JP2024045213A - Foldable display surface protective film - Google Patents

Abstract

To provide a foldable display surface protective film which does not cause distortion of a portion of an image displayed near a crease when the film is repeatedly folded.SOLUTION: A foldable display surface protective film is provided, comprising an adhesive layer formed on one surface of a polyester film having a thickness of 10-75 μm and an intrinsic viscosity of 0.65-1.0 dl/g. Preferably, the foldable display surface protective film has a hard coat layer laminated on a surface of the polyester film opposite the surface with the adhesive layer.SELECTED DRAWING: None

Description

The present invention relates to a surface protection film for a foldable display that is used by being attached to the surface of a foldable display installed in a mobile terminal device, etc. The present invention relates to a surface protection film for a foldable display that is used by being attached to the surface of a foldable display installed in a mobile terminal device, etc., and even if it is repeatedly folded, the image will not change due to the deformation of the surface protection film located on the surface. This invention relates to a surface protection film for foldable displays that is less susceptible to disturbance.

BACKGROUND ART Mobile terminal devices are becoming thinner and lighter, and mobile terminal devices typified by smartphones are becoming widespread. While mobile terminal devices are required to have various functions, they are also required to be convenient. Therefore, the popular mobile terminal devices must have a small screen size of about 6 inches because they can be easily operated with one hand and are intended to be stored in clothes pockets.

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.

In order to achieve these goals, a method has been proposed to make it more compact by connecting multiple displays together, but since the bezel remains, the image is cut off and the visibility deteriorates, so it is not widely used. .

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

The display surface of mobile terminal devices is generally made of glass or hard coat film, making it resistant to scratches, but if it is rubbed against stones or metal, it will inevitably get scratched. In reality, it is used by pasting an adhesive surface protection film afterwards (see Patent Document 1).

Here, for displays and mobile terminal devices that do not have a conventional folding structure, an inexpensive polyester film with high impact resistance is sometimes used as the surface protection film of the display. On the other hand, in a foldable display, the part that corresponds to a certain folding part is repeatedly folded, so the film at that part deforms over time, and if a general polyester film is used, the image displayed on the display may be distorted. There was a problem.

Japanese Patent Application Publication No. 2010-228391

The present invention aims to solve the problems associated with conventional surface protection films for displays as described above, and to provide a surface protection film for folding displays that is free from the risk of distortion of the image displayed on the folded portion after repeated folding.

That is, the present invention comprises the following:
1. A surface protection film for a folding display, comprising a polyester film having a thickness of 10 to 75 μm and an intrinsic viscosity of 0.65 to 1.0 dl/g, and an adhesive layer on one side of the polyester film.
2. The surface protection film for a folding display according to claim 1, wherein a hard coat layer is laminated on the side of the polyester film opposite to the side having the adhesive layer.
3. The surface protection film for a folding display according to claim 1 or 2, wherein the polyester film is a biaxially oriented polyethylene terephthalate film.

The surface protection film for a foldable display of the present invention does not cause image distortion at the folded portion of the display because the surface protection film located on the surface of the foldable display does not undergo deformation after repeated folding. It is. A mobile terminal device equipped with a foldable display as described above provides beautiful images, is rich in functionality, and has excellent convenience such as portability.

(display)
The term "display" used in the present invention generally refers to display devices, and types of displays include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs. , organic EL, and inorganic EL are preferred. Particularly preferred are organic EL and inorganic EL, which can reduce the layer structure, and organic EL, which has a wide color gamut, is even more preferred.

(Foldable display)
The foldable display preferably has a structure in which one continuous display is folded in half when carried, reducing the size by half and improving portability. At the same time, it is desirable that the device be thin and lightweight.

(Organic EL)
The general structure of an organic EL display includes 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 above the organic EL display or between the organic EL layer/retardation plate. At this time, if an impact is applied from above, there is a risk that the circuits of the organic EL and touch panel will be disconnected, so it is common to use a surface protection film affixed during normal use. It is preferable that the surface protective film has a hard coat layer laminated on the surface side.

As a surface protection film, any film with high light transmittance and low haze can be used, such as polyimide film, polyester film, polycarbonate film, acrylic film, TAC film, cycloolefin polymer film, etc. Among them, impact-resistant films can be used. A polyester film is preferred because it has high properties and can be manufactured at low cost.

In the present invention, the polyester film may be a single-layer film made of one or more polyester resins, or when two or more types of polyester are used, it may be a multilayer film or a super 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, biaxially 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 base 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 the base film, the intrinsic viscosity of at least one type of resin pellet is preferably in the range of 0.65 to 1.0 dl/g. If the intrinsic viscosity is 0.65 dl/g or more, the resulting film is less likely to deform after repeated folding, and there is no risk of image quality deteriorating, which is preferable. On the other hand, if the intrinsic viscosity is 1.00 dl/g or less, the filtration pressure of the molten fluid does not increase too much, making it easier to operate the film production stably, which is preferable.

Regardless of whether the film has a single layer structure or a laminated structure, the intrinsic viscosity of the film is preferably 0.65 dl/g or more. More preferably, it is 0.68 dl/g or more. If it is 0.65 dl/g or more, a sufficient bending resistance effect can be obtained. On the other hand, a fill having an intrinsic viscosity of 1.00 dl/g or less is preferable because it can be manufactured 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, the effect of improving the pencil hardness of the hard coat layer when laminated on the opposite side of the adhesive layer is seen, and when the thickness is 75 μm or less, it is advantageous for weight reduction and is also possible. Excellent flexibility, workability, and handling.

The surface of the polyester film of the present invention may be smooth or uneven, but since it is used to protect the surface of a display, low light transmittance is not preferable.

The unevenness can be formed by blending a filler into the polyester resin or by coating a filler-containing coat layer during film formation.

Methods for incorporating particles into polyester films 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.

In particular, after homogeneously dispersing aggregate inorganic particles in a monomer liquid that will become a part of the polyester raw material, the filtered material is added to the remainder of the polyester raw material before, during, or after the esterification reaction. A method of adding is preferred. According to this method, since the monomer liquid has a low viscosity, it is easy to homogeneously disperse the particles and filter the slurry with high precision. Aggregates are also less likely to occur. From this point of view, it is particularly preferable to add it to the remainder of the raw material 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 as long as the film maintains its 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, more preferably 87% or more. A transmittance of 85% or more is preferable because visibility can be sufficiently ensured. It can be said that 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 adhesive layer, hard coat layer, etc.

Examples of surface treatment methods include sandblasting, unevenness treatment using solvent treatment, corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone/ultraviolet irradiation treatment, flame treatment, chromic acid treatment, hot air treatment, etc. Examples include oxidation treatment, which can be used without particular limitation.

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 above-mentioned polyester film is produced by, for example, a polymerization process in which inorganic particles are homogeneously dispersed in a monomer liquid that becomes a part of the polyester raw material, filtered, and then added to the remainder of the polyester raw material to polymerize the polyester. It can be manufactured through a film forming process in which a polyester film is formed by melt-extruding it into a sheet through a filter, cooling it, and stretching it.

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

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, during melt extrusion, it is preferable to perform high-precision filtration at any location where the molten resin is maintained at about 280° C. in order to remove foreign substances contained in the resin. The filter medium used for high-precision filtration of molten resin is not particularly limited, but a filter medium of stainless steel sintered body has excellent ability to remove aggregates and high melting point organic substances mainly composed of Si, Ti, Sb, Ge, and Cu. Therefore, it is preferable.

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.

Specifically, for example, after sufficiently vacuum-drying PET pellets, they are fed to an extruder, melted and extruded at about 280° C. into a sheet, and cooled and solidified to form an unstretched PET sheet. The obtained unstretched sheet is stretched 2.5 to 5.0 times in the longitudinal direction using rolls heated to 80 to 120°C to obtain a uniaxially oriented PET film. Furthermore, the ends of the film are held with clips and introduced into a hot air zone heated to 80 to 180°C, and after drying, the film is stretched 2.5 to 5.0 times in the width direction. Subsequently, the material is introduced into a heat treatment zone at 160 to 240° 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 length direction, if necessary.

(adhesive layer)
A polyester film that is attached to the surface of a folding display to protect the surface of the display is adhered by an adhesive layer laminated on one surface of the polyester film. As the adhesive layer, rubber-based, acrylic-based, urethane-based, polyester-based, silicone-based, etc. can be used without particular limitation. Further, as long as no abnormality is caused in the optical properties, two or more types of materials can be mixed and used, or two or more layers can be used. Further, particles such as fillers, additives, etc. can also be added.

(Thickness of adhesive layer)
The thickness of the adhesive layer is preferably 1 to 25 μm, more preferably 3 to 15 μm. A thickness of more than 1 μm is preferable because it has sufficient adhesive strength and does not peel off even when bent. If it is thinner than 25 μm, deformation of the adhesive when bent is suppressed and image disturbance does not occur, which is preferable.

(Method of applying adhesive layer)
The adhesive layer can be applied by any method including, but not limited to, a comma coater, a knife coater, a die coater, a gravure coater, a Mayer bar coater, etc., which can be appropriately selected depending on the viscosity and film thickness.

(Adhesive layer curing conditions)
The method for curing the adhesive layer is not particularly limited and may be selected appropriately from methods using energy rays such as ultraviolet rays and electron beams, and heat.

(Hard coat layer)
The polyester film that is attached to the surface of the folding display to protect the display surface preferably has a hard coat layer on the surface opposite to the adhesive layer. As the resin forming the hard coat layer, acrylic, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, and epoxy resins can be used without particular limitation. Moreover, two or more types of materials can be mixed and used, and particles such as inorganic filler or organic filler can also be added.

(Thickness of hard coat layer)
The thickness of the hard coat layer is 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 40 μm or less, curling due to cure shrinkage of the hard coat layer can be suppressed, and the handling properties of the film can be improved.

(How to apply hard coat layer)
As a coating method for the hard coat layer, a Mayer bar coater, a gravure coater, a die coater, a knife coater, etc. can be used without particular limitation, and can be appropriately selected depending on the viscosity and film thickness.

(Curing conditions for hard coat layer)
As a method for curing the hard coat layer, energy rays such as ultraviolet rays, electron beams, and heat curing methods can be used, but ultraviolet rays and electron beams are preferable 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. If the hard coat layer has a pencil hardness of B or more, the hard coat layer is not easily scratched and does not reduce visibility. In general, the harder the hard coat layer has, the more preferable the pencil hardness is, but the hard coat layer may have a pencil hardness of 10H or less, or 8H or less, or even 6H or less.

(Type of hard coat layer)
The hard coat layer in the present invention may be added 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, hard coat layers with added functionality such as anti-glare layers, anti-glare anti-reflection layers, anti-reflection layers, low reflection layers and antistatic layers having a certain pencil hardness as described above can also be used in the present invention. is preferably applied.

In the present invention, when a hard coat layer is laminated on the surface of the polyester film opposite to the side on which the adhesive layer is laminated, the total light transmittance of the hard coat layer is preferably 85% or more, and more preferably 87% or more, similar to that of the polyester film. A transmittance of 85% or more is preferable because it ensures sufficient visibility. In the above case, the higher the total light transmittance, the better, but it may be 99% or less, or 97% or less.

Next, the effects of the present invention will be explained using Examples and Comparative Examples. First, the method for evaluating characteristic values used in the present invention is shown below.

(1) 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.

(2) Bending resistance The adhesive surface of the sample is attached to a 50 μm thick polyimide film to prepare a measurement sample with a size of 50 mm in the width direction × 100 mm in the flow direction. Using a no-load U-shaped stretch tester (Yuasa System Co., Ltd., DLDMLH-FS), the bending radius is set to 3 mm, and the sample is bent 50,000 times at a speed of 1 time/second. At that time, the sample is fixed at a position 10 mm from both ends of the long side, and the bending area is 50 mm × 80 mm. After the bending process, the sample is placed on a flat surface with the inside of the bend facing down, and visually inspected. In each of the following examples and comparative examples, the bending resistance of the surface protection film with a hard coat layer is evaluated.
◎: No deformation of the sample was observed.
◯: The sample was deformed, but when placed horizontally, the maximum floating height was less than 5 mm.
×: The sample had a crease or the maximum height of the sample when placed horizontally was 5 mm or more.

(3) Pencil Hardness The surface of the hard coat layer 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.

(4) Total light transmittance, haze Measured using a haze meter (manufactured by Nippon Denshoku Industries, Ltd., NDH5000). In each of the following Examples and Comparative Examples, the total light transmittance and haze of the surface protection film with a hard coat layer were measured.

(5) Adhesive strength The adhesive side of the sample was attached to an SUS plate (SUS304), and after a rubber roller weighing 2 kg was moved back and forth once, the sample was left at room temperature for 20 hours. Thereafter, the force (N/25 mm) required to peel off the adhesive tape from the SUS plate at an angle of 180 degrees at a speed of 300 mm/min was measured using a tensile tester.

(Preparation of hard coat coating liquid 1)
94.9 parts by weight of dipentaerythritol hexaacrylate (manufactured by Shin Nakamura Chemical Co., Ltd., product name: A-DPH, solid content concentration: 100% by weight), photoradical polymerization initiator (manufactured by IGM Resins, product name: Omnirad 907, 5 parts by weight of silicone additive (manufactured by BYK Chemie Japan, BYK-3505, solid content concentration: 40% by weight) were mixed, and MEK/toluene/IPA=1 A 40% hard coat coating solution 1 was prepared by diluting with a solvent of /1/1.

(Preparation of Adhesive Coating Solution 2)
100 parts by weight of an acrylic adhesive (manufactured by Toyo Ink Mfg., product name: Olivine BPS5762K, solids concentration: 45.5% by weight), 1.4 parts by weight of a crosslinking agent (manufactured by Toyo Ink Mfg., product name: BXX5627, solids concentration: 50% by weight), and 30.6 parts by weight of MEK were mixed to prepare an adhesive coating solution 2 with a solids concentration of 35%.

(Preparation of adhesive coating liquid 3)
100 parts by weight of silicone adhesive (manufactured by Shin-Etsu Chemical Co., Ltd., product name: , product name: KS-3802) was diluted with toluene to prepare adhesive coating liquid 3 having a solid content 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))
Polyethylene terephthalate pellets (b) were obtained by adjusting the intrinsic viscosity to 0.580 dl/g in the same manner as in the production process of polyethylene terephthalate pellets (a) except that the residence time of the third esterification reaction was adjusted.

(Preparation of polyethylene terephthalate pellets (c))
Polyethylene terephthalate pellets (a) were subjected to solid phase polymerization using a rotary vacuum polymerization apparatus at 220° C. for different times under a reduced pressure of 0.5 mmHg to produce polyethylene terephthalate pellets (c) with an intrinsic viscosity of 0.67 dl/g. It was created.

(Preparation of polyethylene terephthalate pellets (d))
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 prepare polyethylene terephthalate pellets (d) having an intrinsic viscosity of 0.75 dl/g.

(Preparation of polyethylene terephthalate pellets (e))
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 prepare polyethylene terephthalate pellets (e) having an intrinsic viscosity of 0.83 dl/g.

The polyethylene terephthalate master pellets (a) were dried under reduced pressure (3 Torr) at 180°C for 8 hours, and then the polyethylene terephthalate pellets (a) were fed to an extruder and melted at 285°C. The polymer was filtered through a stainless steel sintered filter medium (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. This unstretched film was stretched 3.4 times in the longitudinal direction at 85°C. This uniaxially stretched film was stretched 4.2 times in the transverse direction at 95°C using a tenter and heat-treated at 220°C for 5 seconds to obtain the polyethylene terephthalate film No. 1 in Table 1. The polyethylene terephthalate master pellets (b) to (e) were fed to a process similar to that described above, with the exception of some adjustments to the conditions, to obtain the polyethylene terephthalate films No. 1 to 7 in Table 1.

(Example 1)
Polyethylene terephthalate film No. Using a Mayer bar, hard coat coating solution 1 was applied to one side of 4 so that the film thickness after drying was 2.0 μm, dried at 80°C for 1 minute, and then irradiated with ultraviolet light (integrated A hard coat layer was laminated with a light intensity of 200 mJ/cm2). Next, adhesive coating liquid 2 was applied to the surface of the polyethylene terephthalate film on the side on which the hard coat layer was not laminated so that the film thickness after drying was 10 μm, and dried at 120° C. for 1 minute to form a surface protection film. It was created.

Example 2
One side of the polyethylene terephthalate film No. 4 was coated with hard coat coating solution 1 using a Mayer bar so that the film thickness after drying was 2.0 μm, and then dried at 80 ° C for 1 minute, and then irradiated with ultraviolet light (integrated light amount 200 mJ / cm 2 ) to laminate a hard coat layer. Next, the adhesive coating solution 3 was coated on the surface of the polyethylene terephthalate film on the side where the hard coat layer was not laminated so that the film thickness after drying was 10 μm, and then dried at 130 ° C for 1 minute to prepare a surface protection film.

(Examples 3-5, Comparative Examples 1-3)
A surface protection film was prepared in the same manner as in Example 1 under the conditions shown in Table 2.

The present invention makes it possible to provide a protective film for a folding display that is less likely to deform after the folding display is repeatedly folded, and is less likely to cause image distortion over time.

Claims (3)

a polyester film having an intrinsic viscosity of 0.73 to 1.0 dl/g and a total light transmittance of 87% or more, and an adhesive layer on one side of the polyester film;
The polyester film is a biaxially oriented polyethylene terephthalate film,
a hard coat layer having a thickness of 1 to 40 μm, a pencil hardness of H or more, and a total light transmittance of 87% or more is laminated on the side of the polyester film opposite to the side having the adhesive layer;
The adhesive layer is formed of an acrylic adhesive and has a thickness of 3 to 15 μm.
The hard coat layer is a surface protection film for foldable displays formed from acrylic resin. organic EL display,
A foldable display comprising : the surface protection film for a foldable display according to claim 1 , which is disposed on an upper part of an organic EL display . A mobile terminal device comprising the foldable display according to claim 2.