Polyester film for protecting foldable displays

Abstract

The present invention relates to a polyester film for protecting foldable displays. More specifically, the present invention relates to a polyester film for protecting foldable displays, which includes a primer layer having a high refractive index on at least one surface of a multi-layered polyester base film containing a UV absorber, in order to ensure foldability and improve visibility by reducing rainbow.

Description

Technical Field

[0001] This invention relates to a polyester film for protecting foldable displays, and more specifically, to a polyester film for protecting foldable displays that can prevent whitening and cracking at the folding points in foldable displays and improve visibility by reducing iris spots. Background Technology

[0002] Polyester films typically possess excellent dimensional stability, thickness uniformity, and optical transparency, and have a very wide range of applications, not only as display devices but also as various industrial materials.

[0003] In recent years, the demand for display technologies driven by technological advancements has surged, leading to the commercialization and sales of various display devices such as curved displays, bendable displays, and foldable displays. In particular, foldable displays offer the advantages of being compact when not in use, enhancing portability, and unfolding to provide a wide screen when in use.

[0004] Compared to conventional flat panel displays, these display devices are more susceptible to external impacts, and to overcome this drawback, a cover window is necessary. Recently, transparent polyimide films or ultra-thin glass have been primarily used for this purpose. However, transparent polyimide films are susceptible to external scratches, and ultra-thin glass has poor shatter resistance to external impacts; therefore, a protective film is applied to their surfaces. The film applied to these foldable display devices is subjected to continuous loads in the folded state, which can cause film deformation, leading to whitening and cracking. Delamination between layers may also occur due to this deformation.

[0005] The primary protective film used is an elastomer-based polymer film that is not easily deformable. In particular, the use of polyester films, which offer durability and cost advantages, is increasing. For application in foldable displays, additional features such as iris reduction and UV blocking are needed for the protection of internal components in normal environments. Summary of the Invention

[0006] Technical issues

[0007] This invention is designed to solve various problems including those mentioned above and to meet conventional requirements. One object of this invention is to provide a polyester film for protecting foldable displays, which exhibits excellent visibility due to reduced iris spots and does not whiten or crack even during prolonged use.

[0008] The above and other objects and advantages of the present invention will become apparent from the following description of preferred embodiments.

[0009] Technical solution

[0010] The above objective is achieved by a polyester film for protecting the foldable display, the polyester film for protecting the foldable display comprising a substrate made of polyester and a primer layer formed on at least one surface of the substrate, while satisfying the following expression 1.

[0011] (Expression 1)

[0012] 0.85 <MDT / TDT<1.05

[0013] Here, MDT represents the tensile strength in the MD direction and TDT represents the tensile strength in the TD direction.

[0014] Preferably, the polyester film used to protect the foldable display can satisfy the following expression 2.

[0015] (Expression 2)

[0016] 2.6 <MDF5 / MDF1<3.4

[0017] Here, MDF5 represents the strength with 5% strain in the MD direction when stretched at a tensile speed of 100 mm / min, and MDF1 represents the strength with 1% strain in the MD direction when stretched at a tensile speed of 100 mm / min.

[0018] Preferably, the polyester film used to protect the foldable display can satisfy Formula 3 below.

[0019] (Expression 3)

[0020] TDE>MDE

[0021] Here, MDE represents the elastic modulus in the MD direction throughout the region up to the yield point of the membrane, and TDE represents the elastic modulus in the TD direction throughout the region up to the yield point of the membrane.

[0022] Preferably, the polyester film used to protect the foldable display can have a strength of 330 kgf / mm². 2 Up to 480 kgf / mm 2 MD elastic modulus and 370 kgf / mm 2 Up to 520 kgf / mm 2 The TD elastic modulus.

[0023] Preferably, the substrate may have at least three or more laminated layers, wherein at least one of the layers other than the outermost layer contains 0.1% to 3.0% by weight of a UV absorber.

[0024] Preferably, the primer layer may contain polymer resin, organic particles, and a curing agent.

[0025] Preferably, the polymer resin may include at least one resin selected from polyurethane-based resins, polyester-based resins, and polyacrylic acid-based resins.

[0026] Preferably, the organic particles may include two or more types of organic particles of the same or different kinds with different average particle sizes.

[0027] Preferably, the organic particles may comprise organic particles composed of one or more types of organic particles selected from polyacrylic acid-based particles, polystyrene-based particles, melamine-based particles, polyphenylguanidine-based particles, polyimide-based particles, and polyester-based particles with block or branched structures.

[0028] Preferably, the curing agent may include a selection from those based on At least one of the following curing agents: zoline-based curing agents, carbodiimide-based curing agents, epoxy-based curing agents, and melamine-based curing agents.

[0029] Beneficial effects

[0030] According to the polyester film for protecting foldable displays according to the present invention, when applied to foldable displays, it provides long-term reliability and improved visibility due to excellent whitening prevention and iris reduction.

[0031] Those skilled in the art will understand that the effects achievable by the present invention are not limited to those specifically described above, and that other advantages of the present invention will become clearer from the foregoing detailed description. Attached Figure Description

[0032] Figure 1 This is a cross-sectional view of a polyester film for protecting a foldable display according to one embodiment of the present invention. Detailed Implementation

[0033] In the following description, exemplary embodiments of the invention will be described more fully with reference to the accompanying drawings, enabling those skilled in the art to readily practice the invention. The invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0034] In the accompanying drawings, the thickness of layers and regions has been enlarged for clarity. The same reference numerals denote the same elements throughout the description. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it may be directly on the other element or there may be intermediate elements present. In contrast, when an element is referred to as being “directly” on another element, there are no intermediate elements present.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In case of conflict, this specification (including definitions) shall prevail. While methods and materials similar to or equivalent to those described herein may be used in the practice or testing of this invention, suitable methods and materials are described herein.

[0036] In this specification, the terms "-based resin," "-based polymer," and / or "-based copolymer" encompass the concepts covering "~ resin," "~ polymer," "~ copolymer," and / or "derivatives of ~ resin, polymer, or copolymer." Furthermore, as used herein, the term "polymer or copolymer crosslinked with these resins" means "polymer or copolymer crosslinked with the aforementioned resins." As used herein, the term "compound" is a comprehensive concept encompassing "monoatomic molecule," "oligomer," and "polymer compounds including both homopolymers and copolymers."

[0037] In this specification, unless expressly stated otherwise, the terms “comprise,” “comprising,” “include,” “including,” “containing,” “characterized in,” “having,” “owning,” or any other variation thereof shall be understood to imply inclusion of the stated element, but do not preclude the presence or addition of one or more other elements.

[0038] Unless otherwise stated, all percentages, parts, ratios, etc., are by weight. When quantities, concentrations, or other values ​​or parameters are given in the form of ranges, preferred ranges, or lists of upper and lower preferred values, this shall be understood as a specific disclosure of all ranges formed by any pair of any upper or preferred range and any lower or preferred range, whether or not the range is disclosed individually.

[0039] Where numerical ranges are described herein, unless otherwise stated, such ranges are intended to include their endpoints and all integers and fractions within that range. It is not intended to limit the scope of the invention to the specific values ​​listed when defining the scope.

[0040] In this specification, each of the components encompasses both the singular and plural forms.

[0041] Figure 1 This is a cross-sectional view of a polyester film for protecting a foldable display according to one embodiment of the present invention. (Refer to...) Figure 1According to one aspect of the invention, a polyester film for protecting a foldable display may include a substrate 10 made of polyester and a primer layer 30 formed on at least one surface of the substrate. The substrate 10 may include a core layer 11 containing a UV absorber and an outermost layer 12 without a UV absorber. Furthermore, as... Figure 1 As shown, the primer layer 30 is located on one surface of the substrate 10, but it can also be located on both surfaces of the substrate 10.

[0042] In one embodiment, the substrate 10 may have an A / B / A three-layer structure consisting of a core layer 11 containing a UV absorber and an outermost layer 12 without a UV absorber. For additional functions of the membrane, it may be a multilayer with five or more layers. In the case of a multilayer structure with five or more layers, the core layer 11 containing a UV absorber and the outermost layer 12 without a UV absorber are arranged alternately, and the outermost layer 12 of the substrate does not contain a UV absorber. Furthermore, the substrate may be formed by mixing a masterbatch containing organic or inorganic particles, other resins compatible with polyester, or recycled polyester flakes.

[0043] The polyester resin constituting the substrate 10 can be formed by polymerizing dicarboxylic acids and diols. Examples of dicarboxylic acids include aromatic dicarboxylic acids such as terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, diphenylcarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenoxyethane dicarboxylic acid, 5-sodium sulfone dicarboxylic acid, phthalic acid, etc.; aliphatic dicarboxylic acids such as hydroxy acids, succinic acid, adipic acid, sebacic acid, dimer acids, maleic acid, fumaric acid, etc.; alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid, etc.; and hydroxycarboxylic acids such as p-hydroxybenzoic acid. Examples of diols include aliphatic diols such as ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, neopentanediol, etc.; polyoxyalkylene diols such as diethylene glycol, polyethylene glycol, polypropylene glycol, etc.; alicyclic diols such as cyclohexanediol, etc.; and aromatic diols such as bisphenol A, bisphenol S, etc.

[0044] Preferably, the core layer 11 containing the UV absorber can use terephthalic acid or naphthalenedicarboxylic acid as the dicarboxylic acid, and is composed of a polyester resin formed by polymerizing ethylene glycol as a diol, and masterbatch flakes obtained by compounding the polyester resin with the UV absorber. However, it is not limited to this, and can contain various organic or inorganic particles and resins compatible with polyester to provide functionality.

[0045] In one embodiment, considering productivity, a method may be included to produce masterbatch flakes containing 5% or more and 20% or less of a UV absorber by compounding with a polyester resin using a twin-screw extruder. During film manufacturing, the masterbatch flakes and polyester resin are blended to produce the film. At least one layer other than the outermost layer 12 may contain 0.1% to 3.0% of the UV absorber. If the UV absorber is less than 0.1% by weight, it may not be able to protect the internal components from UV light, while if it exceeds 3.0% by weight, it reduces the overall light transmittance of the film, making it unsuitable for use as a display protective film.

[0046] The UV absorber used in this invention has low transmittance at a wavelength of 380 nm to protect the substrate, while exhibiting high transmittance at a wavelength of 400 nm to enhance visibility depending on screen brightness.

[0047] Furthermore, preferably, the outermost layer 12, which does not contain UV absorbers, can utilize terephthalic acid or naphthalenedicarboxylic acid as the dicarboxylic acid and is composed of a polyester resin formed by polymerizing ethylene glycol as a diol. However, the outermost layer is not limited to this and can contain various organic or inorganic particles to improve properties such as operating characteristics during product manufacturing and windability during product winding.

[0048] In one embodiment, the primer layer 30 may comprise a polymer resin, organic particles, and a curing agent. Additionally, it may contain additives for further functionality.

[0049] In one embodiment, the polymer resin may include at least one resin selected from polyurethane-based resins, polyester-based resins, and polypropylene-based resins.

[0050] Furthermore, the organic particles can also be used in the form of copolymers, which consist of one or more of the following: block or branched particles selected from polyacrylic acid-based particles, polystyrene-based particles, melamine-based particles, polyphenylguanidine-based particles, polyimide-based particles, and polyester-based particles. Additionally, the organic particles in the primer layer 30 may include two or more types of organic particles of the same or different kinds with different average particle sizes.

[0051] In addition, the curing agent may include those selected from those based on At least one of the following curing agents: zoline-based curing agents, carbodiimide-based curing agents, epoxy-based curing agents, and melamine-based curing agents.

[0052] In this invention, the refractive index of the primer layer 30 is preferably 1.55 to 1.60. If the refractive index of the primer layer 30 is less than 1.55, irising cannot be controlled, resulting in poor visibility. If the refractive index exceeds 1.60, irising may be exacerbated during subsequent processing due to the difference in refractive index with the hard coating.

[0053] In one embodiment, the polyester film used to protect the foldable display preferably satisfies expressions 1 to 3 in order to achieve excellent foldability when used to protect the foldable display.

[0054] (Expression 1)

[0055] 0.85 <MDT / TDT<1.05

[0056] Here, MDT represents the tensile strength in the MD direction and TDT represents the tensile strength in the TD direction. More preferably, Expression 1 can be in the range of 0.9 to 1.0.

[0057] If the value of Expression 1 is 0.85 or less, it may cause deformation during repeated folding, resulting in reduced long-term dimensional stability due to repeated folding. If it is 1.05 or greater, it may reduce folding durability.

[0058] (Expression 2)

[0059] 2.6 <MDF5 / MDF1<3.4

[0060] Here, MDF5 represents the strength with 5% strain in the MD direction when stretched at a tensile speed of 100 mm / min, and MDF1 represents the strength with 1% strain in the MD direction when stretched at a tensile speed of 100 mm / min. More preferably, Expression 2 can be in the range of 2.7 to 3.3.

[0061] If the value of expression 2 is 2.6 or less, folding durability may be reduced, and if it is 3.4 or greater, long-term dimensional stability may be reduced due to repeated folding.

[0062] (Expression 3)

[0063] TDE>MDE

[0064] Here, MDE represents the elastic modulus in the MD direction over the entire region up to the membrane's yield point, and TDE represents the elastic modulus in the TD direction over the entire region up to the membrane's yield point. If Expression 3 is not satisfied, i.e., if the elastic modulus in the MD direction up to the membrane's yield point is equal to or greater than the elastic modulus in the TD direction, it may lead to reduced folding durability or membrane bulging during folding due to increased load at the folding point.

[0065] It should be noted that in the aforementioned expressions 1 to 3, the folding direction refers to the folding in the MD direction of the membrane.

[0066] According to one embodiment of the invention, the polyester film for protecting a foldable display preferably has a strength of 330 kgf / mm². 2 Up to 480 kgf / mm 2 MD elastic modulus and 370 kgf / mm 2 Up to 520 kgf / mm 2 The TD elastic modulus. If the MD elastic modulus is less than 330 kgf / mm². 2 Or the TD elastic modulus is less than 370 kgf / mm² 2 Repeated folding may lead to decreased long-term dimensional stability. If the modulus of elasticity (MD) exceeds 480 kgf / mm², this could be problematic. 2 Or the TD elastic modulus exceeds 520 kgf / mm² 2 This may lead to reduced folding durability.

[0067] A method for manufacturing a polyester film for protecting a foldable display according to another aspect of the present invention comprises: a first step of melt-extruding a resin for forming a core layer A comprising polyester resin and a UV absorber and a resin for forming an outermost layer B comprising polyester resin into a three-layer structure of A / B / A; a second step of rapidly cooling the melt-extruded resin to obtain an unstretched sheet; a third step of stretching the unstretched sheet in a longitudinal direction; a fourth step of applying a primer coating composition to the longitudinally stretched sheet; a fifth step of stretching the sheet in a transverse direction; and a sixth step of heat-treating the biaxially stretched sheet.

[0068] In the third step, the unstretched sheet is preferably stretched 3.0 to 3.3 times in the longitudinal direction at a temperature of 100°C to 140°C. If the longitudinal stretch ratio is less than 3.0 times, visible streaks may result due to longitudinal stretch marks. If it exceeds 3.3 times, the desired mechanical properties may not be achieved.

[0069] In the fifth step, the sheet is preferably stretched 3.3 to 3.7 times in the transverse direction at a temperature of 120°C to 160°C. If the transverse stretch ratio is less than 3.3 times, it may result in poor uniformity of the film in the transverse direction. If it exceeds 3.7 times, it may cause the film to rupture during production due to overstretching, resulting in a lower yield.

[0070] Furthermore, in step six, it is preferable to heat-treat the biaxially stretched sheet at a temperature of 200°C to 225°C for 5 to 20 seconds. At this point, if the heat treatment temperature is 210°C or lower, it may lead to an increase in the overall shrinkage rate of the film. If it exceeds 225°C, it may result in poor uniformity of the film in the transverse direction.

[0071] The configuration and effects of the present invention will be described in more detail below through embodiments and comparative examples. The following embodiments are provided to further illustrate the present invention and are not intended to limit the scope of the invention.

[0072] [Example]

[0073] Preparation Example 1: Preparation of Polyester Resin

[0074] 100 mol% dimethyl terephthalate as the dicarboxylic acid component and 100 mol% ethylene glycol as the diol component were placed in an autoclave equipped with a stirrer and a distillation column. Subsequently, manganese acetate was added as a transesterification catalyst at a concentration of 0.07 wt% relative to dimethyl terephthalate. Afterwards, the reaction was carried out at 2 kg / cm³. 2 The mixture was pressurized and heated to 250°C to initiate a chemical reaction, while removing the byproduct methanol. After esterification, the reaction mixture was transferred to a second reactor equipped with a vacuum system, and trimethyl phosphate was added as a heat stabilizer at 0.02 wt% relative to dimethyl terephthalate. After 10 minutes, antimony trioxide was added as a polymerization catalyst at 0.04 wt% relative to dimethyl terephthalate. Subsequently, polymerization was carried out for 240 minutes by gradually increasing the temperature to 285°C, while excess glycol components were removed by gradually reducing the pressure to below 1 Torr over 50 minutes via a vacuum line, thereby obtaining a polyester resin (polyethylene terephthalate flakes) with an intrinsic viscosity of 0.65 dl / g.

[0075] Preparation Example 2: Preparation of Polyester Masterbatch Containing UV Absorber

[0076] A polyester masterbatch (polyethylene terephthalate flakes) containing UV absorber was prepared by mixing a resin blend containing 90 wt% polyester resin and 10 wt% UV absorber using a twin-screw extruder at 220°C to 280°C, a screw speed of 100 rpm, and a discharge rate of 20 kg / h.

[0077] Preparation Example 3: Preparation of a coating composition for a primer

[0078] Preparation Example 3-1

[0079] The coating composition was prepared by mixing and stirring 10 g of an aqueous solution of anionic surfactant (10% by weight of solids) in 700 g of water, then adding 260 g of urethane-based resin (20% by weight of solids), 20 g of melamine-based curing agent (80% by weight of solids), and 2.5 g and 1.5 g of aqueous solutions (10% by weight of solids) in which polystyrene organic particles with average particle sizes of 200 nm and 400 nm, respectively, were dispersed, and stirring.

[0080] Preparation Example 3-2

[0081] The coating composition was prepared by mixing and stirring 10 g of an aqueous solution of anionic surfactant (10% by weight of solids) in 700 g of water, then adding 260 g of Co-PET-based resin (20% by weight of solids), 20 g of melamine-based curing agent (80% by weight of solids), and 2.5 g and 1.5 g of aqueous solutions (10% by weight of solids) in which polystyrene organic particles with average particle sizes of 200 nm and 400 nm, respectively, were dispersed, and stirring.

[0082] Preparation Example 3-3

[0083] The coating composition was prepared by mixing and stirring 10 g of an aqueous solution of anionic surfactant (10% by weight of solids) in 700 g of water, then adding 260 g of acrylic resin (20% by weight of solids), 20 g of block isocyanate curing agent (80% by weight of solids), and 2.5 g and 1.5 g of aqueous solutions (10% by weight of solids) in which polystyrene organic particles with average particle sizes of 200 nm and 400 nm, respectively, were dispersed, and stirring.

[0084] [Example 1]

[0085] A mixture of polyethylene terephthalate flakes prepared according to Preparation Example 1 and polyethylene terephthalate granular flakes prepared according to Preparation Example 2, in a weight ratio of 85:15, was dried in an extruder associated with layer B (core layer) for use. Only the polyethylene terephthalate flakes prepared according to Preparation Example 1 were dried in an extruder associated with layer A (outermost layer) for use. The polyethylene terephthalate flakes were melt-extruded at 280°C into a three-layer structure of A / B / A, and then rapidly cooled from a T-die by casting or calendering to obtain an unstretched sheet of 500 μm. Subsequently, it was stretched 3.2 times in the longitudinal direction at 120°C, coated with the coating composition prepared in Preparation Example 3-1 using a #3 Mayer rod, stretched 3.6 times in the transverse direction at 120°C, and heat-treated at 210°C for 10 seconds to produce a polyester film with a thickness of 50 μm.

[0086] Example 2

[0087] The polyester film was produced using the same process as in Example 1, except that the coating composition prepared in Preparation Examples 3-2 was used for coating.

[0088] Example 3

[0089] The polyester film was produced using the same process as in Example 1, except that the resulting sheet was stretched 3.1 times in the longitudinal direction.

[0090] Example 4

[0091] The polyester film was produced using the same process as in Example 1, except that the resulting sheet was stretched 3.3 times in the longitudinal direction.

[0092] Example 5

[0093] The polyester film was produced using the same process as in Example 1, except that the resulting sheet was stretched 3.3 times in the longitudinal direction and heat-treated at 200°C for 10 seconds.

[0094] [Comparative Example]

[0095] [Comparative Example 1]

[0096] The polyester film was produced using the same process as in Example 1, except that the resulting sheet was stretched 3.3 times in the longitudinal direction and 3.7 times in the transverse direction and then heat-treated at 230°C for 10 seconds.

[0097] [Comparative Example 2]

[0098] Polyester films are produced using the same process as in Example 1, except that all polyester sheets prepared in Example 1 are used in an extruder associated with layer B (core layer).

[0099] [Comparative Example 3]

[0100] The polyester film was produced using the same process as in Example 1, except that the coating composition prepared in Preparation Examples 3-3 was used for coating.

[0101] [Comparative Example 4]

[0102] The polyester film was produced using the same process as in Example 1, except that the resulting sheet was stretched 3.35 times in the longitudinal direction.

[0103] [Comparative Example 5]

[0104] The polyester film was produced using the same process as in Example 1, except that the resulting sheet was stretched 3.3 times in the longitudinal direction and then heat-treated at 190°C for 10 seconds after being stretched in the transverse direction.

[0105] [Comparative Example 6]

[0106] The polyester film was produced using the same process as in Example 1, except that the resulting sheet was stretched 2.9 times in the longitudinal direction and 3.8 times in the transverse direction.

[0107] The composition, proportions, and manufacturing processes of the substrate layer (core layer and outermost layer) and primer layer in the above embodiments and comparative examples are presented in Table 1 below.

[0108] [Table 1]

[0109]

[0110]

[0111] [Table 2]

[0112]

[0113] Using the polyester films according to Examples 1 to 5 and Comparative Examples 1 to 6 described above, the following experimental examples were conducted to measure physical properties, and the results are shown in Tables 3 and 4 below.

[0114] [Experimental Example]

[0115] 1. Measurement of light transmittance

[0116] Measurements were performed using a COH-300A (haze meter) according to ASTM D1003. For both total transmittance and haze measurements, five polyester film samples from examples and comparative examples were measured, and the average values ​​excluding the maximum and minimum values ​​were calculated as total transmittance and haze.

[0117] 2. Measurement of transmittance

[0118] The transmittance values ​​at wavelengths of 380 nm and 400 nm were calculated by measuring the reflectance at a 5° angle within the wavelength range of 300 nm to 600 nm using a UV-Vis spectrophotometer (UV-3600, Shimadzu).

[0119] 3. Measurement of strength and elastic modulus

[0120] Using a general-purpose testing machine (AG-X plus 500N, Shimadzu), polyester film samples from the examples and comparative examples were cut into dimensions of 100 mm in length and 12.65 mm in width. These samples were then stretched at a tensile speed of 100 mm / min until fracture, and stress-strain curves were obtained. Measurements were performed by defining the strength at 1% strain as F1, the strength at 5% strain as F5, and the strength at fracture as the maximum value. The elastic modulus was calculated by dividing the force applied across the cross-sectional area by the strain rate, according to the machine's calculation equations.

[0121] 4. Assessment of foldability

[0122] Polyester film samples from the examples and comparative examples were subjected to 10,000 folding tests at -20°C using a folding tester. Subsequently, the appearance of the films was repeatedly inspected for a total of 200,000 cycles under the same conditions, and cracks and whitening at the fold locations were visually inspected and evaluated.

[0123] 5. Assessment of iris spots

[0124] The presence of iris spots was visually detected by attaching insulating tape (Nitto) to the opposite side of the coating to be measured. Polyester film samples cut to 15cm x 5cm from the examples and comparative examples were examined under fluorescence reflection.

[0125] [Table 3]

[0126]

[0127]

[0128] [Table 4]

[0129]

[0130] As can be seen from Tables 3 and 4, the polyester films for protecting foldable displays according to Embodiments 1 to 5 of the present invention exhibit excellent bending properties and at the same time show excellent visibility by reducing iris spots.

[0131] On the other hand, it can be seen that, for example, in the case of Comparative Example 1, when Expression 1 and Expression 2 cannot be satisfied, even if Expression 3 is satisfied through inappropriate processing conditions, the membrane cannot achieve sufficient foldability.

[0132] Furthermore, it can be seen that when a suitable UV absorber is not selected as shown in Comparative Example 2, there is no UV blocking effect. In other words, in the case of, for example, Comparative Example 2, when no UV absorber is used or when an unsuitable UV absorber is used, the transmittance at a wavelength of 380 nm increases, and therefore the substrate cannot be protected and may result in poor product visibility.

[0133] Furthermore, in Comparative Example 3, for example, where an appropriate primer layer was not selected, iris spots could not be controlled, resulting in poor visibility.

[0134] Furthermore, as shown in Comparative Example 4, folding durability decreases when the value of Expression 1 exceeds 1.05.

[0135] Furthermore, as shown in Comparative Example 5, when the value of Expression 2 exceeds 3.4, there is a decrease in long-term dimensional stability due to repeated folding.

[0136] Furthermore, as shown in Comparative Example 6, when TDE is less than MDE, this leads to reduced folding durability.

[0137] While the invention has been specifically shown and described with reference to exemplary embodiments thereof, those skilled in the art will understand that various changes in form and detail may be made without departing from the spirit and scope of the invention as defined in the following claims.

[0138] Explanation of reference numerals in the attached figures

[0139] 10: Base

[0140] 11: Core layer (containing UV absorber)

[0141] 12: Outermost layer (without UV absorber)

[0142] 30: Primer layer

Claims

1. A polyester film for protecting a foldable display, comprising: A substrate made of polyester; and A primer layer formed on at least one surface of the substrate. Meanwhile, the polyester film satisfies the following expression 1: (Expression 1) 0.85 <MDT / TDT<1.05, Where MDT represents the tensile strength in the MD direction and TDT represents the tensile strength in the TD direction; and Satisfy the following expression 2: (Expression 2) 2.6 <MDF5 / MDF1<3.4, MDF5 represents the strength when stretched at a tensile speed of 100 mm / min with 5% strain in the MD direction, and MDF1 represents the strength when stretched at a tensile speed of 100 mm / min with 1% strain in the MD direction.

2. The polyester film according to claim 1, satisfying formula 3: (Expression 3) TDE > MDE Where MDE represents the elastic modulus in the MD direction throughout the region up to the yield point of the membrane, and TDE represents the elastic modulus in the TD direction throughout the region up to the yield point of the membrane.

3. The polyester film according to claim 1, having an MD elastic modulus of 330 kgf / mm² to 480 kgf / mm² and a TD elastic modulus of 370 kgf / mm² to 520 kgf / mm².

4. The polyester film of claim 1, wherein the substrate has at least three or more laminated layers, wherein at least one of the layers other than the outermost layer contains 0.1% to 3.0% by weight of a UV absorber.

5. The polyester film according to claim 1, wherein the primer layer comprises a polymer resin, organic particles, and a curing agent.

6. The polyester film according to claim 5, wherein the polymer resin comprises at least one resin selected from polyurethane-based resins, polyester-based resins, and polyacrylic acid-based resins.

7. The polyester film according to claim 5, wherein the organic particles comprise two or more types of organic particles of the same or different kinds having different average particle sizes.

8. The polyester film of claim 7, wherein the organic particles comprise organic particles composed of one or more types of organic particles selected from polyacrylic acid-based particles, polystyrene-based particles, melamine-based particles, polyphenylguanidine-based particles, polyimide-based particles, and polyester-based particles with block or branched structures.

9. The polyester film of claim 5, wherein the curing agent comprises a group selected from those based on... At least one of the following curing agents: zoline-based curing agents, carbodiimide-based curing agents, epoxy-based curing agents, and melamine-based curing agents.

Images