WO2018159285A1

WO2018159285A1 - Polyester film as surface protective film for foldable display and application thereof

Info

Publication number: WO2018159285A1
Application number: PCT/JP2018/004956
Authority: WO
Inventors: 正太郎西尾; 賢二吉野; 有記本郷; 明子西尾
Original assignee: 東洋紡株式会社
Priority date: 2017-03-02
Filing date: 2018-02-14
Publication date: 2018-09-07
Also published as: JP2023153164A; TW201842006A; JP2024161299A; JP2023033345A; JP7247584B2; TWI782952B; JP7327629B2; JP7568005B2; JPWO2018159285A1

Abstract

The present invention provides a foldable display well-suited for mass production and causing little distortion to an image displayed on a folded portion even after repeated folding, and a mobile terminal device equipped with the foldable display. For this purpose, the present invention also provides a polyester film and a hard coat film as a surface protective film. A polyester film serving as a surface protective film for a foldable display has a thickness of 10 through 75 µm and a deformation at 0.2% proof stress of 2.6 through 5.0% at least in either lengthwise or widthwise. A hard coat film, a foldable display, and a mobile terminal device use the polyester film as a surface protective film for a foldable display.

Description

Polyester film for surface protection film of foldable display and its use

The present invention relates to a polyester film for a surface protective film of a foldable display, a hard coat film for a surface protective film of a foldable display, a foldable display, and a portable terminal device. The present invention relates to a foldable display and a portable terminal device in which image disturbance hardly occurs, and a polyester film and a hard coat film for a surface protective film of the foldable display.

As mobile terminal devices are becoming lighter and thinner, mobile terminal devices such as smartphones are widely used. While various functions are required for portable terminal devices, convenience has been sought. For this reason, portable terminal devices that are in widespread use require simple operation with one hand, and are required to be stored in a pocket of clothes or the like.

On the other hand, tablet terminals having a screen size of 7 inches to 10 inches have high functionality for not only video content and music but also business use, drawing use, and reading. However, it cannot be operated with one hand, has poor portability, and has a problem in convenience.

In order to achieve these, a method of making a compact by connecting a plurality of displays has been proposed (Patent Document 1), but since the bezel portion remains, the video is cut off, and the visibility is problematic. It is not popular.

Therefore, in recent years, mobile terminals incorporating a flexible display and a foldable display have been proposed. With this method, the image is not interrupted and can be conveniently carried as a mobile terminal device equipped with a large screen display.

Here, for displays and portable terminal devices that do not have a conventional folding structure, the surface of the display could be protected with a non-flexible material such as glass. In the case of a single-sided display, it is necessary to use a hard coat film that is flexible and can protect the surface. However, in a foldable display, a portion corresponding to a certain folding portion is repeatedly bent, so that there is a problem that a film in the portion is deformed with time and an image displayed on the display is distorted.

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

JP 2010-228391 A JP 2016-155124 A

The present invention is intended to solve the problems of the surface protection member of the conventional display as described above, is excellent in mass productivity, and may cause disturbance in the image displayed at the folded portion after repeated folding. In order to be able to provide a foldable display with no fold and a portable terminal device equipped with such a foldable display, it is intended to provide a polyester film for a surface protective film of a foldable display or a hard coat film for a surface protective film. Is.

That is, the present invention has the following configuration.
1. A foldable display comprising a polyester film having a thickness of 10 to 75 μm and a 0.2% yield strength strain in at least one of a longitudinal direction and a width direction of 2.6 to 5.0% Polyester film for surface protection film.
2. The polyester film for a surface protective film for a foldable display as described in the above item 1, wherein the 0.2% yield strength strain in the bending direction is 2.6 to 5.0%.
(Here, the bending direction means a direction orthogonal to the folding portion when the polyester film is folded.)
3. 3. The polyester film for a surface protective film for a foldable display as described in the above 1 or 2, wherein the intrinsic viscosity of the film is 0.60 to 1.0 dl / g.
4). A surface protective film for a foldable display, comprising a hard coat layer having a thickness of 1 to 50 μm on at least one surface of the polyester film for a surface protective film for a foldable display according to any one of the first to third shifts. Hard coat film.
5). The hard coat film for a surface protective film for a foldable display as described in the above 4, wherein the pencil hardness of the hard coat layer measured at 750 g load in accordance with JIS K5600-5-4: 1999 is H or more .
6). The hard coat film for a surface protective film of the foldable display according to the fourth or fifth aspect is a foldable display arranged as a surface protective film so that the hard coat layer is positioned on the surface, and when folded, A folding display having a bending radius of 5 mm or less.
7). The foldable display according to the sixth aspect, wherein a single continuous hard coat film is disposed through a foldable portion of the foldable display. 8). A portable terminal device having the foldable display according to the sixth or seventh aspect.

The foldable display using the polyester film or hard coat film for the surface protective film of the foldable display of the present invention does not cause deformation after the polyester film or hard coat film is repeatedly folded while maintaining mass productivity. Therefore, the image is not disturbed at the folding portion of the display. A portable terminal device equipped with a foldable display as described above provides beautiful images, is rich in functionality, and has excellent convenience such as portability.

It is a schematic diagram for showing the measurement location of the bending radius at the time of folding in the present invention. It is a schematic diagram for showing the bending direction of the polyester film for surface protection films of the foldable display in the present invention. It is a schematic diagram for demonstrating the 0.2% yield strength strain in this invention.

(display)
The display referred to in the present invention generally refers to a display device, and types of display include LCD, organic EL display, inorganic EL display, LED, FED, etc. 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 more preferable.

(Foldable display)
The foldable display preferably has a structure in which one continuous display is folded in half when carried and the size is reduced by half and the portability is improved. At the same time, it is desirable that the thickness and weight be reduced. Therefore, the bending radius of the foldable display is preferably 5 mm or less, and more preferably 3 mm or less. If the bending radius is 5 mm or less, it is possible to reduce the thickness in a folded state. It can be said that the smaller the bending radius, the better. However, it may be 0.1 mm or more, and may be 0.5 mm or more. Even if it is 1 mm or more, the practicality is sufficiently good as compared with a conventional display having no folding structure. The bending radius at the time of folding is to measure a portion indicated by reference numeral 11 in the schematic diagram of FIG. 1 and means a radius inside the folding portion at the time of folding. In addition, the surface protection film mentioned later may be located in the folded outer side of a foldable display, and may be located inside.

(Organic EL)
A general configuration of the organic EL display includes an organic EL layer composed 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 for a portable terminal device having a touch panel, a touch panel module is disposed on the organic EL display or between the organic EL layer / phase difference plate. At this time, if an impact is applied from above, there is a possibility that the circuit of the organic EL or touch panel may be disconnected. Therefore, a surface protection film is necessary, and the film disposed on the front surface of the display as the surface protection film is at least the surface of the display. It is preferable that a hard coat layer is laminated on the side.

(Surface protection film for folding display)
As the surface protective film, it can be used as long as it is a 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, and a cycloolefin polymer film. Polyimide films and polyester films having high impact resistance and sufficient pencil hardness are preferred, and polyester films that can be produced at low cost are particularly preferred.

In the present invention, the polyester film may be a single-layer film composed of one or more kinds of polyester resins, and when two or more kinds of polyester are used, it may be a multilayer structure film or a super multi-layer laminate film having a repeating structure. Good.

Examples of the polyester resin include polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or a polyester film made of a copolymer mainly composed of components of these resins. Among these, a stretched polyethylene terephthalate film is particularly preferable from the viewpoint of mechanical properties, heat resistance, transparency, price, and the like.

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; terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid. Aromatic dicarboxylic acids such as: 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, and 1,4-cyclohexanedimethanol. And polyethylene glycol having an average molecular weight of 150 to 20,000. The mass ratio of the copolymer component of the preferred copolymer is less than 20% by mass. When it is less than 20% by mass, film strength, transparency and heat resistance are maintained, which is preferable.

In the production of a polyester film, the intrinsic viscosity of at least one kind of resin pellets 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 it is preferable that the internal circuit is not easily broken by an external impact. Moreover, it contributes to the small deformation when bent repeatedly, which is preferable. On the other hand, if the intrinsic viscosity is 1.00 dl / g or less, it is preferable that the film production can be stably operated without excessively increasing the filtration pressure of the molten fluid.

Regardless of whether the film has a single layer structure or a laminated structure, the intrinsic viscosity of the film is preferably 0.60 dl / g or more. More preferably, it is 0.62 dl / g or more. More preferably, it is 0.68 dl / g or more. If it is 0.60 dl / g or more, fatigue resistance can be imparted and a sufficient bending resistance effect can be obtained. On the other hand, a film having an intrinsic viscosity of 1.00 dl / g or less is preferable because it can be produced with good operability.

The thickness of the polyester film is preferably 10 to 75 μm, more preferably 25 to 75 μm. When the thickness is 10 μm or more, an effect of improving pencil hardness is observed, and when the thickness is 75 μm or less, it is advantageous for weight reduction and excellent in flexibility, workability and handling properties.

The surface of the polyester film of the present invention may be smooth or uneven, but since it is used for a display surface cover, a decrease in optical properties due to the unevenness is not preferable. 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 better, but it may be 0.1% or more, or 0.3% or more.

For the purpose of reducing the haze as described above, it is better that the film surface is not very uneven, but in order to give a degree of slipperiness from the viewpoint of handling, as a method of forming the unevenness, the surface polyester resin It can be formed by blending a layer with a filler or coating a coating layer containing a filler during film formation.

As a method of blending particles into the base film, a known method can be adopted. For example, it can be added at any stage for producing polyester, but it is preferably added as a slurry dispersed in ethylene glycol or the like at the stage of esterification or after the end of the transesterification reaction and before the start of the polycondensation reaction. Then, the polycondensation reaction may proceed. Also, a method of blending a slurry of particles dispersed in ethylene glycol or water with a vented kneading extruder and a polyester raw material, or a method of blending dried particles and a polyester raw material using a kneading extruder Etc.

Among these, after the aggregated inorganic particles are homogeneously dispersed in the monomer liquid that is a part of the polyester raw material, the filtered material is used in the remainder of the polyester raw material before, during or after the esterification reaction. The method of adding is preferable. According to this method, since the monomer liquid has a low viscosity, it is easy to perform homogeneous dispersion of particles and high-accuracy filtration of the slurry, and when added to the rest of the raw material, the dispersibility of the particles is good and new Aggregates are also unlikely to occur. From this point of view, it is particularly preferable to add to the remainder of the raw material in a low temperature state before the esterification reaction.

In addition, the number of protrusions on the film surface can be further reduced by a method (master batch method) in which polyester containing particles is obtained in advance and the pellets and pellets not containing particles are kneaded and extruded.

Moreover, the polyester film may contain various additives within a range that maintains a preferable range of the 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 is preferably 85% or more, and more preferably 87% or more. If there is a transmittance of 85% or more, sufficient visibility can be secured. The higher the total light transmittance of the polyester film, the better. However, 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 the adhesion with a resin that forms a hard coat layer or the like.

Surface treatment methods include, for example, sand blast treatment, roughening 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, etc. An oxidation process etc. are mentioned, It can use without limitation.

Also, adhesion can be improved by an adhesion improving layer such as an easy adhesion layer. The easy-adhesion layer can be used without particular limitation, such as an acrylic resin, a polyester resin, a polyurethane resin, or a polyether resin, and can be formed by a general coating technique, preferably a so-called in-line coating formulation.

The polyester film described above includes, for example, a polymerization process in which inorganic particles are homogeneously dispersed in a monomer liquid that is a part of a polyester raw material and filtered, and then added to the remainder of the polyester raw material to polymerize the polyester, and the polyester is added. It can be manufactured through a film-forming step of forming a base film by melting and extruding it into a sheet form through a filter, and cooling and extruding it.

Next, an example of using a polyethylene terephthalate (hereinafter referred to as PET) pellet as a raw material for a base film will be described in detail with respect to a method for producing a biaxially stretched polyester film, but is not limited thereto. Further, the number of layers is not limited, such as a single layer configuration or a multilayer configuration.

After PET pellets are mixed and dried at a predetermined ratio, they are supplied to a known melt laminating 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, a single extruder may be used, but in the case of producing a multilayer film, two or more extruders, two or more manifolds or a merge block (for example, a merge having a square merge portion) A plurality of film layers constituting each outermost layer are laminated using a block, two or more sheets are extruded from the die, and cooled 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 any place where the molten resin is kept 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 removing aggregates and high-melting-point organic substances mainly composed of Si, Ti, Sb, Ge, and Cu. Therefore, it is preferable.

Furthermore, the filter particle size (initial filtration efficiency 95%) of the filter medium is preferably 20 μm or less, and particularly preferably 15 μm or less. When the filter particle size of the filter medium (initial filtration efficiency 95%) exceeds 20 μm, foreign matters having a size of 20 μm or more cannot be sufficiently removed. By performing high-precision filtration of molten resin using a filter medium having a filter particle size (initial filtration efficiency of 95%) of 20 μm or less, productivity may be reduced, but a film with few protrusions due to coarse particles is obtained. Preferred above.

The 0.2% proof stress at least in any one of the longitudinal direction (machine flow direction) and the width direction of the polyester film is preferably 2.6 to 5.0%, more preferably 3.0 to 5.%. More preferably, it is 0%. The 0.2% yield strength point strain is a value used as a substitute for the yield point whether or not the yield point appears in the stress-strain curve, and can be used as an index of the elastic region. FIG. 3 shows a schematic diagram of a stress-strain curve for obtaining 0.2% proof stress. FIG. 3 shows an example where no yield point appears. The 0.2% proof stress is determined by a normal tensile test. In the stress-strain curve, a parallel line is drawn from the point Q where the stress is 0 MPa and the strain is 0.2% to OA where the Hooke's law is established, and the point where the stress-strain curve intersects is defined as the proof stress point P. The value of the strain at the intersection H with the strain axis when the angle is lowered is the 0.2% proof stress. As described above, the 0.2% proof stress strain can be obtained from the stress-strain curve. When a stress-strain curve is obtained by a tensile tester (AUTOGRAPH AG-X Plus 1 kN manufactured by Shimadzu Corporation), If the autograph software TRAPEZIUM X manufactured by Shimadzu Corporation is used, the calculation result of 0.2% proof stress can be directly obtained by setting the strain at 0.2%.

When the 0.2% proof stress is 2.6% or more, it is preferable that deformation is difficult to occur due to strain generated during folding. When it is 5.0% or less, good impact resistance is obtained, which is preferable. The 0.2% proof stress 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 means a direction orthogonal to the folding portion (reference numeral 21) assumed in the use of the surface protection film of the folding display, as indicated by reference numeral 22 on the polyester film (reference numeral 2) in FIG. pointing. The bending direction is not limited to either the longitudinal direction or the width direction of the film.

0.2% proof stress can be adjusted within the above range by adjusting the draw ratio, draw temperature, heat setting temperature, polyester raw material, and the like.

The 0.2% proof stress of the polyester film can be adjusted effectively by adjusting the draw ratio. The unstretched polyester sheet preferably has a stretch ratio in at least one of the longitudinal direction (machine flow direction) and the width direction of 1.0 to 3.4 times, more preferably 1.0 to 3.0 times. . By setting the draw ratio to 1.0 to 3.4 times, the 0.2% yield strength strain can be adjusted to the above-mentioned range, and deformation at the time of repeated folding is small. And it is preferable that the said extending | stretching direction is the said bending direction. The stretching temperature is preferably 80 to 130 ° C, more preferably 90 to 130 ° C. In addition, the heating method at the time of extending | stretching can employ | adopt conventionally well-known means, such as a hot-air heating system, a roll heating system, and an infrared heating system. By setting the stretching temperature to 80 to 130 ° C., thickness unevenness due to stretching at the stretching ratio can be prevented.

From the mechanical characteristics of the film, the draw ratio in the direction perpendicular to the bending direction when folded (the direction of the folding part) is preferably larger than the bending direction. The draw ratio in the direction perpendicular to the bending direction is 2.5-5. .0 can be exemplified. Stable productivity can be obtained by setting the draw ratio to 2.5 times or more, and good impact resistance can be obtained by setting the draw ratio to 5.0 times or less.

The crystallinity of the polyester film is preferably 35 to 54%, more preferably 43 to 54%. By increasing the crystallinity, the 0.2% yield strength strain and impact resistance can be improved. In order to effectively increase the crystallinity, it is preferable to perform heat treatment at 180 to 250 ° C. after biaxial stretching. If it is 180 degreeC or more, a crystallinity degree can be raised effectively, and if it is 250 degrees C or less, there is no possibility that the polyester film surface may melt | dissolve and an external appearance is kept favorable and preferable.

Specifically, for example, PET pellets are sufficiently vacuum-dried, then supplied to an extruder, melted and extruded into a sheet at about 280 ° C., and cooled and solidified to form an unstretched PET sheet. The obtained unstretched sheet is stretched 1.0 to 3.4 times in the longitudinal direction with a roll heated to 80 to 130 ° C. to obtain a uniaxially oriented PET film. Further, the end of the film is gripped with a clip, guided to a hot air zone heated to 80 to 180 ° C., dried, and stretched 2.5 to 5.0 times in the width direction. Subsequently, it is guided to a heat treatment zone at 180 to 250 ° C., and heat treatment is performed for 1 to 60 seconds to complete crystal orientation. During this heat treatment step, a relaxation treatment of 1 to 12% may be performed in the width direction or the longitudinal direction as necessary.

(Hard coat layer)
The polyester film that protects the display by being positioned on the surface of the foldable display preferably has a hard coat layer on the surface. The hard coat layer is preferably used in the display by being positioned on the display surface side on the polyester film. The resin for forming the hard coat layer can be used without particular limitation, such as acrylic, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, and epoxy. In addition, 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.

(Film thickness)
The film thickness of the hard coat layer is preferably 1 to 50 μm. 1 to 40 μm is more preferable. If it is thicker than 1 μm, it is sufficiently cured and a good pencil hardness is obtained. 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. More preferably, it is 2 to 25 μm, particularly preferably 2 to 20 μm, and most preferably 3 to 15 μm.

(Application method)
As a coating method of a hard-coat layer, it can use without limitation, such as a Mayer bar, a gravure coat, a die coater, a knife coater, and can be suitably selected according to a viscosity and a film thickness.

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

(Pencil hardness)
The pencil hardness of the hard coat layer is preferably B or higher, more preferably H or higher, and particularly preferably 2H or higher. If the pencil hardness is greater than or equal to B, it will not be easily scratched and visibility will not be reduced. Generally, the pencil hardness of the hard coat layer is preferably higher, but it may be 10H or less, 8H or less, and 6H or less can be used practically without any problem.

(Type of hard coat layer)
The hard coat layer in the present invention may be provided with other functions as long as it can be used for the purpose of protecting the display by increasing the pencil hardness of the surface as described above. For example, a hard coat layer to which functionality such as an antiglare layer having a certain pencil hardness as described above, an antiglare antireflection layer, an antireflection layer, a low reflection layer, and an antistatic layer is added in the present invention. Is preferably applied.

Next, the effects of the present invention will be described using examples and comparative examples. First, the evaluation method of the characteristic values used in the present invention is shown below.

(1) 0.2% proof stress strain The film was cut into a strip shape with a width of 10 mm in the measuring direction 140 mm and used as a sample. To obtain a stress-strain curve. Thereafter, the autograph software TRAPEZIUM X manufactured by Shimadzu Corporation was used to set the strain at 0.2%, and the 0.2% proof stress was calculated.

(2) Intrinsic viscosity The film or polyester resin was pulverized and dried, and then dissolved in a mixed solvent of phenol / tetrachloroethane = 60/40 (mass ratio). After removing inorganic particles by centrifuging this solution, the flow time of the solution having a concentration of 0.4 (g / dl) and the flow time of the solvent alone were measured at 30 ° C. using an Ubbelohde viscometer. From these time ratios, the intrinsic viscosity was calculated using the Huggins equation, assuming that the Huggins constant was 0.38. In the case of a laminated film, the intrinsic viscosity of each layer was evaluated by scraping the corresponding polyester layer of the film according to the laminated thickness.

(3) Crystallinity The density of the film sample was measured at three points. The average value was defined as the crystallinity.
The resin component is a mixture of completely amorphous and completely crystal, and the density is as described later. The density of the sample is a value obtained by dividing the total mass of each component constituting the sample by the total volume of each component. The crystallinity (weight ratio) of each resin was estimated based on the assumption that The sample density was measured in accordance with a method (density gradient tube method) based on JISK-7112-1980. Moreover, the density of each component used the following value (unit: g / cm3)
Polyethylene terephthalate resin: complete amorphous 1.34, complete crystal 1.46
Polyethylene naphthalate resin: complete amorphous 1.32, complete crystal 1.41

(4) Bending resistance A polyester film was cut into a size of 200 mm (bending direction) × 50 mm (perpendicular direction) to prepare a measurement sample. Spacers were formed by arranging spacers of various thicknesses at the ends of two glass plates having a thickness of 5 mm, and the film was held for 10 seconds. Immediately after that, the film was made to reflect the light of the fluorescent lamp, the folding part was observed, and the interval at which no folding marks were made was recorded.
○: The interval at which no folding marks were made was less than 6.5 mm.
(Triangle | delta): The space | interval with which the folding mark was not attached is 6.5 mm or more and less than 7.0 mm x: The space | interval with which the folding mark was not attached is 7.0 mm or more.

(5) Repeated bending resistance A sample having a size of 50 mm in the width direction and 100 mm in the flow direction is prepared. Using a no-load U-shaped expansion / contraction tester (manufactured by Yuasa System Equipment Co., Ltd., DLDMMLH-FS), a bending radius of 3 mm was set, and bending was performed 50,000 times at a speed of 1 time / second. At that time, the sample was fixed at the position of 10 mm at both ends on the long side, and the bent portion was 50 mm × 80 mm. After completion of the bending process, the sample was placed on a flat surface with the bending inner side down, and a visual inspection was performed.
○: Even when the sample is not deformed or deformed, when it is placed horizontally, the maximum height is less than 3 mm.
Δ: The sample is deformed, and when placed horizontally, the maximum height is 3 mm or more and less than 5 mm.
X: The sample has a crease or when the sample is placed horizontally, the maximum height is 5 mm or more.

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

(Coating liquid 1 for forming a hard coat layer)
To 100 parts by weight of hard coat material (manufactured by JSR, Opstar (registered trademark) Z7503, concentration 75%), 0.1 part by weight of a leveling agent (BYK307, concentration 100%) is added, and methyl ethyl ketone is added. Diluted to prepare a hard coat coating solution 1 having a solid concentration of 40% by weight.

(Coating liquid 2 for forming a hard coat layer)
Urethane acrylate hard coat agent (Arakawa Chemical Industries, Beam Set (registered trademark) 577, solid content concentration 100%) 95 parts by weight, photopolymerization initiator (BASF Japan, Irgacure (registered trademark) 184, solid content Concentration 100%) 5 parts by weight, leveling agent (BYK307, BYK307, solid content concentration 100%) 0.1 part by weight is mixed, diluted with a solvent of toluene / MEK = 1/1, concentration 40 % Coating solution 2 was prepared.

(Preparation of polyethylene terephthalate pellet A)
As the esterification reaction apparatus, a continuous esterification reaction apparatus comprising a three-stage complete mixing tank having a stirrer, a partial condenser, a raw material charging port, and a product take-out port is used. TPA is set to 2 ton / hr, and EG is set to TPA1. The amount of antimony trioxide is 2 mol per mol, the amount of Sb atoms is 160 ppm with respect to the produced PET, and these slurries are continuously supplied to the first esterification reactor of the esterification reactor, 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, supplied to the second esterification reaction can, and distilled from the first esterification reaction can into the second esterification reaction can. EG solution containing 8% by mass of the produced polymer (produced PET) and further containing EG solution containing magnesium acetate in an amount of 65 ppm of Mg atoms relative to the produced PET, and 20 ppm of P atoms relative to the produced PET An EG solution containing TMPA in an amount of 1 was added and allowed to react at 260 ° C. with an average residence time of 1.5 hours at normal pressure. 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. The EG solution was added and reacted at 260 ° C. at normal pressure with an average residence time of 0.5 hours. The esterification reaction product produced in the third esterification reaction can is continuously supplied to a three-stage continuous polycondensation reaction apparatus to carry out polycondensation, and further, a filter medium of a stainless sintered body (nominal filtration accuracy of 5 μm). The particles were filtered with a 90% cut particle) to obtain polyethylene terephthalate pellets A having an intrinsic viscosity of 0.62 dl / g.

(Preparation of polyethylene terephthalate pellet B)
Polyethylene terephthalate pellet A is 0.5 mmH using a rotary vacuum polymerization apparatus.
Under reduced pressure of g, solid-state polymerization was carried out at 220 ° C. for varying times to prepare polyethylene terephthalate pellets B having an intrinsic viscosity of 0.72 dL / g.

(Preparation of polyethylene-2,6-naphthalate pellet C)
Using 0.03 part of manganese acetate tetrahydrate using 100 parts of dimethyl 2,6-naphthalenedicarboxylate and 60 parts of ethylene glycol as a transesterification catalyst and subjecting to transesterification according to a conventional method, triethylphosphonoacetate was added in an amount of 0. 042 parts were added to substantially complete the transesterification reaction. Subsequently, 0.024 part of antimony trioxide was added, and then a polymerization reaction was performed in a conventional manner under high temperature and high vacuum to obtain polyethylene-2,6-naphthalate (PEN) having an intrinsic viscosity of 0.60 dl / g. . Then, after preliminary drying at 150 to 160 ° C. for 3 hours, solid state polymerization was performed in an atmosphere of nitrogen gas at 210 ° C. and 13 kPa to obtain polyethylene-2,6-naphthalate pellets C having an intrinsic viscosity of 0.72 dl / g. Obtained.

Example 1
The polyethylene terephthalate master pellet B is dried under reduced pressure at 150 ° C. for 8 hours (
3 Torr), polyethylene terephthalate pellets B were fed to the extruder and melted at 285 ° C. This polymer is filtered with a filter material of stainless sintered body (nominal filtration accuracy 10 μm particles 95% cut), extruded into a sheet form from the die, and then applied to a casting drum having a surface temperature of 30 ° C. using an electrostatic application casting method. It was brought into contact and cooled and solidified to produce an unstretched film. This unstretched film was uniformly heated to 75 ° C. using a heating roll and heated to 100 ° C. with a non-contact heater to perform 3.4 times roll stretching (longitudinal stretching). The resulting uniaxially stretched film is guided to a tenter, heated to 140 ° C, stretched to 4.0 times, fixed in width, heat treated at 240 ° C for 5 seconds, and further relaxed by 4% in the width direction at 210 ° C. Thus, a polyethylene terephthalate film having a thickness of 50 μm was obtained.

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

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

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

(Example 10)
A polyester film was obtained in the same manner as in Example 1 except that the polyethylene terephthalate master pellet A was changed.

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

(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.

Using a Meyer bar on one side of the polyester films obtained in Examples 1 to 10 and Comparative Examples 1 to 3 above, the film thickness after drying the hard coat layer forming coating solution 1 is 5.0 μm. And dried at 80 ° C. for 1 minute, and then irradiated with ultraviolet rays (high pressure mercury lamp, integrated light quantity 200 mJ / cm ² ) to obtain a hard coat film. In Example 11, a hard coat film was obtained in the same manner as in Example 1 except that the polyester film obtained in Example 1 was applied so that the film thickness after drying was 10.0 μm. In Example 12, a hard coat film was obtained in the same manner as in Example 1, except that the hard coat layer forming coating solution 2 was applied to the polyester film obtained in Example 1.

A smartphone-type foldable display in which these hard coat films are bonded to an organic EL module via an adhesive layer with a thickness of 25 μm and can be folded in half at the center of the entire 3 mm radius corresponding to the bending radius in FIG. It was created. The hard coat film is disposed on the surface of one continuous display through the folding portion, and the hard coat layer is disposed on the surface of the display. What used the hard coat film of each Example satisfy | filled operation | movement and visibility as a smart phone which can be folded and folded in the center part and carried. On the other hand, the foldable display using the hard coat film of each comparative example felt that image distortion occurred at the folding portion of the display as the frequency of use increased, and was not very preferable.

According to the foldable display using the polyester film or the hard coat film for the surface protective film of the foldable display of the present invention, the polyester film or the hard coat film positioned on the surface of the foldable display while maintaining mass productivity. Since the image is not deformed after being repeatedly folded, the image is not disturbed at the folded portion of the display. A portable terminal device equipped with a foldable display using the polyester film or hard coat film of the present invention as a surface protective film provides a beautiful image, is rich in functionality, and has excellent convenience such as portability. .

1: Folding display 11: Bending radius 2: Polyester film for surface protection film of folding display 21: Folding part 22: Bending direction (direction orthogonal to the folding part)
O: Origin A: Point farthest from O where Hook's law is established P: Point parallel to OA line drawn from Q and intersected with stress-strain curve Q: Point of stress 0MPa, elongation (strain) 0.2% H: A point where a parallel line is drawn from P to the vertical axis and intersects the horizontal axis (0.2% proof stress strain (%))
σ _0.2 : Stress value of P (MPa)

Claims

A foldable display comprising a polyester film having a thickness of 10 to 75 μm and a 0.2% yield strength strain in at least one of a longitudinal direction and a width direction of 2.6 to 5.0% Polyester film for surface protection film.
2. The polyester film for a surface protective film for a foldable display according to claim 1, wherein the 0.2% yield strength strain in the bending direction is 2.6 to 5.0%.
(Here, the bending direction means a direction orthogonal to the folding portion when the polyester film is folded.)
The polyester film for a surface protective film for a foldable display according to claim 1 or 2, wherein the intrinsic viscosity of the film is 0.60 to 1.0 dl / g.
4. A surface protection film for a foldable display, comprising a hard coat layer having a thickness of 1 to 50 μm on at least one side of the polyester film for a surface protection film of a foldable display according to claim 1. Hard coat film.
The hard coat film for a surface protective film of a foldable display according to claim 4, wherein the pencil hardness of the hard coat layer measured at 750 g load in accordance with JIS K5600-5-4: 1999 is H or more. .
6. The foldable display according to claim 4 or 5, wherein the hard coat film for a surface protective film of the 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 bent when folded. A folding display having a radius of 5 mm or less.
The foldable display according to claim 6, wherein a continuous single hard coat film is arranged through a foldable portion of the foldable display.
A portable terminal device having the foldable display according to claim 6 or 7.