Polyester film for protecting foldable display

Abstract

The present invention relates to a polyester film for protecting a foldable display. More specifically, the present invention relates to a polyester film for protecting a foldable display comprising a primer layer having a high refractive index on at least one surface of a multilayer polyester base film containing a UV absorber so as to ensure foldability and improve visibility by reducing rainbow spots.

Description

Technical Field

[0001] The present invention relates to a polyester film for protecting a foldable display, and more particularly, to a polyester film for protecting a foldable display that can prevent whitening and cracking at a folding point in the foldable display and improve visibility by reducing rainbow spots. Background Art

[0002] Generally, polyester films have excellent dimensional stability, thickness uniformity, and optical transparency, and have very wide uses not only as display devices but also as various industrial materials.

[0003] In recent years, the demand for display technology driven by technological progress has surged, resulting in the commercialization and sales of various display devices such as curved displays, bent displays, and foldable displays. In particular, foldable displays offer the advantages of being compact when not in use, improving portability, and being unfolded to provide a wide screen when in use.

[0004] Compared with conventional flat-panel displays, these display devices are susceptible to external impacts, and in order to overcome such shortcomings, a cover window is necessary. Recently, for this purpose, transparent polyimide films or ultra-thin glass have been mainly used. However, transparent polyimide films are susceptible to external scratches, and ultra-thin glass has poor anti-shatter properties against external impacts, so a protective film is applied to its surface. The film applied to these foldable display devices is subjected to continuous loads in the folded state, which may cause deformation of the film, resulting in whitening and cracks. Due to this deformation, delamination between layers may also occur.

[0005] As protective films, elastomer-based polymer films that are not easily deformed are mainly used. In particular, the use of polyester films, which have durability and cost advantages, is increasing. In order to apply it to foldable displays, additional functions such as rainbow spot reduction and UV blocking are required for internal component protection in normal environments. Summary of the invention

[0006] Technical issues

[0007] The present invention is designed to solve various problems including the aforementioned problems and meet conventional requirements. An object of the present invention is to provide a polyester film for protecting a foldable display, which exhibits excellent visibility due to rainbow spot reduction and does not whiten and form cracks even during long-term use.

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

[0009] Technical Solution

[0010] The above object is achieved by a polyester film for protecting a foldable display, which includes a substrate composed 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 means tensile strength in the MD direction and TDT means tensile strength in the TD direction.

[0014] Preferably, the polyester film for protecting the foldable display may satisfy Expression 2 below.

[0015] (Expression 2)

[0016] 2.6 <MDF5 / MDF1<3.4

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

[0018] Preferably, the polyester film for protecting the foldable display may satisfy Formula 3 below.

[0019] (Expression 3)

[0020] TDE > MDE

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

[0022] Preferably, the polyester film for protecting the foldable display may have a strength of 330 kgf / mm 2 Up to 480kgf / mm 2 MD elastic modulus and 370kgf / mm 2 Up to 520kgf / mm 2 TD elastic modulus.

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

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

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

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

[0027] Preferably, the organic particles may include organic particles consisting of a single type or two or more types of organic particles selected from polyacrylic acid-based particles, polystyrene-based particles, polymelamine-based particles, polybenzoguanamine-based particles, polyimide-based particles, and polyester-based particles having a block or branched structure.

[0028] Preferably, the curing agent may include a At least one curing agent selected from the group consisting of an oxazoline-based curing agent, a carbodiimide-based curing agent, an epoxy-based curing agent, and a melamine-based curing agent.

[0029] Beneficial Effects

[0030] According to the polyester film for protecting a foldable display according to the present invention, when it is applied to a foldable display, long-term reliability and improved visibility are provided due to excellent whitening prevention and rainbow spot reduction.

[0031] Those skilled in the art will appreciate that the effects that can be achieved by the present invention are not limited to the effects that have been specifically described above, and other advantages of the present invention will be more clearly understood from the foregoing detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a cross-sectional view of a polyester film for protecting a foldable display according to an embodiment of the present invention. DETAILED DESCRIPTION

[0033] Hereinafter, exemplary embodiments of the present invention will be described more fully with reference to the accompanying drawings so that the present invention can be easily put into practice by those skilled in the art. The present invention can 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 is exaggerated for clarity. The same reference numerals represent 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 can be directly on the other element or there can be intermediate elements. In contrast, when an element is referred to as being "directly" "on" another element, there are no intermediate elements.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those of ordinary skill in the art to which the invention belongs. In the event of a conflict, the present specification (including definitions) shall prevail. Although methods and materials similar or equivalent to the methods and materials described herein can be used in the practice or testing of the present invention, suitable methods and materials are described herein.

[0036] In the present specification, the terms "O-based resin", "O-based polymer" or / and "O-based copolymer" include concepts covering "O-resin", "O-polymer", "O-copolymer" or / and "derivatives of O-resin, polymer or copolymer". In addition, as used herein, the term "polymer or copolymer cross-linked with these resins" means "polymer or copolymer cross-linked with the aforementioned resin". The term "compound" as used herein is a comprehensive concept covering "monoatomic molecule", "oligomer" and "polymer compound including both homopolymer and copolymer".

[0037] In this specification, unless explicitly described to the contrary, the terms "comprise", "comprising", "include", "including", "containing", "characterized by", "having", "having" or any other variations thereof, will be understood to imply the inclusion of stated elements, but not excluding the existence or addition of one or more other elements.

[0038] Unless otherwise indicated, all percentages, parts, ratios, etc. are by weight. When an amount, concentration, or other value or parameter is given in the form of a range, a preferred range, or a list of upper preferred values ​​and lower preferred values, this will be understood to specifically disclose all ranges formed by any pair of any upper range or preferred value and any lower range or preferred value, whether or not a range is disclosed individually.

[0039] Where numerical ranges are recited herein, unless otherwise indicated, the ranges are intended to include their endpoints and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values ​​recited when defining a range.

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

[0041] Figure 1 is a cross-sectional view of a polyester film for protecting a foldable display according to one embodiment of the present invention. Figure 1According to one aspect of the present invention, a polyester film for protecting a foldable display may include a substrate 10 composed 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 including a UV absorber and an outermost layer 12 not including a UV absorber. In addition, as Figure 1 As shown, the primer layer 30 is located on one surface of the substrate 10 , but it may also be located on both surfaces of the substrate 10 .

[0042] In one embodiment, the substrate 10 may have a three-layer structure of A / B / A consisting of a core layer 11 containing a UV absorber and an outermost layer 12 containing no UV absorber. For the additional functions of the film, it may be a multilayer having five or more layers. In the case of a multilayer structure having five or more layers, the core layer 11 containing a UV absorber and the outermost layer 12 containing no UV absorber are alternately arranged, and the outermost layer 12 of the substrate does not contain a UV absorber. In addition, the substrate may be formed by mixing a masterbatch containing organic or inorganic particles, or other resins compatible with polyester, or recycled polyester chips.

[0043] The polyester resin constituting the substrate 10 may be formed by polymerizing a dicarboxylic acid and a diol. Here, examples of the dicarboxylic acid may include aromatic dicarboxylic acids such as terephthalic acid, naphthalene dicarboxylic acid, isophthalic acid, diphenyl carboxylic acid, diphenyl sulfone dicarboxylic acid, diphenoxy ethane dicarboxylic acid, 5-sodium sulfone dicarboxylic acid, phthalic acid, etc.; aliphatic dicarboxylic acids such as hydroxy acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, fumaric acid, etc.; alicyclic dicarboxylic acids such as cyclohexane dicarboxylic acid, etc.; and hydroxycarboxylic acids such as p-hydroxybenzoic acid. Examples of the diol may include aliphatic diols such as ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, neopentyl glycol, etc.; polyoxyalkylene glycols such as diethylene glycol, polyethylene glycol, polypropylene glycol, etc.; alicyclic diols such as cyclohexanedimethanol, etc.; and aromatic diols such as bisphenol A, bisphenol S, etc.

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

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

[0046] The UV absorbent used in the present invention has low transmittance at a wavelength of 380 nm to protect the substrate, and exhibits high transmittance at a wavelength of 400 nm to enhance visibility according to screen brightness.

[0047] In addition, preferably, the outermost layer 12 containing no UV absorber may utilize terephthalic acid or naphthalene dicarboxylic acid as dicarboxylic acid and be composed of a polyester resin formed by polymerizing ethylene glycol as diol. However, the outermost layer is not limited thereto and may contain various organic or inorganic particles to improve characteristics such as running characteristics during product production and windability during product winding.

[0048] In one embodiment, the primer layer 30 may include a polymer resin, organic particles, and a curing agent. In addition, it may further include additives for additional functions.

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

[0050] In addition, the organic particles may also be used in the form of a copolymer, the organic particles being composed of a single one or two or more selected from polyacrylic acid-based particles, polystyrene-based particles, polymelamine-based particles, polybenzoguanamine-based particles, polyimide-based particles, and polyester-based particles of a block or branched structure. In addition, the organic particles in the primer layer 30 may include two or more types of organic particles of the same kind or different kinds having different average particle sizes.

[0051] In addition, the curing agent may include a At least one curing agent selected from the group consisting of an oxazoline-based curing agent, a carbodiimide-based curing agent, an epoxy-based curing agent, and a melamine-based curing agent.

[0052] In the present 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, rainbow spots cannot be controlled, resulting in poor visibility. If the refractive index exceeds 1.60, rainbow spots may be aggravated during subsequent processing due to the refractive index difference with the hard coating layer.

[0053] In one embodiment, the polyester film for protecting a foldable display preferably satisfies Expressions 1 to 3 in order to achieve excellent foldability when used to protect a 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 may be in the range of 0.9 to 1.0.

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

[0058] (Expression 2)

[0059] 2.6 <MDF5 / MDF1<3.4

[0060] Here, MDF5 represents the strength when there is 5% strain in the MD direction when stretched at a stretching speed of 100 mm / min, and MDF1 represents the strength when there is 1% strain in the MD direction when stretched at a stretching speed of 100 mm / min. More preferably, Expression 2 may 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 decrease, and if it is 3.4 or more, long-term dimensional stability decreases due to repeated folding.

[0062] (Expression 3)

[0063] TDE > MDE

[0064] Here, MDE represents the elastic modulus in the MD direction in the entire region until the yield point of the film, and TDE represents the elastic modulus in the TD direction in the entire region until the yield point of the film. If Expression 3 is not satisfied, that is, if the elastic modulus in the MD direction is equal to or greater than the elastic modulus in the TD direction until the yield point of the film, it may cause a decrease in folding durability or bulging of the film due to an increase in load at the folding point during folding.

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

[0066] The polyester film for protecting a foldable display according to one embodiment of the present invention preferably has a strength of 330 kgf / mm 2 Up to 480kgf / mm 2 MD elastic modulus and 370kgf / mm 2 Up to 520kgf / mm 2 If the TD elastic modulus is less than 330kgf / mm 2 Or TD elastic modulus is less than 370kgf / mm 2 If the MD elastic modulus exceeds 480kgf / mm 2 Or TD elastic modulus exceeds 520kgf / mm 2 , it may lead to reduced folding durability.

[0067] According to another aspect of the present invention, a method for manufacturing a polyester film for protecting a foldable display includes: a first step of melt-extruding a resin for forming a core layer A comprising a polyester resin and a UV absorber and a resin for forming an outermost layer B comprising a polyester resin into a three-layer structure of A / B / A; a second step of obtaining an unstretched sheet by rapidly cooling the melt-extruded resin; 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. At this time, if the longitudinal stretch ratio is less than 3.0 times, visible streaks may be caused due to longitudinal stretch marks. If it exceeds 3.3 times, 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. At this time, if the transverse stretching 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 film breakage during production due to excessive stretching, resulting in lower productivity.

[0070] Furthermore, in the sixth step, the biaxially stretched sheet is preferably heat-treated at a temperature of 200° C. to 225° C. for 5 to 20 seconds. At this time, if the heat treatment temperature is 210° C. or lower, it may cause an increase in the shrinkage of the entire film. If it exceeds 225° C., it may cause poor uniformity in the transverse direction of the film.

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

[0072] [Example]

[0073] Preparation Example 1: Preparation of polyester resin

[0074] 100 mol% of dimethyl terephthalate as a dicarboxylic acid component and 100 mol% of ethylene glycol as a 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. Thereafter, the reaction mixture was heated to 200 ℃ and then heated to 400 ℃. 2 The mixture was pressurized and the temperature was raised to 250°C to carry out a chemical reaction while removing the byproduct methanol. After the esterification reaction was completed, 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% by weight relative to dimethyl terephthalate. After 10 minutes, antimony trioxide was added as a polymerization catalyst at 0.04% by weight relative to dimethyl terephthalate. Subsequently, the polymerization was carried out for 240 minutes by gradually raising the temperature to 285°C while removing excess glycol components through a vacuum line by gradually reducing the pressure to less than 1 Torr within 50 minutes to obtain a polyester resin (polyethylene terephthalate flakes) having an intrinsic viscosity of 0.65 dl / g.

[0075] Preparation Example 2: Preparation of polyester masterbatch containing UV absorber

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

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

[0078] Preparation Example 3-1

[0079] The coating composition is prepared by mixing and stirring 10 g of an anionic surfactant aqueous solution (solid content of 10 wt%) in 700 g of water, and then adding 260 g of a urethane-based resin (solid content of 20 wt%), 20 g of a melamine-based curing agent (solid content of 80 wt%), and 2.5 g and 1.5 g of an aqueous solution (solid content of 10 wt%) in which polystyrene organic particles having average particle sizes of 200 nm and 400 nm, respectively, are dispersed and stirred.

[0080] Preparation Example 3-2

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

[0082] Preparation Example 3-3

[0083] The coating composition was prepared by mixing and stirring 10 g of an anionic surfactant aqueous solution (solid content of 10 wt%) in 700 g of water, and then adding 260 g of an acrylic-based resin (solid content of 20 wt%), 20 g of a blocked isocyanate curing agent (solid content of 80 wt%), and 2.5 g and 1.5 g of an aqueous solution (solid content of 10 wt%) in which polystyrene organic particles having average particle diameters of 200 nm and 400 nm, respectively, were dispersed and stirred.

[0084] [Example 1]

[0085] A mixture of polyethylene terephthalate chips prepared according to Preparation Example 1 and polyethylene terephthalate particle chips 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 chips prepared according to Preparation Example 1 were dried in an extruder associated with layer A (outermost layer) for use. The polyethylene terephthalate chips 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 a temperature of 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 a temperature of 120°C, and heat treated at a temperature of 210°C for 10 seconds to produce a polyester film with a thickness of 50 μm.

[0086] Example 2

[0087] A polyester film was produced by the same procedure as in Example 1, except that the coating composition prepared in Preparation Example 3-2 was used for coating.

[0088] Example 3

[0089] A polyester film was produced by the same procedure as in Example 1, except that the obtained sheet was stretched 3.1 times in the longitudinal direction.

[0090] Example 4

[0091] A polyester film was produced by the same procedure as in Example 1, except that the obtained sheet was stretched 3.3 times in the longitudinal direction.

[0092] Example 5

[0093] A polyester film was produced by the same procedure as in Example 1, except that the obtained sheet was stretched 3.3 times in the longitudinal direction and heat-treated at a temperature of 200° C. for 10 seconds.

[0094] [Comparative Example]

[0095] [Comparative Example 1]

[0096] A polyester film was produced by the same procedure as in Example 1, except that the obtained sheet was stretched 3.3 times in the longitudinal direction and 3.7 times in the transverse direction and heat-treated at a temperature of 230° C. for 10 seconds.

[0097] [Comparative Example 2]

[0098] A polyester film was produced by the same process as in Example 1, except that all the polyester chips prepared in Preparation Example 1 were used in the extruder associated with Layer B (core layer).

[0099] [Comparative Example 3]

[0100] A polyester film was produced by the same procedure as in Example 1, except that the coating composition prepared in Preparation Example 3-3 was used for coating.

[0101] [Comparative Example 4]

[0102] A polyester film was produced by the same procedure as in Example 1, except that the obtained sheet was stretched 3.35 times in the longitudinal direction.

[0103] [Comparative Example 5]

[0104] A polyester film was produced by the same procedure as in Example 1, except that the obtained sheet was stretched 3.3 times in the longitudinal direction and heat-treated at a temperature of 190° C. for 10 seconds after the transverse stretching.

[0105] [Comparative Example 6]

[0106] A polyester film was produced by the same procedure as in Example 1, except that the obtained sheet was stretched 2.9 times in the longitudinal direction and 3.8 times in the transverse direction.

[0107] The constituent components of the base layer (core layer and outermost layer) and the primer layer in the above-mentioned Examples and Comparative Examples, their proportions and the manufacturing process 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 performed 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] The measurement was performed using COH-300A (haze meter) according to ASTM D1003. For both total light 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 light transmittance and haze.

[0117] 2. Transmittance measurement

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

[0119] 3. Measurement of strength and elastic modulus

[0120] The polyester film samples from Examples and Comparative Examples were cut into a size of 100 mm in length and 12.65 mm in width using a universal testing machine (AG-X plus 500N, Shimadzu). These samples were then stretched at a stretching speed of 100 mm / min until fracture occurred, and a stress-strain curve was obtained. The measurement was performed by defining the strength at 1% strain as F1, the strength at 5% strain as F5, and the strength at the time of fracture as the maximum value. The elastic modulus was calculated by dividing the force applied to the cross-sectional area by the strain rate according to the calculation equation of the device.

[0121] 4. Evaluation of foldability

[0122] The polyester film samples from Examples and Comparative Examples were subjected to 10,000 folding tests at -20°C using a folding tester. Thereafter, the appearance of the film was repeatedly inspected for a total of 200,000 cycles under the same conditions, and cracks and whitening at the folded positions were visually inspected and evaluated.

[0123] 5. Evaluation of rainbow spots

[0124] The polyester film samples from Examples and Comparative Examples cut to a size of 15 cm x 5 cm were examined under fluorescent reflection to visually detect the presence of rainbow spots by attaching an insulating tape (Nitto) to the opposite side of the coating to be measured.

[0125] [Table 3]

[0126]

[0127]

[0128] [Table 4]

[0129]

[0130] As can be seen from Table 3 and Table 4, the polyester films for protecting a foldable display according to Examples 1 to 5 of the present invention exhibited excellent bending characteristics and simultaneously showed excellent visibility by reducing rainbow spots.

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

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

[0133] Furthermore, in the case of, for example, Comparative Example 3 in which an appropriate primer layer was not selected, rainbow spots could not be controlled, resulting in poor visibility.

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

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

[0136] Furthermore, as shown in Comparative Example 6, when TDE is smaller than MDE, this results in a decrease in folding durability.

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

[0138] Description of Reference Numerals

[0139] 10: Base

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

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

[0142] 30: Primer layer

Claims

1. A polyester film for protecting a foldable display, include: A base composed of polyester; and forming a primer layer on at least one surface of the substrate, At the same time, the polyester film satisfies the following expression 1: (Expression 1) 0.85 <MDT / TDT<1.05, Wherein MDT represents the tensile strength in the MD direction and TDT represents the tensile strength in the TD direction.

2. The polyester film according to claim 1, satisfying the following Expression 2: (Expression 2) 2.6 <MDF5 / MDF1<3.4, Here, MDF5 represents the strength when there is a 5% strain in the MD direction when stretched at a stretching speed of 100 mm / min, and MDF1 represents the strength when there is a 1% strain in the MD direction when stretched at a stretching speed of 100 mm / min.

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

4. The polyester film according to claim 1, having a strength of 330 kgf / mm 2 Up to 480kgf / mm 2 MD elastic modulus and 370kgf / mm 2 Up to 520kgf / mm 2 TD elastic modulus. 5 . The polyester film according to claim 1 , wherein the substrate has at least three or more laminated layers, wherein at least one of the layers except the outermost layer comprises 0.1 to 3.0 wt % of a UV absorber. The polyester film according to claim 1 , wherein the primer layer comprises a polymer resin, organic particles, and a curing agent. 7 . The polyester film according to claim 6 , wherein the polymer resin comprises at least one resin selected from the group consisting of a polyurethane-based resin, a polyester-based resin, and a polyacrylic acid-based resin. 8 . The polyester film according to claim 6 , wherein the organic particles include two or more types of organic particles of the same kind or different kinds having different average particle diameters.

9. The polyester film according to claim 8, wherein the organic particles comprise organic particles consisting of a single type or two or more types of organic particles selected from polyacrylic acid-based particles, polystyrene-based particles, polymelamine-based particles, polybenzoguanamine-based particles, polyimide-based particles, and polyester-based particles of a block or branched structure.

10. The polyester film according to claim 6, wherein the curing agent comprises a At least one curing agent selected from the group consisting of an oxazoline-based curing agent, a carbodiimide-based curing agent, an epoxy-based curing agent, and a melamine-based curing agent.

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