Due to the nature of the in-line coating, these are preferably coated with an aqueous coating liquid, and the resin or cross-linking agent is preferably a water-soluble or water-dispersible resin or compound.

It is preferable to add particles to the easy-adhesion layer in order to impart slipperiness. The average particle size of the fine particles is preferably 2 μm or less. When the average particle size of the particles exceeds 2 μm, the particles are likely to fall off from the easy-adhesion layer. The particles contained in the easy-adhesion layer include, for example, titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectrite, zirconia, tungsten oxide, lithium fluoride, etc. Examples thereof include inorganic particles such as calcium fluoride and organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles. These may be added to the easy-adhesion layer alone, or may be added in combination of two or more.

Further, as a method of applying the coating liquid, a known method can be used in the same manner as the above-mentioned coating layer. For example, the reverse roll coating method, the gravure coating method, the kiss coating method, the roll brush method, the spray coating method, the air knife coating method, the wire bar coating method, the pipe doctor method, etc. can be mentioned, and these methods can be used alone. Alternatively, it can be performed in combination.

(Hard coat layer)
When the hard coat film of the present invention is used as a surface protective film for protecting a display by locating it on the surface of a foldable display, it is necessary to have a hard coat layer on at least one surface of the polyester film as a base material. preferable. The hard coat layer is preferably located on the surface side of the display on the polyester film and used in the display. The hard coat layer may be provided on both sides in order to suppress curl. As the resin forming the hard coat layer, acrylic type, siloxane type, inorganic hybrid type, urethane acrylate type, polyester acrylate type, epoxy type and the like can be used without particular limitation. Further, two or more kinds of materials can be mixed and used, and particles such as an inorganic filler and an organic filler can be added.

(Thickness of hard coat layer)
The thickness of the hard coat layer is preferably 1 to 50 μm. When it is 1 μm or more, it is sufficiently cured and the pencil hardness becomes high, which is preferable. Further, by setting the thickness to 50 μm or less, curling due to curing shrinkage of the hard coat can be suppressed, and the handleability of the film can be improved.

(Applying method)
As a method for applying the hard coat layer, a Meyer bar, a gravure coater, a die coater, a knife coater and the like can be used without particular limitation, and can be appropriately selected according to the viscosity and the film thickness.

(Curing conditions)
As a curing method of the hard coat layer, energy rays such as ultraviolet rays and electron beams and a curing method by heat can be used, and in order to reduce damage to the film, a curing method using ultraviolet rays and electron beams is preferable.

(Pencil hardness)
The pencil hardness of the hard coat layer is preferably 3H or higher, and more preferably 4H or higher. If the pencil has a hardness of 3H or more, it will not be easily scratched and the visibility will not be deteriorated. Generally, it is preferable that the pencil hardness of the hard coat layer is high, but it may be 9H or less, 8H or less, or 6H or less without any problem in practical use.

(Characteristics of hard coat film)
The hard coat layer in the present invention can be used for the purpose of increasing the pencil hardness of the surface as described above to protect the display, and preferably has a high transmittance. The total light transmittance of the hard coat film is preferably 85% or more, more preferably 88% or more. When the transmittance is 85% or more, sufficient visibility can be obtained. Generally, the higher the total light transmittance of the hard coat film, the more preferable, but from the viewpoint of stable production, it is preferably 99% or less, and may be 97% or less. The haze of the hard coat film is generally preferably low, preferably 3% or less. The haze of the hard coat film is more preferably 2% or less, and most preferably 1% or less. When the haze is 3% or less, the visibility of the image can be improved. Generally, the lower the haze, the better, but from the viewpoint of stable production, 0.1% or more is preferable, and 0.3% or more may be used.

The hard coat layer may be further provided with other functions. For example, a hard coat layer to which functionality such as an antiglare layer having a certain pencil hardness, an antiglare antireflection layer, an antireflection layer, a low reflection layer, an anti-scratch layer, and an anti-static layer as described above is added. Is also preferably applied in the present invention.

A hard coat layer may also be provided when used as a base film for a touch panel module. When, for example, an ITO layer is used as the transparent electrode layer of the touch panel module, it is preferable to provide a depression rate adjusting layer between the base film and the transparent electrode layer in order to make the electrode pattern difficult to see. In this case, the hard coat layer itself may also serve as the refractive index adjusting layer, and the refractive index adjusting may be separately laminated.

Next, the present invention will be described with reference to Examples and Comparative Examples. First, the evaluation method of the characteristic value carried out in the present invention is shown below.

(1) Extreme Viscosity of Polyester A polyester film or polyester resin was crushed and dried, and then dissolved in a mixed solvent of phenol / tetrachloroethane = 60/40 (mass ratio). After centrifuging this solution to remove inorganic particles, the flow time of the solution at a concentration of 0.4 (g / dl) at 30 ° C. and the flow time of the solvent alone were measured using an Ubbelohde viscometer. From those time ratios, the ultimate viscosity was calculated using the Huggins formula, assuming that the Huggins constant was 0.38.

(2) Bending resistance of polyester film sample (bending radius 1.5 mm)
Prepare a polyester film sample having a size of 20 mm in the width direction and 110 mm in the flow direction. Using a no-load U-shaped expansion / contraction tester (DLDMLLH-FS manufactured by Yuasa System Co., Ltd.), the bending radius was set to 1.5 mm, and the bending was performed 200,000 times at a speed of 1 time / second. At that time, the positions of both ends of the sample on the long side were fixed at 10 mm, and the bent portion was set to 20 mm × 90 mm. Here, FIG. 1 is a schematic view for showing the bending radius when the foldable display is folded, and in consideration of the case where the polyester film is arranged on the inner surface of the folded form, FIG. The bending test is performed as a model assuming that the location of reference numeral 11 is set to 1.5 mm. After the bending treatment was completed, the sample was placed on a flat surface with the inside of the bending facing down, and visually observed.
◯: No cracks or deformation can be confirmed in the sample.
×: The sample has cracks or creases, and when placed horizontally, the maximum height is 5 mm or more.

(3) Bending resistance of hard coat film sample (bending radius 3.0 mm)
By the same method as the above bending test, the hard coat film was bent 200,000 times at a bending radius of 3.0 mm and a speed of 1 time / sec. When there was only one hard coat layer, a test with the hard coat layer on the inside and a test with the hard coat layer on the outside were performed respectively.
◯: There is no crack in the hard coat layer, and deformation cannot be confirmed in the hard coat film.
X: Broken, cracked in the hard coat layer, or deformed in the hard coat film can be confirmed.

(4) Refractive index of polyester film Using an Abbe refractive index meter (manufactured by Atago, NAR-4T, measurement wavelength 589 nm) in accordance with JIS K 7142: 2014 "Refractive index measurement method for plastics (Method A)". , The refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction were determined.

(5) Pencil hardness Using a hard coat film as a sample, the measurement was performed at a load of 750 g and a speed of 1.0 mm / s according to JIS K 5600-5-4: 1999. In the present invention, 3H or more was regarded as acceptable.

(6) Total light transmittance was measured using a haze haze meter (NDH5000, manufactured by Nippon Denshoku Kogyo Co., Ltd.).

(7) Density The density was measured according to a method (density gradient tube method) conforming to JIS K 7112: 1999. (Unit: g / cm ³ ).

(8) Test force Pushing depth after unloading The sample is cut into a square of about 2 cm, and the opposite side of the measurement surface is glued (Cemedine (registered trademark) high) on the micro cover glass 18 x 18 mm (manufactured by Matsunami Glass Co., Ltd.). It was fixed at Super 30). After sticking and fixing, leave it at room temperature for 12 hours or more, and then use a dynamic ultra-micro hardness tester "DUH-111" (manufactured by Shimadzu Corporation) under the following conditions to push in depth after unloading the test force ( μm) was measured.
≪Measurement conditions≫
Test mode: Load-unload test
Indenter used: Ridge angle 115 degrees, triangular pyramid indenter
Indenter modulus: 1.140 × 10 ⁶ N / mm ²
Indenter Poisson's ratio: 0.07
Test power: 50mN
Load speed: 4.44 mN / sec
Load holding time: 2 sec
Unloading retention time: 0 sec

(9) Maximum heat shrinkage rate The sample film was cut into a length of 10 mm and a width of 250 mm, marked at intervals of 200 mm according to the direction in which the long side was to be measured, and the interval A of the marks was measured under a constant tension of 5 g. Subsequently, the sample film was left unloaded in an oven at 150 ° C. for 30 minutes, then removed from the oven and cooled to room temperature. Then, the mark interval B was determined under a constant tension of 5 g, and the heat shrinkage rate (%) was determined by the following formula. The heat shrinkage rate is measured at a position divided into three equal parts in the width direction of the sample film, and the average value of the three points is taken as the heat shrinkage rate (%).
Heat shrinkage rate (%) = [(AB) x 100] / A
The sample film is cut and measured separately in both the bending direction and the folding direction so that the vertical and horizontal directions are different, and the data in the direction in which the measured value is large is defined as the maximum heat shrinkage rate (%).

(10) Tensile elastic modulus (Young's modulus (unit: GPa))
The tensile elastic modulus in the bending direction and the folding direction of the polyester film was measured at 23 ° C. according to JIS K7127: 1999.

(Preparation of polyethylene terephthalate pellet (a))
As the esterification reaction device, a continuous esterification reaction device consisting of a three-stage complete mixing tank having a stirrer, a splitter, a raw material charging port and a product extraction port is used, the TPA is 2 tons / hr, and the EG is TPA1. The amount of antimony trioxide is 2 mol per mol, the amount of Sb atom is 160 ppm with respect to the produced PET, and these slurries are continuously supplied to the first esterification reaction can of the esterification reaction apparatus at normal pressure. The reaction was carried out at 255 ° C. with an average residence time of 4 hours.
Next, the reaction product in the first esterification reaction can is continuously taken out of the system and supplied to the second esterification reaction can, and distilled from the first esterification reaction can in the second esterification reaction can. 8% by mass of the EG to be produced is supplied to the produced polymer (produced PET), and an EG solution containing magnesium acetate in an amount of 65 ppm of Mg atoms with respect to the produced PET and 20 ppm of P atoms with respect to the produced PET. An EG solution containing an amount of TMPA was added, and the reaction was carried out at normal pressure at an average residence time of 1.5 hours and at 260 ° C. Next, the reaction product in the second esterification reaction can is continuously taken out of the system and supplied to the third esterification reaction can, and further contains TMPA in an amount of 20 ppm of P atoms with respect to the produced PET. An EG solution was added, and the reaction was carried out at normal pressure at an average residence time of 0.5 hours and at 260 ° C. The esterification reaction product produced in the third esterification reaction can is continuously supplied to a three-stage continuous polycondensation reaction apparatus to perform polycondensation, and further, a filter medium of a stainless sintered body (nominal filtration accuracy of 5 μm). The particles were filtered through 90% of the particles) to obtain polyethylene terephthalate pellets (a) having an ultimate viscosity of 0.62 dl / g.

(Preparation of polyethylene terephthalate pellet (b))
Regarding the manufacturing process of the polyethylene terephthalate pellet (a), the intrinsic viscosity was adjusted to 0.580 dl / g by the same method except that the residence time of the third esterification reaction was adjusted to obtain the polyethylene terephthalate pellet (b). ..

(Preparation of polyethylene terephthalate pellet (c))
The polyethylene terephthalate pellet (a) was subjected to solid phase polymerization at 220 ° C. under a reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus, and the polyethylene terephthalate pellet (c) having an ultimate viscosity of 0.75 dl / g was obtained. It was created.

(Polymerization of urethane resin)
In a four-necked flask equipped with a stirrer, Dimroth condenser, nitrogen introduction tube, silica gel drying tube, and thermometer, 72.96 parts by mass of 1,3-bis (isocyanis methyl) cyclohexane and 12.60 parts of dimethylol propionic acid. A nitrogen atmosphere was added with 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of a polycarbonate diol having a number average molecular weight of 2000, and 85.00 parts by mass of acetonitrile and 5.00 parts by mass of N-methylpyrrolidone as a solvent. The mixture was stirred at 75 ° C. for 3 hours, and it was confirmed that the reaction solution reached a predetermined amine equivalent. Next, the reaction solution was cooled to 40 ° C., and then 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer D solution. Next, 450 g of water was added to a reaction vessel equipped with a homodisper capable of high-speed stirring, adjusted to 25 ° C., and while stirring and mixing at 2000 min-1, the isocyanate group-terminated prepolymer was added and dispersed in water. did. Then, under reduced pressure, acetonitrile and a part of water were removed to prepare a water-soluble polyurethane resin (A) having a solid content of 35% by mass.

(Polymerization of water-soluble carbodiimide compound)
200 parts by mass of isophorone diisocyanate and 4 parts by mass of 3-methyl-1-phenyl-2-phospholene-1-oxide of carbodiimidization catalyst in a flask equipped with a thermometer, a nitrogen gas introduction tube, a reflux condenser, a dropping funnel, and a stirrer. Was charged and stirred at 180 ° C. for 10 hours under a nitrogen atmosphere to obtain an isocyanate-terminated isophorone carbodiimide (degree of polymerization = 5). Then, 111.2 g of the obtained carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100 ° C. for 24 hours. Water was gradually added thereto at 50 ° C. to obtain a yellow transparent water-soluble carbodiimide compound (B) having a solid content of 40% by mass.

(Preparation of coating liquid for easy adhesive layer formation)
The following coating agents were mixed to prepare a coating solution.
Water 16.97 parts by mass Isopropanol 21.96 parts by mass Polyurethane resin (A) 3.27 parts by mass Water-soluble carbodiimide compound (B) 1.22 parts by mass Particles 0.51 parts by mass (Silica sol with an average particle size of 40 nm, solid content Concentration 40% by mass)
Surfactant 0.05 parts by mass (silicone type, solid content concentration 100% by mass)

(Preparation of hard coat coating liquid a)
To 100 parts by mass of the hard coat material (JSR, Opstar (registered trademark) Z7503, concentration 75%), 0.1 part by mass of the leveling agent (Big Chemie Japan, BYK307, concentration 100%) was added, and the mixture was made with methyl ethyl ketone. A hard coat coating liquid a having a solid content concentration of 40% by mass was prepared by diluting.

(Preparation of hard coat coating liquid b)
Pentaerythritol triacrylate (manufactured by Shin-Nakamura Chemical Industry Co., Ltd., A-TMM-3, solid content concentration 100%) 95 parts by mass, photopolymerization initiator (manufactured by BASF Japan, Irgacure (registered trademark) 907, solid content concentration 100%) ) 5 parts by mass and 0.1 part by mass of a leveling agent (BYK307, solid content concentration 100%) manufactured by Big Chemie Japan Co., Ltd. are mixed and diluted with a solvent of toluene / MEK = 1/1 to a concentration of 40% by mass. A hard coat coating solution b was prepared.

(Example 1)
The polyethylene terephthalate pellet (a) was supplied to the extruder and melted at 285 ° C. This polymer is filtered through a stainless steel sintered filter medium (nominal filtration accuracy of 10 μm particles 95% cut), extruded into a sheet from the base, and then cast into a casting drum with a surface temperature of 30 ° C. using an electrostatic application casting method. The unstretched film was prepared by contacting and cooling and solidifying. This unstretched film was uniformly heated to 75 ° C. using a heating roll and heated to 85 ° C. with a non-contact heater to perform 1.4 times roll stretching (longitudinal stretching). The above-mentioned coating liquid for forming an easy-adhesion layer was applied to both sides of the obtained uniaxially stretched film by a roll coating method, and then dried at 80 ° C. for 20 seconds. The final (after biaxial stretching) coating amount after drying was adjusted to 0.06 g / m ² . Then, it is guided to a tenter, preheated at 105 ° C., laterally stretched 4.0 times at 95 ° C., fixed in width, heat-fixed at 230 ° C. for 5 seconds, and further relaxed by 4% in the width direction at 180 ° C. A polyethylene terephthalate film having a thickness of 50 μm was obtained. Using a Meyer bar, the hard coat coating liquid a was applied to one surface of the produced film so that the film thickness after drying was 5 μm, dried at 80 ° C. for 1 minute, and then irradiated with ultraviolet rays (integration). A hard coat film with a light intensity of 200 mJ / cm ² ) was obtained. The evaluation results are shown in Table 1.

(Examples 2 to 3)
A polyester film was obtained in the same manner as in Example 1 except that the draw ratio was changed in the longitudinal direction shown in Table 1, to obtain a hard coat film.

(Example 4)
A polyester film was obtained in the same manner as in Example 1 except that the stretching ratio in the width direction was changed to 4.4 times and the heat fixing temperature was changed to 220 ° C. to obtain a hard coat film.

(Examples 5 to 6)
A polyester film was obtained in the same manner as in Example 4 except that the stretching ratio was changed in the longitudinal direction as shown in Table 1, and a hard coat film was obtained.

(Example 7)
A polyester film was obtained in the same manner as in Example 1 except that the stretching ratio in the width direction was changed to 5.5 times and the heat fixing temperature was changed to 190 ° C. to obtain a hard coat film.

(Examples 8 to 9)
A polyester film was obtained in the same manner as in Example 7 except that the stretching ratio was changed in the longitudinal direction as shown in Table 1, and a hard coat film was obtained.

(Example 10)
In the production process of Example 5, a polyester film was obtained in the same manner as in Example 5 except that the film was stretched in the longitudinal direction and then subjected to a relaxation heat treatment of 10% at 100 ° C. to obtain a hard coat film.

(Example 11)
In the manufacturing process of Example 5, a polyester film was obtained in the same manner as in Example 5 except that the clip was opened at 200 ° C. after heat fixing and relaxation heat treatment was performed in the longitudinal direction and the width direction to obtain a hard coat film. .. In the longitudinal direction, the tenter speed and the take-up roll speed were adjusted so that the relaxation rate was 3%. Relaxation in the width direction was left free.

(Example 12)
A polyester film was obtained in the same manner as in Example 1 except that the temperature at the time of stretching in the longitudinal direction was changed to 75 ° C. and the heat fixing temperature was changed to 220 ° C. to obtain a hard coat film.

(Example 13)
The temperature at the time of stretching in the longitudinal direction was changed to 75 ° C., the stretching ratio was changed to 1.2 times, and then the stretching ratio was changed to 5.0 times in the width direction, and the stretching was performed in the same manner as in Example 1. To obtain a polyester film and a hard coat film.

(Example 14)
The stretching in the longitudinal direction of Example 3 was set to two-step stretching, the stretching ratio of the first step was set to 1.2 times, and the stretching ratio of the second step was set to 1.67 times in the same manner as in Example 3. A polyester film was obtained and a hard coat film was obtained. The total elongation ratio in the longitudinal direction is about 2.0 times.

(Example 15)
A polyester film was obtained in the same manner as in Example 5 except that the preheating temperature at the time of stretching in the width direction was changed to 95 ° C. and the heat fixing temperature was changed to 190 ° C. to obtain a hard coat film.

(Example 16)
The stretching in the width direction of Example 2 was defined as two-stage stretching, the first-stage stretching ratio was 1.5 times, the second-stage stretching ratio was 4.0 times, and the heat fixing temperature was changed to 190 ° C. Other than that, a polyester film was obtained in the same manner as in Example 2 to obtain a hard coat film. The total draw ratio in the width direction is 6.0 times.

(Examples 17-18)
A polyester film was obtained in the same manner as in Example 2 except that the thickness was changed as shown in Table 1, to obtain a hard coat film.

(Example 19)
A polyester film was obtained in the same manner as in Example 1 except that the relaxation heat treatment in the width direction was not performed in the manufacturing process of Example 1, to obtain a hard coat film.

(Example 20)
After preparing an unstretched film in the same manner as in Example 1, the unstretched film was preheated at 75 ° C. with a tenter and laterally stretched 1.4 times at 85 ° C. The above-mentioned coating liquid for forming an easy-adhesion layer was applied to both sides of the obtained uniaxially stretched film by a roll coating method, and then dried at 80 ° C. for 20 seconds. The final (after biaxial stretching) coating amount after drying was adjusted to 0.06 g / m ² . Uniformly heat to 105 ° C. using a heating roll and heat to 95 ° C. with a non-contact heater. Roll stretching (longitudinal stretching) was performed 4.0 times. The width was fixed and heat-fixed at 230 ° C. for 5 seconds to obtain a polyethylene terephthalate film having a thickness of 50 μm. A hard coat film was obtained in the subsequent steps in the same manner as in Example 1.

(Example 21)
A polyethylene terephthalate film having a thickness of 50 μm was obtained in the same manner as in Example 1, and then a hard coat film coated with the hard coat coating liquid b was obtained.

(Comparative Example 1)
A polyester film was obtained in the same manner as in Example 1 except that the film was stretched only in the width direction and stretched in the lateral uniaxial direction without stretching in the longitudinal direction to obtain a hard coat film.

(Comparative Example 2)
A polyester film was obtained in the same manner as in Example 7 except that the film was stretched only in the width direction and stretched in the lateral uniaxial direction without stretching in the longitudinal direction to obtain a hard coat film.

(Comparative Examples 3 to 7)
A polyester film was obtained in the same manner as in Example 1 except that the heat fixing temperature was changed to 220 ° C. and the PET pellets and thicknesses shown in Table 1 were used to obtain a hard coat film.
Comparative Examples 3 to 7 are a combination of each condition level which is not the best in the condition range in which the heat fixing temperature is lower than that of Example 1 and the stretching ratio in the longitudinal direction and the width direction is preferable as described above. As described in No. 1, the refractive index in the thickness direction increased, the pushing depth after unloading the test force was large, and the pencil hardness after laminating the hard coat layer was smaller than in each example.

(Comparative Example 8)
A polyester film was obtained in the same manner as in Example 1 except that the stretching ratio in the longitudinal direction was changed to 2.7 times and the heat fixing temperature was changed to 220 ° C. to obtain a hard coat film.

(Comparative Example 9)
A polyester film was obtained in the same manner as in Example 1 except that the draw ratio in the longitudinal direction was changed to 3.4 times, and a hard coat film was obtained.

(Comparative Example 10)
A polyester film was obtained in the same manner as in Example 4 except that the heat fixing temperature was changed to 100 ° C. to obtain a hard coat film.

(Comparative Example 11)
A polyester film was obtained in the same manner as in Example 13 except that the stretching temperature in the longitudinal direction was changed to 130 ° C. to obtain a hard coat film.

(Comparative Example 12)
A polyester film was obtained in the same manner as in Example 1 except that the preheating temperature in the width direction was changed to 120 ° C. to obtain a hard coat film.

A hard coat film is attached to an organic EL module via an adhesive layer with a thickness of 25 μm to create a smartphone-type foldable display that can be folded in half at the center of the whole where the radius corresponding to the bending radius in FIG. 1 is 3 mm. did. The hard coat film is arranged on the surface of one continuous display via the folding portion, and the hard coat layer is arranged so as to be located on the surface of the display. The one using the hard coat film of each example satisfied the operation and visibility as a smartphone that can be folded in half at the center and carried. In addition, the surface was not dented by an external force. On the other hand, the foldable display using the hard coat film of each comparative example seems to have caused image distortion at the foldable portion of the display as the frequency of use increases, which is not very preferable. In addition, some were dented on the surface and scratches were confirmed.

The foldable display using the hard coat film for a foldable display of the present invention deforms after the polyester film or hard coat film located on the surface of the foldable display is repeatedly folded while maintaining mass productivity. Since it does not occur, the image is not distorted at the folded portion of the display. In particular, a mobile terminal device or an image display device equipped with a foldable display using the hard coat flume of the present invention as a surface protective film provides beautiful images, is rich in functionality, and is excellent in convenience such as portability. Is.

1: Folding display 11: Bending radius 2: Base material polyester film constituting the hard coat film for foldable display 21: Folding part 22: Bending direction (direction orthogonal to the folding part)

Claims (6)

A hard coat film having a hard coat layer on at least one side of a polyester film having a thickness of 10 to 80 μm and having a total light transmittance of 85% or more, wherein the polyester film satisfies the following conditions.
(1) The refractive index in the bending direction is 1.590 to 1.620.
(2) The refractive index in the direction of the folding part is 1.670 to 1.700.
(3) Refractive index in the thickness direction is 1.520 or less (4) Density is 1.380 g / cm ³ or more
(Here, the bending direction means a direction orthogonal to the folding portion when the polyester film is folded.) The hard coat film for a foldable display according to claim 1, wherein the polyester film has an elastic modulus in the bending direction of 2.7 GPa or less and an elastic modulus in the folding portion direction of 4.5 GPa or more. The hard coat film for a foldable display according to claim 1 or 2, wherein the haze is 3% or less. The hard coat film for a foldable display according to any one of claims 1 to 3, wherein the thickness of the hard coat layer is 1 to 50 μm. The hard coat film for a foldable display according to claim 4 is a foldable display arranged as a surface protective film so that the hard coat layer is located on the surface, and is continuous through a foldable portion of the foldable display. A foldable display with a single hard coat film. A mobile terminal device having the foldable display according to claim 5.