JP7568005B2 - Polyester film for surface protection of folding displays and its applications - Google Patents

Description

The present invention relates to a polyester film for surface protection film of a folding display, a hard coat film for surface protection film of a folding display, a folding display, and a mobile terminal device, and relates to a folding display and a mobile terminal device that are unlikely to suffer from image distortion due to deformation of the film located on the surface even when folded repeatedly, and to a polyester film and a hard coat film for surface protection film of the folding display.

As mobile terminal devices become thinner and lighter, smartphones and other mobile terminal devices are becoming more and more widespread. While mobile terminal devices are required to have a variety of functions, they are also required to be convenient to use. For this reason, the mobile terminal devices that are currently in widespread use must be able to perform simple operations with one hand, and must have a small screen size of around 6 inches, since they are assumed to be stored in a clothing pocket, etc.

On the other hand, tablet devices with screen sizes between 7 and 10 inches are highly functional and are intended for use not only for video content and music, but also for business purposes, drawing, reading, etc. However, they cannot be operated with one hand, are less portable, and have issues with convenience.

To achieve these goals, a method has been proposed to make the display more compact by connecting multiple displays together (Patent Document 1), but this method has not become widespread due to the remaining bezel, which results in the image being cut off and reduced visibility.

In recent years, mobile devices incorporating flexible and foldable displays have been proposed. With this method, the image is uninterrupted and the device can be conveniently carried around as a mobile device equipped with a large-screen display.

In conventional displays and mobile terminal devices that do not have a folding structure, the surface of the display can be protected with an inflexible material such as glass, but in the case of a folding display, when a full-surface display is to be created via a folding section, it is necessary to use a hard coat film or the like that is flexible and can protect the surface. However, in a folding display, the area that corresponds to a certain folding section is repeatedly folded, and as a result, the film in that area deforms over time, causing problems such as distorting the image displayed on the display.

A method has been proposed to change the film thickness partially (see Patent Document 2), but this has the problem of poor mass productivity.

JP 2010-228391 A JP 2016-155124 A

The present invention seeks to solve the problems associated with conventional surface protection members for displays as described above, and aims to provide a foldable display that is easy to mass-produce and has no risk of image distortion at the folded portion after repeated folding, as well as a mobile terminal device equipped with such a foldable display, by providing a polyester film for surface protection films for foldable displays and a hard coat film for surface protection films.

That is, the present invention comprises the following:
1. A polyester film for use as a surface protection film for a folding display, the polyester film having a thickness of 10 to 75 μm and a 0.2% yield point strain in at least one of the longitudinal direction and the width direction of 2.6 to 5.0%.
2. The polyester film for surface protection of a folding display according to claim 1, characterized in that the 0.2% proof stress strain in the bending direction is 2.6 to 5.0%.
(Here, the bending direction refers to the direction perpendicular to the fold when folding the polyester film.)
3. The polyester film for surface protection of a folding display according to the above 1 or 2, characterized in that the intrinsic viscosity of the film is 0.60 to 1.0 dl/g.
4. A hard coat film for a surface protective film of a folding display, comprising the polyester film for a surface protective film of a folding display according to any one of the first to third aspects described above, and a hard coat layer having a thickness of 1 to 50 μm on at least one side of the polyester film for a surface protective film of a folding display.
5. The hard coat film for surface protection of a folding display according to the above 4, wherein the hard coat layer has a pencil hardness of H or more as measured under a load of 750 g in accordance with JIS K5600-5-4:1999.
6. A foldable display in which the hard coat film for a surface protective film of a foldable display according to the above 4 or 5 is arranged as a surface protective film so that the hard coat layer is positioned on the surface, and the foldable display has a bending radius of 5 mm or less when folded.
7. The foldable display according to the above item 6, wherein a single continuous hard coat film is disposed across the folding portion of the foldable display.
8. A mobile terminal device having the foldable display according to claim 6 or 7.

A folding display using the polyester film or hard coat film for surface protection of a folding display of the present invention maintains mass productivity, and the polyester film or hard coat film does not deform after repeated folding, so that no image distortion occurs at the folded portion of the display. A mobile terminal device equipped with such a folding display provides beautiful images, is highly functional, and has excellent convenience such as portability.

FIG. 2 is a schematic diagram showing the measurement points of the bending radius when folded in the present invention. FIG. 2 is a schematic diagram showing a bending direction of a polyester film for a surface protection film of a folding display according to the present invention. FIG. 2 is a schematic diagram for explaining the 0.2% proof stress point strain in the present invention.

(display)
The display in the present invention generally refers to a display device, and the types of displays include LCD, organic EL display, inorganic EL display, LED, FED, etc., but LCD, organic EL, and inorganic EL, which have a foldable structure, are preferred. In particular, organic EL and inorganic EL, which can reduce the layer structure, are particularly preferred, and organic EL, which has a wide color gamut, is even more preferred.

(Foldable display)
The folding display is preferably a structure in which a single continuous display is folded in half when carried, thereby reducing the size by half and improving portability. At the same time, it is desirable that the display is thin and lightweight. Therefore, the bending radius of the folding display is preferably 5 mm or less, and more preferably 3 mm or less. If the bending radius is 5 mm or less, the display can be made thin when folded. It can be said that the smaller the bending radius, the better, but it may be 0.1 mm or more, or 0.5 mm or more. Even if it is 1 mm or more, the practicality is sufficiently good compared to conventional displays that do not have a folding structure. The bending radius when folded is measured at the location of the symbol 11 in the schematic diagram of FIG. 1, and means the radius of the inside of the folded part when folded. The surface protective film described later may be located on the outside or inside of the folded display.

(Organic EL)
A typical organic EL display is composed of an organic EL layer consisting of an electrode/electron transport layer/light emitting layer/hole transport layer/transparent electrode, a retardation plate for improving image quality, and a polarizing plate.

(Mobile terminal device with touch panel)
When an organic EL display is used in a mobile terminal device having a touch panel, a touch panel module is placed on the top of the organic EL display or between the organic EL layer and the retardation plate. In this case, if an impact is applied from above, the circuits of the organic EL and the touch panel may be broken, so a surface protection film is necessary, and it is preferable that a film placed on the front surface of the display as a surface protection film has a hard coat layer laminated on at least the front surface side of the display.

(Surface protection film for foldable displays)
As the surface protection film, any film having high light transmittance and low haze, such as a polyimide film, a polyester film, a polycarbonate film, an acrylic film, a triacetyl cellulose film, or a cycloolefin polymer film, can be used. Among these, polyimide films and polyester films having high impact resistance and sufficient pencil hardness are preferred, and polyester films which can be produced inexpensively are particularly preferred.

In the present invention, the polyester film may be a single-layer film made of one or more types of polyester resin, or when two or more types of polyester are used, it may be a multilayer film or an ultra-multilayer laminate film with a repeating structure.

Examples of polyester resins include polyester films made of polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or copolymers whose main components are the constituent components of these resins. Among these, stretched polyethylene terephthalate films are particularly preferred in terms of mechanical properties, heat resistance, transparency, price, etc.

When a polyester copolymer is used for the polyester film, examples of the dicarboxylic acid component of the polyester include aliphatic dicarboxylic acids such as adipic acid and sebacic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; and 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; alicyclic glycols such as 1,4-cyclohexanedimethanol; and polyethylene glycols having an average molecular weight of 150 to 20,000. The mass ratio of the copolymerization components 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.

In addition, in the production of polyester films, the intrinsic viscosity of at least one type of resin pellet is preferably in the range of 0.60 to 1.0 dl/g. When the intrinsic viscosity is 0.60 dl/g or more, the impact resistance of the obtained film is improved, and disconnection of the internal circuit due to external impact is less likely to occur, which is preferable. It is also preferable because it contributes to minimizing deformation when repeatedly bent. On the other hand, when the intrinsic viscosity is 1.00 dl/g or less, the increase in filtration pressure of the molten fluid does not become too large, making it easier to operate the film production stably, which is preferable.

Regardless of whether the film has a single-layer or laminated structure, the intrinsic viscosity of the film is preferably 0.60 dl/g or more. It is more preferably 0.62 dl/g or more. Even more preferably, it is 0.68 dl/g or more. If it is 0.60 dl/g or more, it can impart fatigue resistance and can sufficiently achieve the effect of bending resistance. On the other hand, a film with an intrinsic viscosity of 1.00 dl/g or less can be produced with good operability and is therefore preferable.

The thickness of the polyester film is preferably 10 to 75 μm, and more preferably 25 to 75 μm. A thickness of 10 μm or more improves pencil hardness, while a thickness of 75 μm or less is advantageous for weight reduction and is excellent in flexibility, processability, and handling properties.

The surface of the polyester film of the present invention may be smooth or uneven, but since it is used as a surface cover for a display, deterioration of optical properties due to unevenness is undesirable. The haze is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. If the haze is 3% or less, the visibility of the image can be improved. The lower limit of the haze is preferably as small as possible, but it may be 0.1% or more, or 0.3% or more.

As mentioned above, in order to reduce haze, it is better for the film surface to not have too much unevenness, but in order to provide a certain degree of slipperiness from the viewpoint of handling, unevenness can be formed by blending a filler into the polyester resin layer on the surface or by coating a coat layer containing filler during film formation.

Methods for incorporating particles into the base film can be known. For example, they can be added at any stage of polyester production, but preferably they can be added as a slurry dispersed in ethylene glycol or the like at the esterification stage, or at a stage after the completion of the transesterification reaction and before the start of the polycondensation reaction, to advance the polycondensation reaction. Alternatively, they can be added by using a vented kneading extruder to blend a slurry of particles dispersed in ethylene glycol or water with the polyester raw material, or by using a kneading extruder to blend dried particles with the polyester raw material.

Among these, a method in which aggregated inorganic particles are homogeneously dispersed in a monomer liquid that becomes part of the polyester raw material, and then the filtered product is added to the remainder of the polyester raw material before, during, or after the esterification reaction is preferred. With this method, the monomer liquid has a low viscosity, so that homogeneous dispersion of the particles and highly accurate filtration of the slurry can be easily performed, and when added to the remainder of the raw material, the particles have good dispersibility and new aggregates are unlikely to be generated. From this perspective, it is particularly preferred to add the monomer liquid to the remainder of the raw material that is in a low-temperature state before the esterification reaction.

In addition, after obtaining polyester containing particles in advance, the number of protrusions on the film surface can be further reduced by a method such as kneading and extruding the pellets with pellets that do not contain particles (masterbatch method).

The polyester film may also contain various additives within a range that maintains the preferred range of total light transmittance. Examples of additives include antistatic agents, UV absorbers, and stabilizers.

The total light transmittance of the polyester film is preferably 85% or more, and more preferably 87% or more. A transmittance of 85% or more ensures sufficient visibility. The higher the total light transmittance of the polyester film, the better, but it may be 99% or less, or 97% or less.

The surface of the polyester film of the present invention can be treated to improve adhesion with the resin that forms the hard coat layer, etc.

Surface treatment methods include, for example, roughening treatment using sandblasting or solvent treatment, and oxidation treatments such as corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone or ultraviolet irradiation treatment, flame treatment, chromic acid treatment, and hot air treatment, and can be used without any particular limitations.

In addition, adhesion can be improved by using an adhesion-improving layer such as an easy-adhesion layer. The easy-adhesion layer can be made of any resin, including acrylic resin, polyester resin, polyurethane resin, and polyether resin, and can be formed by a general coating method, preferably a so-called in-line coating method.

The polyester film described above can be manufactured, for example, through a polymerization process in which inorganic particles are homogeneously dispersed in a monomer liquid that becomes part of the polyester raw material, filtered, and then added to the remainder of the polyester raw material to polymerize the polyester, and a film formation process in which the polyester is melt-extruded through a filter into a sheet, which is cooled and stretched to form a base film.

Next, a method for producing a biaxially stretched polyester film will be described in detail using an example in which polyethylene terephthalate (hereinafter, referred to as PET) pellets are used as the raw material for the base film, but the present invention is not limited thereto. In addition, the number of layers, such as a single layer structure or a multilayer structure, is not limited thereto.

After mixing and drying PET pellets in a specified ratio, they are fed into a known melt lamination extruder, extruded into a sheet from a slit die, and cooled and solidified on a casting roll to form an unstretched film. A single extruder is sufficient for a single layer, but when producing a multi-layered film, two or more extruders, two or more manifolds or merging blocks (e.g., merging blocks with rectangular merging sections) can be used to laminate the multiple film layers that make up the outermost layers, extrude a sheet of two or more layers from a die, and cool it on a casting roll to form an unstretched film.

In this case, it is preferable to perform high-precision filtration to remove foreign matter contained in the resin at any location where the molten resin is kept at about 280°C during melt extrusion. There are no particular limitations on the filter material used for high-precision filtration of the molten resin, but sintered stainless steel filter material is preferable because it has excellent performance in removing aggregates and high-melting-point organic matter whose main components are Si, Ti, Sb, Ge, and Cu.

Furthermore, the filtration particle size of the filter medium (initial filtration efficiency 95%) is preferably 20 μm or less, and particularly preferably 15 μm or less. If the filtration particle size of the filter medium (initial filtration efficiency 95%) exceeds 20 μm, foreign matter of a size of 20 μm or more cannot be sufficiently removed. Although the productivity may decrease by performing high-precision filtration of molten resin using a filter medium with a filtration particle size (initial filtration efficiency 95%) of 20 μm or less, this is preferable in terms of obtaining a film with fewer protrusions due to coarse particles.

The 0.2% proof stress strain in at least one of the longitudinal direction (machine flow direction) and the transverse direction of the polyester film is preferably 2.6 to 5.0%, and more preferably 3.0 to 5.0%. The 0.2% proof stress strain is a value used as a substitute for the yield point, whether or not a yield point appears in the stress-strain curve, and can be used as an index of the elastic region. Figure 3 shows a schematic diagram of a stress-strain curve for determining the 0.2% proof stress strain. Figure 3 shows an example of a case where a yield point does not appear. The 0.2% proof stress strain is determined by a normal tensile test. On the stress-strain curve, a parallel line is drawn from point Q, where the stress is 0 MPa and the strain is 0.2%, to OA, where Hooke's law holds true, and the point where it intersects with the stress-strain curve is the proof stress point P. The value of the strain at point H where a parallel line is drawn from the vertical axis and intersects with the strain axis is the 0.2% proof stress point strain. The 0.2% proof stress point strain can be obtained from the stress-strain curve as described above, but if the stress-strain curve is obtained using a tensile testing machine (Shimadzu AUTOGRAPH AG-X Plus 1kN), the calculated result of the 0.2% proof stress point strain can then be obtained directly by setting the strain to 0.2% using Shimadzu's autograph software TRAPEZIUM X.

When the 0.2% proof stress point strain is 2.6% or more, deformation due to strain generated during folding is unlikely to occur, which is preferable. When it is 5.0% or less, good impact resistance is obtained, which is preferable. The 0.2% proof stress point strain in the bending direction of the polyester film is preferably 2.6 to 5.0%, and more preferably 3.0 to 5.0%. Here, the bending direction refers to the direction perpendicular to the folding part (reference number 21) assumed in the application of the surface protection film for a folding display, as shown by reference number 22 on the polyester film (reference number 2) in Figure 2. The bending direction is not limited to either the longitudinal direction or the transverse direction of the film.

The 0.2% yield point strain can be adjusted to within the above range by adjusting the stretch ratio, stretching temperature, heat setting temperature, polyester raw material, etc.

The 0.2% proof stress point strain of the polyester film can be effectively adjusted by adjusting the stretching ratio. The stretching ratio of the unstretched polyester sheet in at least one of the longitudinal direction (machine flow direction) and the width direction is preferably 1.0 to 3.4 times, more preferably 1.0 to 3.0 times. By setting the stretching ratio to 1.0 to 3.4 times, the 0.2% proof stress point strain can be adjusted to a high range as described above, and deformation when repeatedly folded is small. The stretching direction is preferably the bending direction as described above. The stretching temperature is preferably 80 to 130°C, more preferably 90 to 130°C. The heating method during stretching can be a conventionally known method such as a hot air heating method, a roll heating method, or an infrared heating method. By setting the stretching temperature to 80 to 130°C, thickness unevenness due to stretching at the above stretching ratio can be prevented.

In terms of the mechanical properties of the film, it is preferable that the stretching ratio in the direction perpendicular to the bending direction (the direction of the folded part) is larger than the bending direction, and an example of the stretching ratio in the direction perpendicular to the bending direction is 2.5 to 5.0 times. A stretching ratio of 2.5 times or more can provide stable productivity, and a stretching ratio of 5.0 times or less can provide good impact resistance.

The degree of crystallinity of the polyester film is preferably 35 to 54%, and more preferably 43 to 54%. Increasing the degree of crystallinity can improve the 0.2% proof stress strain and impact resistance. In order to effectively increase the degree of crystallinity, it is preferable to perform a heat treatment at 180 to 250°C after biaxial stretching. If the temperature is 180°C or higher, the degree of crystallinity can be effectively increased, and if the temperature is 250°C or lower, there is no risk of the polyester film surface melting and the appearance is maintained in a good condition, which is preferable.

Specifically, for example, PET pellets are thoroughly vacuum dried, then fed to an extruder, melt-extruded into a sheet at about 280°C, and cooled to solidify, forming an unstretched PET sheet. The resulting unstretched sheet is stretched 1.0 to 3.4 times in the longitudinal direction with rolls heated to 80 to 130°C to obtain a uniaxially oriented PET film. The ends of the film are then held with clips and guided to a hot air zone heated to 80 to 180°C, where it is dried and stretched 2.5 to 5.0 times in the width direction. It is then guided to a heat treatment zone at 180 to 250°C, where it is heat-treated for 1 to 60 seconds to complete the crystal orientation. During this heat treatment process, a relaxation treatment of 1 to 12% may be performed in the width or length direction as necessary.

(Hard Coat Layer)
The polyester film that is placed on the surface of the foldable display to protect the display preferably has a hard coat layer on its surface. The hard coat layer is preferably placed on the display surface side of the polyester film and used in the display. The resin that forms the hard coat layer can be acrylic, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, epoxy, or other resins without any particular limitation. Two or more types of materials can be mixed and used, or particles such as inorganic filler or organic filler can be added.

(Film Thickness)
The thickness of the hard coat layer is preferably 1 to 50 μm. More preferably 1 to 40 μm. If the thickness is more than 1 μm, the hard coat layer is sufficiently cured and a good pencil hardness is obtained. Furthermore, by making the thickness 50 μm or less, curling due to cure shrinkage of the hard coat can be suppressed, and the handling properties of the film can be improved. More preferably, the thickness is 2 to 25 μm, particularly preferably 2 to 20 μm, and most preferably 3 to 15 μm.

(Application method)
The hard coat layer can be applied by any method including, but not limited to, a Mayer bar, a gravure coater, a die coater, a knife coater, etc., and can be appropriately selected depending on the viscosity and film thickness.

(Curing conditions)
The hard coat layer can be cured by energy rays such as ultraviolet rays and electron beams, or by heat, but curing methods using ultraviolet rays or electron beams are preferred in order to reduce damage to the film.

(Pencil hardness)
The pencil hardness of the hard coat layer is preferably B or more, more preferably H or more, and particularly preferably 2H or more. If the hard coat layer has a pencil hardness of B or more, it is not easily scratched and does not reduce visibility. In general, the pencil hardness of the hard coat layer is preferably high, but it may be 10H or less, 8H or less, or even 6H or less can be used without any practical problems.

(Type of hard coat layer)
The hard coat layer in the present invention may have other functions added thereto, so long as it can be used for the purpose of increasing the pencil hardness of the surface and protecting the display as described above.For example, hard coat layers with added functions such as antiglare layers, antiglare antireflection layers, antireflection layers, low reflection layers, and antistatic layers having a certain pencil hardness as described above are also preferably used in the present invention.

Next, the effects of the present invention will be explained using examples and comparative examples. First, the evaluation methods for the characteristic values used in the present invention are shown below.

(1) 0.2% proof stress strain The film was cut into a strip of 10 mm width with a measurement direction of 140 mm to prepare a sample, and a tensile test was performed at a tensile speed of 100 mm/min using a tensile tester (AUTOGRAPH AG-X Plus 1kN manufactured by Shimadzu Corporation) to obtain a stress-strain curve. Then, the 0.2% proof stress strain was calculated using Shimadzu Corporation's autograph software TRAPEZIUM X, with a strain setting of 0.2%.

(2) Intrinsic Viscosity After pulverizing and drying the film or polyester resin, it was dissolved in a mixed solvent of phenol/tetrachloroethane = 60/40 (mass ratio). After the solution was centrifuged to remove inorganic particles, the flow time of the solution with a concentration of 0.4 (g/dl) at 30 ° C. and the flow time of the solvent alone were measured using an Ubbelohde viscometer, and the intrinsic viscosity was calculated from the time ratio using the Huggins formula and assuming that the Huggins constant is 0.38. In the case of a laminated film, the intrinsic viscosity of each layer alone was evaluated by scraping off the corresponding polyester layer of the film according to the laminate thickness.

(3) Crystallinity The density of the film sample was measured at three points. The average value was taken as the crystallinity. The resin component was a mixture of completely amorphous and completely crystalline components, and its density was as described below. Based on the assumption that the density of the sample was the sum of the masses of each component constituting the sample divided by the sum of the volumes of each component, the crystallinity (weight ratio) of each resin was estimated. The density of the sample was measured according to a method (density gradient tube method) in accordance with JIS K-7112-1980. The density of each component alone was calculated using the following values (unit: g/cm3):
Polyethylene terephthalate resin: completely amorphous 1.34, completely crystalline 1.46
Polyethylene naphthalate resin: completely amorphous 1.32, completely crystalline 1.41

(4) Bending resistance A polyester film was cut into a size of 200 mm (bending direction) x 50 mm (orthogonal direction) to prepare a measurement sample. Spacers of each thickness were placed at the ends of two 5 mm thick glass plates to create a space, and the film was sandwiched and held for 10 seconds. Immediately after that, the film was made to reflect the light of a fluorescent lamp, the folded part was observed, and the interval at which no folding marks were left was recorded.
◯: The interval where no folding marks were left was less than 6.5 mm.
△: The interval where no folding marks were left was 6.5 mm or more and less than 7.0 mm. ×: The interval where no folding marks were left was 7.0 mm or more.

(5) Repeated bending resistance A sample measuring 50 mm in width direction x 100 mm in machine direction was prepared. Using a no-load U-shaped stretch tester (Yuasa System Co., Ltd., DLDMLH-FS), the bending radius was set to 3 mm, and the sample was bent 50,000 times at a speed of 1 time/second. At that time, the sample was fixed at a position 10 mm from both ends of the long side, and the bending area was 50 mm x 80 mm. After the bending process was completed, the sample was placed on a flat surface with the inside of the bend facing down, and visual inspection was performed.
◯: The sample was not deformed, or even if it was deformed, the maximum height of the rise when placed horizontally was less than 3 mm.
△: The sample is deformed, and when placed horizontally, the maximum height of the rise is 3 mm to 5 mm.
less than.
×: The sample had a crease or the maximum height of the sample when placed horizontally was 5 mm or more.

(6) Pencil Hardness The prepared polyester film with a hard coat was measured in accordance with JIS K 5600-5-4:1999 under a load of 750 g and a speed of 0.5 mm/s.

(Coating solution 1 for forming hard coat layer)
To 100 parts by weight of a hard coat material (Opstar (registered trademark) Z7503, concentration 75%, manufactured by JSR Corporation), 0.1 parts by weight of a leveling agent (BYK307, concentration 100%, manufactured by BYK Japan K.K.) was added, and the mixture was diluted with methyl ethyl ketone to prepare hard coat coating solution 1 having a solid content concentration of 40% by weight.

(Coating solution 2 for forming hard coat layer)
95 parts by weight of a urethane acrylate hard coat agent (Beamset (registered trademark) 577, solids concentration 100%, manufactured by Arakawa Chemical Industries, Ltd.), 5 parts by weight of a photopolymerization initiator (Irgacure (registered trademark) 184, solids concentration 100%, manufactured by BASF Japan Ltd.), and 0.1 parts by weight of a leveling agent (BYK307, solids concentration 100%, manufactured by BYK Japan Ltd.) were mixed and diluted with a solvent of toluene/MEK=1/1 to prepare a coating solution 2 with a concentration of 40%.

(Preparation of polyethylene terephthalate pellets A)
As the esterification reaction apparatus, a continuous esterification reaction apparatus consisting of a three-stage complete mixing tank having an agitator, a partial condenser, a raw material inlet, and a product outlet was used. TPA was set at 2 tons/hr, EG was set at 2 moles per mole of TPA, and antimony trioxide was set at an amount such that the Sb atom concentration in the produced PET was 160 ppm. These slurries were continuously supplied to a first esterification reaction vessel of the esterification reaction apparatus and reacted at normal pressure for an average residence time of 4 hours at 255° C. Next, the reaction product in the first esterification reactor was continuously taken out of the system and fed to a second esterification reactor, EG distilled off from the first esterification reactor was fed into the second esterification reactor in an amount of 8% by mass relative to the produced polymer (produced PET), and further an EG solution containing magnesium acetate in an amount such that the produced PET had 65 ppm Mg atoms and an EG solution containing TMPA in an amount such that the produced PET had 20 ppm P atoms were added, and reacted at normal pressure for an average residence time of 1.5 hours at 260° C. Next, the reaction product in the second esterification reactor was continuously taken out of the system and fed to a third esterification reactor, and further an EG solution containing TMPA in an amount such that the produced PET had 20 ppm P atoms were added, and reacted at normal pressure for an average residence time of 0.5 hours at 260° C. The esterification reaction product produced in the third esterification reactor was continuously supplied to a three-stage continuous polycondensation reactor to carry out polycondensation, and was further filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 90% cut of 5 μm particles) to obtain polyethylene terephthalate pellets A having an intrinsic viscosity of 0.62 dl/g.

(Preparation of polyethylene terephthalate pellets B)
The polyethylene terephthalate pellets A were subjected to solid-phase polymerization at 220° C. for various times under a reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus to produce polyethylene terephthalate pellets B having an intrinsic viscosity of 0.72 dL/g.

(Preparation of polyethylene-2,6-naphthalate pellets C)
100 parts of dimethyl 2,6-naphthalenedicarboxylate, 60 parts of ethylene glycol, and 0.03 parts of manganese acetate tetrahydrate as an ester exchange catalyst were used to carry out an ester exchange reaction according to a conventional method, and then 0.042 parts of triethylphosphonoacetate was added to substantially terminate the ester exchange reaction. Next, 0.024 parts of antimony trioxide was added, and a polymerization reaction was subsequently carried out in a conventional manner at high temperature under high vacuum to obtain polyethylene-2,6-naphthalate (PEN) having an intrinsic viscosity of 0.60 dl/g. Thereafter, the mixture was pre-dried at 150 to 160°C for 3 hours, and then solid-phase polymerization was carried out at 210°C, 13 kPa, and in a nitrogen gas atmosphere to obtain polyethylene-2,6-naphthalate pellets C having an intrinsic viscosity of 0.72 dl/g.

Example 1
The polyethylene terephthalate master pellet B was dried under reduced pressure (3 Torr) at 150°C for 8 hours, and then the polyethylene terephthalate pellet B was fed to the extruder and melted at 285°C. The polymer was filtered through a stainless steel sintered filter material (nominal filtration accuracy 10 μm particles 95% cut), extruded into a sheet from a die, and then contacted with a casting drum with a surface temperature of 30°C using an electrostatic casting method to cool and solidify, producing an unstretched film. The unstretched film was uniformly heated to 75°C using a heating roll, heated to 100°C with a non-contact heater, and roll-stretched (longitudinal stretching) 3.4 times. The obtained uniaxially stretched film was introduced into a tenter, heated to 140°C, stretched laterally 4.0 times, fixed in width, heat-treated at 240°C for 5 seconds, and further relaxed in the width direction at 210°C to obtain a polyethylene terephthalate film with a thickness of 50 μm.

(Examples 2 to 6)
A polyester film was obtained in the same manner as in Example 1 except that the stretching ratio and heat treatment temperature were changed to those shown in Table 1.

(Examples 7 and 8)
A polyester film was obtained in the same manner as in Example 1 except that the thickness was changed to that shown in Table 1.

Example 9
A polyester film was obtained in the same manner as in Example 1, except that polyethylene-2,6-naphthalate pellets C were used as shown in Table 1 and the temperature was adjusted.

Example 10
A polyester film was obtained in the same manner as in Example 1, except that polyethylene terephthalate master pellets A were used instead.

(Comparative Example 1)
A polyester film was obtained in the same manner as in Example 1, except that the longitudinal stretching ratio was changed to 3.5 times.

(Comparative Examples 2 and 3)
A polyester film was obtained in the same manner as in Example 1, except that the heat setting temperature was changed to that shown in Table 1.

The hard coat layer forming coating solution 1 was applied to one side of the polyester film obtained in the above Examples 1 to 10 and Comparative Examples 1 to 3 using a Mayer bar so that the film thickness after drying would be 5.0 μm, and then dried at 80 ° C. for 1 minute, followed by irradiation with ultraviolet light (high pressure mercury lamp, integrated light amount 200 mJ / cm 2 ) to obtain a hard coat film. In Example 11, the polyester film obtained in Example 1 was coated so that the film thickness after drying would be 10.0 μm, but the hard coat film was obtained in the same manner as in Example 1. In Example 12, the polyester film obtained in Example 1 was coated with the hard coat layer forming coating solution 2 to obtain a hard coat film in the same manner as in Example 1, but the hard coat film was obtained in the same manner as in Example 1.

These hard coat films were attached to an organic EL module via a 25 μm thick adhesive layer to create a smartphone-type foldable display that can be folded in half at the center of the entire display with a radius of 3 mm, which corresponds to the bending radius in Figure 1. The hard coat film was placed on the surface of a single continuous display through the folding part, and the hard coat layer was placed so as to be located on the surface of the display. The displays using the hard coat films of each example satisfied the operation and visibility as a smartphone that can be folded in half at the center and carried around. On the other hand, the foldable displays using the hard coat films of each comparative example seemed to develop image distortion at the folding part of the display as the frequency of use increased, and were not very preferable.

According to a folding display using the polyester film or hard coat film for surface protection film of the folding display of the present invention, the polyester film or hard coat film located on the surface of the folding display does not deform after repeated folding while maintaining mass productivity, so that no image distortion occurs at the folded part of the display. A mobile terminal device equipped with a folding display using the polyester film or hard coat film of the present invention as a surface protection film provides beautiful images, is highly functional, and has excellent convenience such as portability.

1: Folding display 11: Bend radius 2: Polyester film for surface protection film of folding display 21: Folding portion 22: Bend direction (direction perpendicular to the folding portion)
O: Origin A: The farthest point from O where Hooke's law holds true P: A line parallel to the OA line drawn from Q intersects with the stress-strain curve Q: Stress 0 MPa, elongation (strain) 0.2% H: A line parallel to the vertical axis drawn from P intersects with the horizontal axis (0.2% yield point strain (%))
σ0.2: Stress value of P (MPa)

Claims (8)

The crystallinity is 35 to 54%,
The total light transmittance is 85% or more and 99% or less,
A polyester film for displays, characterized in that the 0.2% yield point strain in the machine direction is 2.6 to 5.0%. 2. The polyester film for displays according to claim 1 , wherein the machine direction is a bending direction perpendicular to a fold when the polyester film is folded . 2. The polyester film for displays according to claim 1, which has a thickness of 10 to 75 μm and an intrinsic viscosity of 0.60 to 1.0 dl/g. 2. The polyester film for displays according to claim 1, wherein the haze is from 0.1% to 3% . In a no-load U-shaped stretch test in which the cable was bent 50,000 times at a bending radius of 3 mm and a speed of 1 time per second, after bending, the cable was placed horizontally on a flat surface with the inside of the bend facing down.
2. The polyester film for displays according to claim 1, which has a maximum lift height of less than 5 mm or is not deformed. The polyester film for display according to any one of claims 1 to 5 has a hard coat layer having a thickness of 1 to 50 μm on at least one surface thereof,
A hard coat film for displays, characterized in that the pencil hardness of the hard coat layer measured under a load of 750 g in accordance with JIS K5600-5-4:1999 is H or more. A foldable display in which the hard coat film for a display according to claim 6 is arranged as a surface protective film so that the hard coat layer is positioned on a surface thereof, the foldable display has a bending radius of 5 mm or less when folded,
A foldable display comprising a single continuous hard coat film disposed across the foldable portion of the display. A mobile terminal device having the foldable display according to claim 7.