Surface protective film for foldable display

Abstract

To provide a surface protective film for a folding type display which has no possibility of causing disturbance in an image displayed on a folded part after the surface protective film for the folding type display is repeatedly folded.SOLUTION: The surface-protecting film for a foldable display has an adhesive layer on one surface side of a polyester having a thickness of 10 to 75 μm and an intrinsic viscosity of 0.65 to 1.1 0dl / g. Preferably, a hard coat layer is laminated on the side opposite to the side having the adhesive layer of the polyester film.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a surface protective film for a folding display that is attached to the surface of a folding display equipped in a mobile terminal device or the like, and relates to a surface protective film for a folding display that is less likely to cause image distortion due to deformation of the surface protective film located on the surface, even when the display is folded repeatedly. [Background technology]

[0002] As mobile terminal devices become thinner and lighter, smartphones and other mobile terminal devices are becoming more and more popular. While mobile terminal devices are required to have a variety of functions, they are also required to be convenient. For this reason, the most popular mobile terminal devices must be able to perform simple operations with one hand and have a small screen size of around 6 inches, as they are designed to be stored in a pocket or similar.

[0003] On the other hand, tablet devices with screen sizes of 7 to 10 inches are highly functional and are intended for not only video content and music, but also business use, drawing, reading, etc. However, they cannot be operated with one hand, are less portable, and have issues with convenience.

[0004] To achieve these goals, a method has been proposed of connecting multiple displays to make them more compact, but this method has not become widespread because it leaves a bezel, which means the image is cut off and visibility is reduced.

[0005] In recent years, mobile devices incorporating flexible and foldable displays have been proposed, allowing users to conveniently carry around large-screen mobile devices without image interruption.

[0006] The display surface of mobile terminal devices is generally made of glass or hard-coated film, which makes it scratch-resistant, but scratches are inevitable when rubbed against stones or metal, so in reality, an adhesive surface protection film is attached after use (see Patent Document 1).

[0007] Here, for conventional displays and mobile terminal devices that do not have a folding structure, some display surface protection films are made of inexpensive, impact-resistant polyester film. On the other hand, for folding displays, the area corresponding to the fold is repeatedly folded, so the film in that area deforms over time, and when a typical polyester film is used, there are problems such as distortion of the image displayed on the display. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-228391 Summary of the Invention [Problem to be solved by the invention]

[0009] 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 a folding display that does not pose a risk of distortion of the image displayed on the folded portion after repeated folding. [Means for solving the problem]

[0010] 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 thereof. 2. A surface protection film for a foldable 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. A surface protection film for a folding display according to claim 1 or 2, wherein the polyester film is a biaxially oriented polyethylene terephthalate film. [Effects of the Invention]

[0011] The surface protection film for a foldable display of the present invention is a surface protection film located on the surface of the foldable display that does not deform after repeated folding, and therefore does not cause image distortion at the folded portion of the display. Mobile terminal devices equipped with such a foldable display provide beautiful images, are highly functional, and are convenient in terms of portability and other factors. DETAILED DESCRIPTION OF THE INVENTION

[0012] (display) The term "display" as used herein refers to a general display device, and examples of the display include LCD, organic EL display, inorganic EL display, LED, and FED. However, LCD, organic EL, and inorganic EL, which have a bendable 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.

[0013] (foldable display) A foldable display is a single continuous display that can be folded in half when carried, reducing its size by half and improving portability. At the same time, it is desirable for the display to be thin and lightweight.

[0014] (Organic EL) The general structure of an organic EL display consists of an organic EL layer consisting of an electrode / electron transport layer / light-emitting layer / hole transport layer / transparent electrode, a retardation plate to improve image quality, and a polarizing plate.

[0015] (Mobile terminal device with touch panel) When an organic EL display is used in a mobile terminal device with a touch panel, a touch panel module is placed on top of the organic EL display or between the organic EL layer and the retardation film. In this case, if an impact is applied from above, the circuits of the organic EL and the touch panel may be disconnected, so it is common to attach a surface protection film during normal use. It is preferable that the surface protection film has a hard coat layer laminated on the surface side.

[0016] Any film with high light transmittance and low haze, such as a polyimide film, polyester film, polycarbonate film, acrylic film, TAC film, or cycloolefin polymer film, can be used as the surface protection film. Among these, polyester film is preferred because it has high impact resistance and can be produced inexpensively.

[0017] 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 an ultra-multilayer laminate film with a repeating structure.

[0018] Examples of polyester resins include polyester films made of polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or copolymers containing these resin components as main components. Among these, biaxially stretched polyethylene terephthalate films are particularly preferred in terms of mechanical properties, heat resistance, transparency, cost, etc.

[0019] 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 in the copolymer is preferably less than 20% by mass. A mass ratio of less than 20% by mass is preferred because film strength, transparency, and heat resistance are maintained.

[0020] 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. When the intrinsic viscosity is 0.65 dL / g or more, the resulting film is less likely to deform after repeated folding, which is preferable because there is no risk of image quality degradation. On the other hand, when the intrinsic viscosity is 1.00 dL / g or less, the filtration pressure of the molten fluid does not increase too much, which is preferable because film production can be easily and stably operated.

[0021] Regardless of whether the film has a single-layer or multilayer structure, the intrinsic viscosity of the film is preferably 0.65 dL / g or more. It is more preferably 0.68 dL / g or more. A film with an intrinsic viscosity of 0.65 dL / g or more provides sufficient flex resistance. On the other hand, a film with an intrinsic viscosity of 1.00 dL / g or less is preferable because it can be produced with good operability.

[0022] The thickness of the polyester film is preferably 10 to 75 μm, and more preferably 25 to 75 μm. When the thickness is 10 μm or more, the pencil hardness of the hard coat layer is improved when the hard coat layer is laminated on the surface opposite the adhesive layer, and when the thickness is 75 μm or less, it is advantageous for weight reduction and is excellent in flexibility, processability, handleability, etc.

[0023] The surface of the polyester film of the present invention may be smooth or may have irregularities, but since the film is used to protect the surface of a display, it is not preferable that the film have low light transmittance.

[0024] The unevenness can be formed by blending a filler into the polyester resin or by coating a coat layer containing a filler during film formation.

[0025] The method of incorporating particles into a polyester film can be a known method. 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 stage of esterification, or after the completion of the transesterification reaction and before the start of the polycondensation reaction, to proceed with 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.

[0026] Among these, a method in which aggregate inorganic particles are homogeneously dispersed in a monomer liquid that will become a 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. This method facilitates homogeneous dispersion of the particles and high-precision filtration of the slurry, since the monomer liquid has a low viscosity. Furthermore, when the monomer liquid is added to the remainder of the raw material, the particles are well dispersible and new aggregates are unlikely to form. From this perspective, it is particularly preferred to add the monomer liquid to the remainder of the raw material at a low temperature before the esterification reaction.

[0027] Furthermore, the number of protrusions on the film surface can be further reduced by a method (masterbatch method) in which a polyester containing particles is prepared in advance and then the pellets are kneaded and extruded with pellets containing no particles.

[0028] The polyester film may contain various additives, such as antistatic agents, UV absorbers, and stabilizers, as long as the polyester film maintains its light transmittance.

[0029] 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 is preferable because sufficient visibility can be ensured. The higher the total light transmittance of the polyester film, the better, but it may be 99% or less, or even 97% or less.

[0030] The surface of the polyester film of the present invention may be subjected to a treatment to improve adhesion with a resin forming an adhesive layer, a hard coat layer, or the like.

[0031] Examples of surface treatment methods include roughening treatments such as sandblasting and solvent treatment, and oxidation treatments such as corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone / ultraviolet irradiation treatment, flame treatment, chromic acid treatment, and hot air treatment, and any of these methods can be used without particular limitation.

[0032] Furthermore, adhesion can be improved by an adhesion-improving layer such as an easy-adhesion layer. The easy-adhesion layer can be made of any resin, such as an acrylic resin, a polyester resin, a polyurethane resin, or a polyether resin, and can be formed by a general coating method, preferably a so-called in-line coating method.

[0033] The polyester film can be produced, for example, through a polymerization step in which inorganic particles are homogeneously dispersed in a monomer liquid that becomes a part of the polyester raw material, the resulting dispersion is filtered, and the resulting dispersion is added to the remainder of the polyester raw material to polymerize the polyester; and a film formation step in which the resulting polyester is melt-extruded into a sheet through a filter, cooled, and stretched to form a polyester film.

[0034] 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 polyethylene terephthalate (hereinafter sometimes referred to as PET) pellets 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 or a multilayer structure, is not limited.

[0035] After mixing and drying PET pellets in a predetermined ratio, the mixture is fed into a known melt lamination extruder, extruded through a slit die into a sheet, and cooled and solidified on a casting roll to form an unstretched film. While a single extruder is sufficient for a single-layer film, multilayer films can be produced using two or more extruders and two or more manifolds or merging blocks (e.g., merging blocks with rectangular merging sections) to laminate the multiple film layers that make up the outermost layers, extrude a two or more layer sheet from the die, and cool it on a casting roll to form an unstretched film.

[0036] 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 maintained at about 280°C during melt extrusion. The filter material used for high-precision filtration of the molten resin is not particularly limited, but a stainless steel sintered filter material is preferred because it has excellent performance in removing aggregates mainly composed of Si, Ti, Sb, Ge, and Cu and high-melting-point organic matter.

[0037] Furthermore, the filtration particle size (initial filtration efficiency 95%) of the filter material is preferably 20 μm or less, particularly preferably 15 μm or less. If the filtration particle size (initial filtration efficiency 95%) of the filter material exceeds 20 μm, foreign matter of 20 μm or more in size cannot be sufficiently removed. Although high-precision filtration of molten resin using a filter material with a filtration particle size (initial filtration efficiency 95%) of 20 μm or less may reduce productivity, it is preferable to obtain a film with fewer protrusions due to coarse particles.

[0038] Specifically, for example, PET pellets are thoroughly vacuum-dried, fed into an extruder, melt-extruded into a sheet at approximately 280°C, and cooled to solidify, forming an unstretched PET sheet. The resulting 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. The film is then gripped at its edges with clips and introduced into a hot air zone heated to 80 to 180°C, where it is dried and then stretched 2.5 to 5.0 times in the transverse direction. The film is then introduced into a heat treatment zone at 160 to 240°C and 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 transverse or longitudinal direction, if necessary.

[0039] (Adhesive layer) The polyester film that is attached to the surface of a folding display to protect the display surface is adhered by an adhesive layer laminated on one surface of the polyester film. The adhesive layer can be made of any material, including rubber, acrylic, urethane, polyester, and silicone. Furthermore, as long as the optical properties are not affected, two or more materials can be mixed or multi-layered. Filler particles and additives can also be added.

[0040] (Adhesive layer thickness) The thickness of the adhesive layer is preferably 1 to 25 μm, more preferably 3 to 15 μm. If it is thicker than 1 μm, it has sufficient adhesive strength and does not peel off even when bent, which is preferable. If it is thinner than 25 μm, it is preferable because deformation of the adhesive when bent is suppressed and image distortion does not occur.

[0041] (Method for applying adhesive layer) The adhesive layer can be applied by any method, including a comma coater, knife coater, die coater, gravure coater, and Meyer bar coater, without any particular limitation, and can be appropriately selected depending on the viscosity and film thickness.

[0042] (Adhesive layer curing conditions) The curing method for curing the adhesive layer is not particularly limited and may be selected appropriately, for example, by using energy rays such as ultraviolet rays or electron beams, or by using heat.

[0043] (Hard coat layer) The polyester film that is attached to the surface of a foldable display to protect the display surface preferably has a hard coat layer on the surface opposite the adhesive layer. The resin that forms the hard coat layer can be any resin, including acrylic, siloxane, inorganic hybrid, urethane acrylate, polyester acrylate, and epoxy. Two or more materials can be mixed, or particles of inorganic filler or organic filler can be added.

[0044] (Thickness of hard coat layer) The thickness of the hard coat layer is preferably 1 to 40 μm. If the thickness is greater than 1 μm, the hard coat layer will cure sufficiently and provide good pencil hardness. Furthermore, by keeping the thickness at 40 μm or less, curling due to cure shrinkage of the hard coat can be suppressed, improving the handling properties of the film.

[0045] (Hard Coating Layer Coating Method) The hard coat layer can be applied by any method, including a Mayer bar coater, a gravure coater, a die coater, and a knife coater, without any particular limitation, and can be appropriately selected depending on the viscosity and film thickness.

[0046] (Curing conditions for hard coat layer) The hard coat layer can be cured by energy rays such as ultraviolet rays and electron beams, or by heat, but ultraviolet rays and electron beams are preferred in order to reduce damage to the film.

[0047] (Pencil hardness) The pencil hardness of the hard coat layer is preferably B or higher, and more preferably H or higher. A pencil hardness of B or higher prevents scratches and does not reduce visibility. Generally, a high pencil hardness of the hard coat layer is preferable, but a hard coat layer having a pencil hardness of 10H or lower, 8H or lower, or even 6H or lower can be used without any practical problems.

[0048] (Type of hard coat layer) The hard coat layer in the present invention may have other functions added thereto, as 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 having the above-mentioned certain pencil hardness and added functions such as an antiglare layer, an antiglare antireflection layer, an antireflection layer, a low reflection layer, and an antistatic layer are also preferably used in the present invention.

[0049] 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, more preferably 87% or more, as with 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 also be 99% or less, or even 97% or less. [Example]

[0050] Next, the effects of the present invention will be described using examples and comparative examples. First, the evaluation methods of the characteristic values ​​used in the present invention will be described below.

[0051] (1) Intrinsic viscosity After crushing and drying the film or polyester resin, it was dissolved in a 60 / 40 (mass ratio) phenol / tetrachloroethane mixed solvent. This solution was centrifuged to remove inorganic particles, and then an Ubbelohde viscometer was used to measure the flow time of a 0.4 (g / dL) solution at 30°C and the flow time of the solvent alone. The intrinsic viscosity was calculated from the ratio of these times using the Huggins equation, assuming a Huggins constant of 0.38. For laminated films, the intrinsic viscosity of each individual layer was evaluated by scraping off the corresponding polyester layer of the film according to the laminate thickness.

[0052] (2) Flexibility The adhesive side of the sample was attached to a 50 μm-thick polyimide film to prepare a measurement sample measuring 50 mm in width and 100 mm in machine direction. Using a no-load U-shaped stretch tester (Yuasa System Co., Ltd., DLDMLH-FS), the sample was bent 50,000 times at a rate of 1 bend / second with a bending radius of 3 mm. The sample was fixed at 10 mm positions on both long sides, and the bending area was 50 mm x 80 mm. After the bending process, the sample was placed on a flat surface with the inside of the bend facing down and visually inspected. The bending resistance of the surface protection film with a hard coat layer was evaluated in each of the following examples and comparative examples. ◎: No deformation of the sample was observed. ○: The sample is deformed, but when placed horizontally, the maximum lift height is less than 5 mm. ×: The sample has creases or the maximum lift height is 5 mm or more when placed horizontally.

[0053] (3) Pencil hardness The surface of the hard coat layer was measured in accordance with JIS K 5600-5-4:1999 at a load of 750 g and a speed of 0.5 mm / s.

[0054] (4) Total light transmittance, haze Measurement was performed using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.) 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.

[0055] (5) Adhesive strength The adhesive side of the sample was attached to a SUS304 plate, and a 2 kg rubber roller was rolled back and forth over it once. After that, the tape was left at room temperature for 20 hours. The force (N / 25 mm) required to peel the tape from the SUS plate at an angle of 180° at a speed of 300 mm / min was measured using a tensile tester.

[0056] (Preparation of hard coat coating solution 1) 94.9 parts by weight of dipentaerythritol hexaacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., product name: A-DPH, solids concentration: 100% by weight), 5 parts by weight of a photoradical polymerization initiator (manufactured by IGM Resins, product name: Omnirad907, solids concentration: 100% by weight), and 0.25 parts by weight of a silicone additive (manufactured by BYK Japan, BYK-3505, solids concentration: 40% by weight) were mixed and diluted with a solvent of MEK / toluene / IPA=1 / 1 / 1 to prepare a 40% hard coat coating solution 1.

[0057] (Preparation of adhesive coating solution 2) 100 parts by weight of an acrylic adhesive (manufactured by Toyo Ink Mfg., product name: Olivine BPS5762K, solid content: 45.5% by weight), 1.4 parts by weight of a crosslinking agent (manufactured by Toyo Ink Mfg., product name: BXX5627, solid content: 50% by weight), and 30.6 parts by weight of MEK were mixed to prepare adhesive coating solution 2 with a solid content of 35%.

[0058] (Preparation of adhesive coating solution 3) 100 parts by weight of silicone adhesive (manufactured by Shin-Etsu Chemical Co., Ltd., product name: X-40-3237), 0.5 parts by weight of catalyst (manufactured by Shin-Etsu Chemical Co., Ltd., product name: PL-50T), and 0.5 parts by weight of crosslinking agent (manufactured by Shin-Etsu Chemical Co., Ltd., product name: KS-3802) were diluted with toluene to prepare adhesive coating solution 3 with a solids concentration of 40%.

[0059] (Preparation of polyethylene terephthalate pellets (a)) A continuous esterification reactor consisting of a three-stage complete mixing vessel equipped with an agitator, a partial condenser, a raw material inlet, and a product outlet was used. TPA was supplied at 2 tons / hr, EG at 2 moles per mole of TPA, and antimony trioxide in an amount sufficient to provide 160 ppm Sb atoms relative to the PET product. These slurries were continuously fed into the first esterification reactor of the esterification reactor and reacted at atmospheric pressure for an average residence time of 4 hours at 255°C. The reaction product in the first esterification reactor was then continuously removed from the system and fed to the second esterification reactor. EG distilled from the first esterification reactor was fed into the second esterification reactor at 8% by mass relative to the polymer (PET product). An EG solution containing magnesium acetate in an amount sufficient to provide 65 ppm Mg atoms relative to the PET product, and an EG solution containing TMPA in an amount sufficient to provide 20 ppm P atoms relative to the PET product, were then added, and the reaction was continued at atmospheric pressure for an average residence time of 1.5 hours at 260°C. Next, the reaction product in the second esterification reactor was continuously removed from the system and fed to a third esterification reactor, and an EG solution containing TMPA in an amount such that the P atom concentration was 20 ppm relative to the produced PET was added, followed by reaction at atmospheric 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 fed to a three-stage continuous polycondensation reactor to carry out polycondensation, and then filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 90% cutoff of 5 μm particles) to obtain polyethylene terephthalate pellets (a) with an intrinsic viscosity of 0.62 dL / g.

[0060] (Preparation of polyethylene terephthalate pellets (b)) The intrinsic viscosity was adjusted 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, to obtain polyethylene terephthalate pellets (b).

[0061] (Preparation of polyethylene terephthalate pellets (c)) The polyethylene terephthalate pellets (a) were subjected to solid-state polymerization at 220°C under a reduced pressure of 0.5 mmHg for various times using a rotary vacuum polymerization apparatus to produce polyethylene terephthalate pellets (c) with an intrinsic viscosity of 0.67 dl / g.

[0062] (Preparation of polyethylene terephthalate pellets (d)) The polyethylene terephthalate pellets (a) were subjected to solid-state polymerization using a rotary vacuum polymerization apparatus under a reduced pressure of 0.5 mmHg at 220°C for various times to produce polyethylene terephthalate pellets (d) with an intrinsic viscosity of 0.75 dl / g.

[0063] (Preparation of polyethylene terephthalate pellets (e)) The polyethylene terephthalate pellets (a) were subjected to solid-state polymerization at 220°C under a reduced pressure of 0.5 mmHg for various times using a rotary vacuum polymerization apparatus to produce polyethylene terephthalate pellets (e) with an intrinsic viscosity of 0.83 dl / g.

[0064] The polyethylene terephthalate master pellets (a) were dried under reduced pressure (3 Torr) at 180°C for 8 hours, and then fed into an extruder and melted at 285°C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% cutoff of 10 μm particles) and extruded into a sheet form through a die. The sheet was then contacted with a casting drum at a surface temperature of 30°C using an electrostatic casting method, where it was cooled and solidified to produce 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, yielding the polyethylene terephthalate film No. 1 in Table 1. The polyethylene terephthalate master pellets (b) to (e) were fed into an extruder using a process similar to that described above, with some adjustments made to the conditions, to obtain the polyethylene terephthalate films No. 1 to 7 in Table 1.

[0065] Example 1 Hard coat coating solution 1 was applied to one side of polyethylene terephthalate film No. 4 using a Mayer bar so that the film thickness after drying would be 2.0 μm, and the film was dried at 80°C for 1 minute, after which it was irradiated with ultraviolet light (cumulative light dose 200 mJ / cm 2 Next, adhesive coating solution 2 was applied to 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 would be 10 μm, and the coating was dried at 120° C. for 1 minute to prepare a surface protection film.

[0066] Example 2 Hard coat coating solution 1 was applied to one side of polyethylene terephthalate film No. 4 using a Mayer bar so that the film thickness after drying would be 2.0 μm, and the film was dried at 80°C for 1 minute, after which it was irradiated with ultraviolet light (cumulative light dose 200 mJ / cm 2 Next, adhesive coating solution 3 was applied to 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 would be 10 μm, and the coating was dried at 130° C. for 1 minute to prepare a surface protection film.

[0067] (Examples 3 to 5, Comparative Examples 1 to 3) A surface protection film was prepared under the conditions in Table 2 in the same manner as in Example 1.

[0068] [Table 1]

[0069] [Table 2] [Industrial Applicability]

[0070] According to the present invention, it is 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

1. A polyester film having a thickness of 50 to 75 μm and an adhesive layer on one side thereof, a hard coat layer is laminated on the surface of the polyester film opposite to the surface having the adhesive layer; The adhesive layer is formed of an acrylic adhesive and has a thickness of 1 to 25 μm. The hard coat layer is formed of an acrylic resin and has a thickness of 1 to 40 μm. 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, A surface protection film for a folding display that, after bending, is placed horizontally on a flat surface with the inside of the bend facing downwards, has a maximum lift of less than 5 mm or does not deform.

2. A surface protection film for a foldable display as described in claim 1, wherein the polyester film has an intrinsic viscosity of 0.65 to 1.0 dl / g.

3. an organic EL display; a touch panel disposed above the organic EL display; A foldable display comprising: the surface protective film for a foldable display according to claim 1 or 2, which is disposed on an upper portion of a touch panel.

4. A mobile terminal device comprising the foldable display according to claim 3.

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