Polyester protective film for flexible display device

Abstract

A protective film, according to one embodiment, achieves very low reflectance through a multilayer structure in which the refractive index is controlled, and does not cause the occurrence of whitening and cracking even in the case of frequent bending or folding thereof. Accordingly, the protective film is applied to a cover of a flexible display device, particularly a foldable display device, thereby exhibiting excellent optical and mechanical characteristics.

Description

[0001] This patent application is a divisional application of an invention patent application with an application date of September 4, 2020, application number 202080062464.9, publication number CN114341675 A, and invention name “Polyester protective film for flexible display devices”. Technical Field

[0002] The present invention relates to a polyester protective film for a flexible display device. More particularly, the present invention relates to a protective film, wherein its modulus at room temperature is controlled, and a laminate and a flexible display device including the protective film. Background Art

[0003] With the development of information technology equipment, display technology has been continuously developed driven by demand. The technology of curved displays and bent displays has been commercialized. In recent years, in the field of mobile devices that require both large screens and portability, flexible display devices that can be flexibly bent or folded in response to external forces have been favored. In particular, foldable display devices have great advantages: that is, when not in use, it is folded into a small size to enhance its portability, and when in use, it is unfolded to form a large screen.

[0004] refer to Figure 4 and Figure 5 Such foldable display devices are developed into an inward folding type (1) and an outward folding type (2). In the inward folding type (1), the screen is located inside the folding direction, and in the outward folding type (2), the screen is located outside the folding direction. As a transparent substrate (200) for the cover window of these foldable display devices, for example, a polyimide-based film is used in the inward folding type, and an ultra-thin glass (UTG) is used in the outward folding type. In addition, a protective film (100) is also attached to the surface of the transparent substrate (200) to achieve the purpose of shock absorption, anti-scattering, and anti-scratch. Recently, attempts have been made to use polyester resins to prepare protective films for flexible display devices.

[0005] Furthermore, in recent years, various surface treatment technologies have been developed to minimize the reflectivity of a film attached to a cover of a display device to prevent visibility from being reduced due to reflection of external light from the front surface of the display device. Summary of the invention

[0006] Technical issues

[0007] In materials used in flexible display devices, it is as important as flexibility that the original properties do not deteriorate despite frequent bending or folding. When conventional materials are fully folded and then unfolded, traces are left behind, making it almost impossible to restore them to their original state. Therefore, the development of materials used in flexible display devices should be accompanied by research to overcome this limitation.

[0008] Specifically, due to Figure 4 The deformation caused by the compressive stress applied to the small inward fold point (a) in the inward folding type (1) shown, and the deformation caused by the compressive stress applied to the small inward fold point (a) as shown in FIG. Figure 5 The deformation caused by the tensile stress applied to the point (b) where the outward folding is large in the outward folding type (2) shown generates whitening or cracks in the protective film (100) and the like, which deteriorates its characteristics. Such whitening and cracks can usually be solved when the modulus of the protective film is small at room temperature. Polyester resins such as polyethylene terephthalate (PET) generally have a large modulus at room temperature, so they have a problem in that whitening and cracks are easily generated when applied to flexible display devices.

[0009] Meanwhile, a technique of forming various functional layers on the surface of a protective film is known in order to reduce the reflectivity of the protective film used in a display device. However, the newly formed functional layer generates optical interference with the existing film layer, which makes it difficult to control the reflectivity or impairs the optical properties. In addition, a polyester film made of, for example, polyethylene terephthalate has a very large birefringence. As a result, the polarization state between the polarizer and the liquid crystal is distorted, resulting in rainbow spots, etc., which seriously impairs visibility.

[0010] The present inventors have conducted research to solve these problems and found that the multilayer structure can be designed in combination with the refractive index and the modulus of the film can be adjusted to improve its optical and mechanical properties.

[0011] Therefore, the present invention is directed to providing a protective film that does not cause whitening and cracks even if frequently bent or folded while achieving very low reflectivity through a multilayer structure with a controlled refractive index, and a laminate and a flexible display device including the same.

[0012] Solution to the problem

[0013] According to one embodiment, a protective film is provided, comprising: a base layer comprising a polyester resin; and a primer layer disposed on the base layer, wherein the protective film has a reflectivity of 5% or less for light having a wavelength of 550 nm, until a whitening strain occurs in the base layer of 10% or greater, and satisfies the following relationship (1).

[0014] 0.7≤n2 / n1≤1.0(1)

[0015] In the above relationship, n1 and n2 refer to the refractive indices of the base layer and primer layer, respectively.

[0016] According to another embodiment, a laminate is provided, which includes a transparent cover for a flexible display device; and a protective film, the protective film including a base layer disposed on the transparent cover and containing a polyester resin; and a primer layer disposed on the base layer, wherein the protective film has a reflectivity of 5% or less for light with a wavelength of 550nm, until the strain of whitening occurring in the transparent cover and the base layer is 10% or greater, and the following relationship (1) is satisfied.

[0017] 0.7≤n2 / n1≤1.0(1)

[0018] In the above relationship, n1 and n2 refer to the refractive indices of the base layer and primer layer, respectively.

[0019] Advantageous Effects of the Invention

[0020] The protective film according to the embodiment, in which the refractive indices between the constituent layers are combined, has a very low reflectivity so that its visibility is not impaired by external light.

[0021] Furthermore, in the protective film according to this embodiment, the modulus at room temperature is controlled so that whitening and cracking can be significantly suppressed despite frequent bending or folding.

[0022] With these features, the protective film can be applied to flexible display devices, especially covers of foldable display devices, to exhibit excellent optical and mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A cross-sectional view of a protective film having a base layer and a primer layer is shown.

[0024] Figure 2 A cross-sectional view of a protective film having a base layer, a primer layer, a hard coat layer, a first refractive layer, and a second refractive layer is shown.

[0025] Figure 3 A cross-sectional view of a laminate having a transparent cover and a protective film for a flexible display device is shown.

[0026] Figure 4 A cross-sectional view of an inward-folding type flexible display device is shown.

[0027] Figure 5 A cross-sectional view of an outward-folding type flexible display device is shown.

[0028] Description of Reference Numerals

[0029] 1: Inward folding flexible display device

[0030] 2: Outward folding flexible display device

[0031] 10: Laminated body 100: Protective film

[0032] 110: Base layer 102: Primer layer

[0033] 130: Hard coating layer 140: First refractive layer

[0034] 150: Second refractive layer 200: Transparent cover

[0035] 300: body of the flexible display device a, b: folding point DETAILED DESCRIPTION

[0036] Throughout the description of the embodiments, when mentioning that each film, panel or layer is formed “on” or “under” another film, panel or layer, this means not only that one element is directly formed on or under another element, but also that one element is indirectly formed on or under another element with other elements interposed therebetween.

[0037] Furthermore, the size of each element in the drawings may be exaggeratedly depicted for the purpose of description and does not indicate the actual size.

[0038] Furthermore, unless otherwise indicated, all numbers expressing physical properties, dimensions, and so forth of components used herein are to be understood as being modified by the term "about."

[0039]

Protective film

[0040] Figure 1 is a cross-sectional view of a protective film ( 100 ) according to an embodiment, and shows the protective film including a base layer ( 110 ) and a primer layer ( 120 ).

[0041] The protective film may further include one or more additional coating layers in addition to the primer layer (120) on the base layer (110).

[0042] For example, the protective film may further include a functional coating for hardness enhancement, antistatic, anti-scattering, refractive index adjustment, surface protection, and the like.

[0043] refer to Figure 2 , the protective film (100) may further include a hard coating layer (130), a first refractive layer (140), and a second refractive layer (150) disposed on the primer layer (120).

[0044] In addition, the protective film (100) may further include a coating having an antistatic or anti-glare function disposed on the primer layer (120).

[0045] [Refractive index and thickness of each layer]

[0046] The polyester film according to the embodiment satisfies the following relationship (1).

[0047] 0.7≤n2 / n1≤1.0(1)

[0048] In the above relationship, n1 and n2 refer to the refractive indices of the base layer and primer layer, respectively.

[0049] Specifically, the ratio of n2 / n1 may be 0.8 to 1.0, 0.9 to 1.0, 0.7 to 0.9, 0.7 to 0.8, or 0.8 to 0.9. Alternatively, the ratio of n2 / n1 may be 0.7 or more to less than 1.0, 0.8 or more to less than 1.0, or 0.9 to less than 1.0.

[0050] According to another embodiment, the protective film may further satisfy the following relationship (2).

[0051] 0≤n1–n2≤0.10(2)

[0052] In the above relationship, n1 and n2 refer to the refractive indices of the base layer and primer layer, respectively.

[0053] According to still another embodiment, the protective film may further include a hard coating layer disposed on the primer layer and further satisfy the following relational expressions (3) to (5).

[0054] n3 <n2<n1(3)

[0055] 0.05≤n1–n3≤0.20(4)

[0056] 0 <n2–n3≤0.13(5)

[0057] In the above relationship, n1, n2, and n3 are the refractive indices of the base layer, primer layer, and hard coating layer, respectively.

[0058] The refractive index (n1) of the base layer may be 1.61 to 1.69. Alternatively, the refractive index (n1) of the base layer may be 1.63 to 1.68, 1.63 to 1.67, or 1.63 to 1.66.

[0059] The refractive index (n2) of the primer layer may be 1.50 to 1.68. Alternatively, the refractive index (n2) of the bottom layer may be 1.54 to 1.63, 1.54 to 1.58, 1.54 to 1.62, 1.55 to 1.61, or 1.58 to 1.63.

[0060] The refractive index (n3) of the hard coating layer may be 1.40 to 1.67. Alternatively, the refractive index (n3) of the hard coating layer may be 1.45 to 1.60 or 1.50 to 1.53.

[0061] The base layer may have a thickness of 10 μm to 100 μm. Alternatively, the base layer may have a thickness of 10 μm to 80 μm, 20 μm to 60 μm, or 40 μm to 60 μm.

[0062] The primer layer may have a thickness of 10 nm to 200 nm. Alternatively, the primer layer may have a thickness of 50 nm to 120 nm, 80 nm to 95 nm, 80 nm to 90 nm, or 80 nm to 85 nm.

[0063] The hard coating layer may have a thickness of 0.5 μm to 100 μm. Alternatively, the hard coating layer may have a thickness of 1 μm to 10 μm, 1 μm to 8 μm, 1 μm to 5 μm, or 1.5 μm to 3.5 μm.

[0064] According to a specific example, the base layer may have a refractive index (n1) of 1.61 to 1.69 and a thickness of 20 μm to 60 μm, the primer layer may have a refractive index (n2) of 1.54 to 1.63 and a thickness of 20 nm to 120 nm, and the hard coating layer may have a refractive index (n3) of 1.50 to 1.53 and a thickness of 1 μm to 5 μm.

[0065] Within the above-mentioned preferred range, when it is applied to an optical component of a display device or the like, it may be more advantageous for suppressing the occurrence of rainbow spots and enhancing visibility.

[0066] According to yet another embodiment, the protective film may further include a first refractive layer disposed on the hard coating layer and a second refractive layer disposed on the first refractive layer, and further satisfy the following relational expressions (6) and (7).

[0067] n5 <n3<n2<n1<n4(6)

[0068] 0.25≤n4–n5≤0.60(7)

[0069] In the above relationship, n1, n2, n3, n4, and n5 represent the refractive indices of the base layer, the primer layer, the hard coating layer, the first refractive layer, and the second refractive layer, respectively.

[0070] The refractive index (n4) of the first refractive layer may be 1.60 to 1.80, 1.60 to 1.75, 1.70 to 1.80, or 1.70 to 1.75.

[0071] In addition, the refractive index (n5) of the second refractive layer may be 1.10 to 1.40 or 1.25 to 1.35.

[0072] In addition, the refractive index difference (n1-n3) between the base layer and the hard coating layer may be 0.05 to 0.20, 0.09 to 0.16, 0.05 to 0.15, or 0.10 to 0.20.

[0073] In addition, a refractive index difference (n4-n5) between the first refractive layer and the second refractive layer may be 0.30 to 0.60, 0.40 to 0.50, 0.30 to 0.50, or 0.40 to 0.60.

[0074] The thickness of the first refractive layer may be 5 nm to 800 nm, 60 nm to 800 nm, 30 nm to 500 nm, 50 nm to 300 nm, 50 nm to 200 nm, 5 nm to 150 nm, 10 nm to 130 nm, or 20 nm to 100 nm.

[0075] The second refractive layer may have a thickness of 10 nm to 1 μm, 80 nm to 120 nm, 80 nm to 100 nm, 90 nm to 110 nm, or 100 nm to 120 nm.

[0076] According to a more specific example, the base layer may have a refractive index (n1) of 1.61 to 1.69 and a thickness of 20 μm to 60 μm, the primer layer may have a refractive index (n2) of 1.54 to 1.63 and a thickness of 20 nm to 120 nm, the hard coating layer may have a refractive index (n3) of 1.50 to 1.53 and a thickness of 1 μm to 5 μm, the first refractive layer may have a refractive index (n4) of 1.60 to 1.75 and a thickness of 60 nm to 800 nm, and the second refractive layer may have a refractive index (n5) of 1.25 to 1.35 and a thickness of 80 nm to 120 nm.

[0077] Within the above preferred range, when applied to optical components of display devices and the like, the reflectivity can be low and other optical characteristics can be enhanced.

[0078] Properties of the base layer

[0079] The base layer may have a strain of 10% or more, 12% or more, 15% or more, or 20% or more until whitening occurs. Within the above range, it can be advantageously applied to a flexible display device because whitening does not occur even though the film is deformed when folded frequently. Strain refers to the ratio of the changed size of the film to the initial size. Here, the changed size may be an increased size or a decreased size.

[0080] The base layer can withstand repeated folding at an angle of 135° for 100 times or more, 1000 times or more, 10000 times or more, 50000 times or more, 100000 times or more, 150000 times or more, or 200000 times or more until it breaks. Within the above preferred range, it can be advantageously applied to a flexible display device because it does not break even when folded frequently.

[0081] In addition, the base layer may have an elongation retention of 80% or more, 85% or more, or 90% or more when treated at 1.2 atm and 120° C. for 72 hours.

[0082] In addition, the base layer may have a 10 g / m 2 Day to 100g / m 2 Day, 10g / m 2 Day to 50g / m 2 day or 10g / m 2 Day to 30g / m 2 moisture permeability.

[0083] In addition, the base layer may have a transmittance of 10% or less, 5% or less, or 3% or less at a wavelength of 380 nm.

[0084] The base layer may have a crystallinity of 35% to 55%. Within the above preferred range, excessive crystallization can be prevented while excellent mechanical properties are also excellent in terms of tensile strength and the like.

[0085] In addition, the base layer may have a surface hardness of 5B or more, 1H or more, 2H or more, or 3H or more, which may be a surface hardness when treated with a hard coating layer having a thickness of 2 μm to 4 μm.

[0086] Under the conditions of 85° C. and 24 hours, the base layer may have a ratio (s2 / s1) of a heat shrinkage rate in the second direction (s2) to a heat shrinkage rate in the first direction (s1) of 1 to 10. Specifically, the ratio (s2 / s1) of the heat shrinkage rate may be 1.4 to 9, 1.5 to 8, or 1.6 to 7.

[0087] In addition, s1 may be 1% or less, 0.8% or less, 0.6% or less, 0.4% or less, or 0.2% or less. For example, s1 may be 0% to 1.0%, 0% to 0.8%, 0% to 0.6%, 0% to 0.4%, or 0% to 0.2%.

[0088] In addition, s2 may be 3% or less, 2% or less, 1.5% or less, 1.2% or less, 1% or less, 0.8% or less, or 0.7% or less. For example, s2 may be 0.2% to 3%, 0.3% to 2%, 0.4% to 1%, or 0.5% to 0.8%.

[0089] Since the base layer has the above-mentioned heat shrinkage characteristics, it is possible that the rise and fall caused by shrinkage may not occur under high temperature conditions. Therefore, spots, flashes, interlayer delamination, cracks, etc. can be prevented in advance.

[0090] Modulus of the base layer

[0091] The base layer has a low level of modulus (m1, m2) in both a first direction and a second direction perpendicular to each other within a plane at 25°C.

[0092] In particular, the difference between m1 and m2 of the film is small (|m1–m2|) when the base layer is at 25°C. That is, the ratio m2 / m1 may be close to 1 at 25°C.

[0093] As such, the protective film can be advantageously applied to a flexible display device because whitening does not occur despite frequent bending or folding.

[0094] At 25°C, m1 and m2 may be 5 GPa or less, 4.5 GPa or less, 4 GPa or less, 3.5 GPa or less, or 3 GPa or less. Specifically, at 25°C, m1 and m2 may be 2 GPa to 5 GPa, 2.5 GPa to 5 GPa, 2 GPa to 4.5 GPa, or 2 GPa to 4 GPa.

[0095] In addition, at 25° C., the difference between m1 and m2 (|m1–m2|) may be 1.0 GPa or less. Specifically, at 25° C., the difference between m1 and m2 (|m1–m2|) may be 0.7 GPa or less, 0.5 GPa or less, 0.3 GPa or less, 0.2 GPa or less, or 0.1 GPa or less.

[0096] Furthermore, at 25°C, the ratio of m2 / m1 may be 0.7 to 1.3, 0.8 to 1.2, or 0.9 to 1.1.

[0097] For example, the first direction may be the transverse direction (TD) of the film, and the second direction may be the machine direction (MD) of the film.

[0098] In this case, at 25°C, the ratio of m2 / m1 may be 0.7 to 1.0, 0.8 to 1.0, 0.9 to 1.0, 0.7 to 0.9, 0.7 to 0.8 or 0.8 to 0.9.

[0099] Alternatively, at 25°C, the ratio of m2 / m1 may be 1.0 to 1.3, 1.0 to 1.2, 1.0 to 1.1, 1.1 to 1.3, 1.2 to 1.3, or 1.1 to 1.2.

[0100] Due to these characteristics, the protective film can be applied to flexible display devices, especially covers of foldable display devices, and can prevent the deterioration of characteristics caused by deformation at a small inward folding point in the inward folding type and deformation at a large outward folding point in the outward folding type.

[0101] Furthermore, at 85° C., m1 and m2 may be 3.5 GPa or less, 3.0 GPa or less, 2.5 GPa or less, 2.0 GPa or less, or 1.5 GPa or less.

[0102] Furthermore, the difference between m1 at 25°C and 85°C may be 1 GPa to 4 GPa, 2 GPa to 4 GPa, or 1 GPa to 3 GPa.

[0103] Furthermore, at 25°C and 85°C, the difference between m2 may be 1 GPa to 4 GPa, 2 GPa to 4 GPa, or 1 GPa to 3 GPa.

[0104] As a specific example, when defining a first direction and a second direction perpendicular to each other in a plane, the ratio (m2 / m1) of the modulus (m2) in the second direction to the modulus (m1) in the first direction of the base layer at 25°C is 0.7 to 1.3. At this time, at 25°C, m1 and m2 may be 4.5 GPa or less. In addition, at 85°C, both m1 and m2 may be 2.5 GPa or less.

[0105] Phase difference of the base layer

[0106] The base layer may have an in-plane phase difference (Ro) of 600 nm or less, 500 nm or less, 400 nm or less, 300 nm or less, or 200 nm or less. Within the above preferred range, the occurrence of rainbow spots can be minimized.

[0107] Meanwhile, the lower limit of the in-plane phase difference of the base layer may be 0 nm. Alternatively, in order to balance the optical characteristics and the mechanical characteristics, the lower limit of the in-plane phase difference (Ro) may be 10 nm or more, 30 nm or more, or 50 nm or more.

[0108] In addition, the base layer may have a thickness direction retardation (Rth) of 4,000 nm or more, 5,000 nm or more, or 5,500 nm or more.

[0109] The thickness direction phase difference can be a value measured based on a thickness of 40 μm to 50 μm. In the above preferred range, the molecular orientation degree is high, which promotes crystallization and is preferred from the viewpoint of mechanical properties. In addition, as the thickness direction phase difference (Rth) becomes larger, the ratio (Rth / Ro) of the thickness direction phase difference (Rth) to the in-plane phase difference (Ro) becomes larger, thereby effectively suppressing rainbow spots.

[0110] Meanwhile, considering the cost and thickness limitation for eliminating rainbow spots in the base layer, the upper limit of the thickness direction retardation (Rth) may be 16,000 nm or less, 15,000 nm or less, or 14,000 nm or less.

[0111] Among them, the in-plane retardation (Ro) is a parameter defined by the product (Δ) of the refractive index anisotropy of two mutually perpendicular axes on the film and the film thickness (d), which is a measure of the degree of optical isotropy and anisotropy.

[0112] In addition, the thickness direction retardation is a parameter defined by the product of the average value of two birefringences Δout (|nx thickness direction) and Δout (|ny direction) observed in the cross section in the film thickness direction and the film thickness (d).

[0113] In addition, the ratio (Rth / Ro) of the thickness direction phase difference (Rth) to the in-plane phase difference (Ro) of the base layer may be 10 or more, 15 or more, or 20 or more. The smaller the in-plane phase difference (Ro), the larger the thickness direction phase difference (Rth), which is more conducive to preventing rainbow spots. Therefore, it is preferred that the ratio (Rth / Ro) of the two values ​​be kept large.

[0114] The composition of the base

[0115] The base layer comprises a polyester resin.

[0116] The polyester resin may be a homopolymer resin or a copolymer resin obtained by polycondensation of dicarboxylic acid and diol. In addition, the polyester resin may be a mixed resin obtained by mixing homopolymer resins or copolymer resins.

[0117] Examples of dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, 2,5-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, 1,4-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, dibenzoic acid, diphenoxyethane dicarboxylic acid, dibenzenesulfonic acid, anthracene dicarboxylic acid, 1,3-cyclopentane dicarboxylic acid, 1,3-cyclohexane dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2,2-dimethylglutaric acid, adipic acid, 2-methyladipic acid, pimelic acid, azelaic acid, sebacic acid, suberic acid, dodecanedicarboxylic acid, and the like.

[0118] Furthermore, examples of the diol include ethylene glycol, propylene glycol, hexylene glycol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, dodecanol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)sulfone and the like.

[0119] Preferably, the polyester resin may be an aromatic polyester resin having excellent crystallinity. For example, it may have polyethylene terephthalate (PET) resin as a main component.

[0120] As an example, the base layer may include a polyester resin, specifically a PET resin, in an amount of at least about 85% by weight, more specifically at least 90% by weight, at least 95% by weight, or at least 99% by weight. As another example, the base layer may further include a polyester resin other than the PET resin. Specifically, the base layer may further include a polyethylene naphthalate (PEN) resin of up to about 15% by weight. More specifically, the base layer may further include a PEN resin in an amount of about 0.1% by weight to 10% by weight or about 0.1% by weight to 5% by weight.

[0121] Due to its high crystallinity and excellent mechanical properties, the base layer is preferably a stretched film. Specifically, the base layer can be a biaxially stretched polyester film. For example, it can be a film stretched with a stretch ratio of 2.0 to 5.0 in the longitudinal direction (MD) and the transverse direction (TD), respectively.

[0122] In addition, since the base layer includes polyester as a main component, the base layer may have increased crystallinity and enhanced mechanical properties such as tensile strength in a process of preparing the base layer by heating, stretching, etc.

[0123] Primer layer

[0124] The primer layer is used to enhance the adhesion between the base layer and other functional coatings.

[0125] The primer layer may be formed of a coating composition comprising a thermosetting resin. For example, the coating composition may comprise a polyester-based resin, a polyurethane-based resin, or a mixture thereof.

[0126] The primer layer can be formed from a composition obtained by mixing and dispersing a raw material resin and, as required, a photoinitiator and other additives in a solvent. Water can be preferably used as a solvent. Therefore, the composition for the primer coating can be prepared in the form of an aqueous coating liquid such as an aqueous solution, an aqueous dispersion or an emulsion. In addition, an organic solvent can be used in a small amount.

[0127] For example, the composition for the primer layer preferably has a solid content of 3% by weight to 20% by weight or 4% by weight to 10% by weight.

[0128] Hard coating

[0129] The hard coating is used to enhance the hardness of the protective film surface.

[0130] As an example, the one or more coating layers may include a hard coating layer having a thickness of 2 μm to 4 μm, and the protective film may have a surface hardness of 2H or higher.

[0131] The hard coating layer may contain a photocurable resin. Examples of the photocurable resin include compounds having one or two or more unsaturated bonds, such as acrylate-based compounds.

[0132] In addition, the composition for the hard coat layer may include a thermosetting resin. Examples of thermosetting resins include phenolic resins, urea resins, diallyl phthalate resins, melamine resins, guanamine resins, unsaturated polyester resins, polyurethane resins, epoxy resins, aminoalkyd resins, melamine-urea copolymer resins, polysiloxane resins, and the like.

[0133] The first refractive layer and the second refractive layer

[0134] The first refractive layer and the second refractive layer serve to reduce reflectivity by a combination of the refractive index and the lower coating layer.

[0135] The first refractive layer may include a resin containing metal oxide fine particles having a refractive index of 1.70 to 2.80.

[0136] The metal oxide particles can be selected from, for example, titanium oxide (TiO2), zirconium oxide (ZrO2), cerium oxide (CeO2), tin oxide (SnO2), antimony oxide (Sb2O5), zinc antimonate (ZnSb2O6), antimony tin oxide (ATO), indium tin oxide (ITO), phosphide tin oxide (PTO), aluminum zinc oxide (AZO), gallium zinc oxide (GZO) and combinations thereof.

[0137] The second refractive layer may include any of the following, for example, (i) a resin containing low-refractive index inorganic particles, such as silicon dioxide or magnesium fluoride, (ii) a fluorine-based resin as a low-refractive index resin, (iii) a fluorine-based resin containing low-refractive index inorganic particles such as silicon dioxide, magnesium fluoride, and (iv) a low-refractive index inorganic material such as silicon dioxide, magnesium fluoride. Silicon dioxide is preferably hollow silicon dioxide particles.

[0138] The fluorine-based resin may be a polymerizable compound containing fluorine atoms at least in its molecule or a polymer thereof. In this case, the polymerizable compound is not particularly limited, but it preferably has a curable reactive group, such as a photocurable functional group, a thermosetting polar group, etc.

[0139] Examples of polymerizable compounds having photocurable functional groups and thermosetting polar groups include partially or fully fluorinated alkyl, alkenyl and aryl esters of acrylic or methacrylic acid, fully or partially fluorinated vinyl ethers, fully or partially fluorinated vinyl esters, fully or partially fluorinated vinyl ketones, and the like.

[0140] As a specific example, the base layer may include polyethylene terephthalate resin, the primer layer may include thermosetting polyurethane resin, the hard coating layer may include photocurable acrylate-based resin, the first refractive layer may include titanium oxide (TiO2) or zirconium oxide (ZrO2), and the second refractive layer may include silica particles.

[0141] Performance of protective film

[0142] The protective film according to the embodiment, in which the refractive indices between the constituent layers are combined, has a very low reflectivity with respect to visible light, so that its visibility is not impaired by external light.

[0143] That is, for light having a wavelength of 550 nm, the protective film may have a reflectivity of 5% or less, 4% or less, 3% or less, or 2% or less, more specifically, 0% to 5%, 3% to 5%, or 0% to 3%. Therefore, the protective film has almost no reflection of external light, thereby improving visibility.

[0144] Method for preparing protective film

[0145] The protective film may be prepared by a method comprising extruding a composition including a polyester resin to obtain an unstretched film; stretching the unstretched film in longitudinal and transverse directions; heat-setting the stretched film to prepare a base layer; and forming one or more coating layers on the base layer.

[0146] More specifically, the protective film can be prepared by a method including extruding a polyester resin to obtain an unstretched sheet; preheating the unstretched sheet at 70° C. to 90° C. and then stretching it at a longitudinal stretch ratio of 2.0 to 5.0 and a transverse stretch ratio of 2.0 to 5.0; heat-setting the stretched sheet at 150° C. to 250° C. to prepare a base layer; and sequentially forming a primer layer, a hard coating layer, a first refractive layer, and a second refractive layer on the base layer.

[0147] In addition, the protective film may further include a coating having an antistatic or antiglare function disposed on the primer layer as an additional functional layer.

[0148]

Laminate

[0149] Figure 3 A cross-sectional view of a laminate having a transparent cover and a protective film for a flexible display device is shown.

[0150] Reference Figure 3 , a laminate (10) according to an embodiment includes a transparent cover (200) for a flexible display device; and a protective film (100) including a base layer disposed on the transparent cover (200) and including a polyester resin and a primer layer disposed on the base layer.

[0151] The protective film in the laminate has substantially the same configuration and performance as those of the protective film according to the above-described embodiment.

[0152] Transparent cover

[0153] The transparent cover may be a cover window of the flexible display device.

[0154] The transparent cover may be a polymer film or a glass substrate. Specifically, the transparent cover may be a polyimide-based film or ultra-thin glass (UTG).

[0155] For example, the transparent cover may include polyimide resin.

[0156] The transparent cover may have a surface hardness of HB or more and a light transmittance of 80% or more at a wavelength of 550 nm. In addition, the transparent cover may have a yellowness index of 5 or less and a haze of 2% or less based on a thickness of 50 μm.

[0157] The transparent cover may have a strain of 10% or more, 12% or more, 15% or more, or 20% or more until whitening occurs. Within the above range, it can be advantageously applied to flexible display devices because whitening does not occur despite deformation during frequent folding. Strain refers to the ratio of the size of the film change to the initial size. Here, the changed size may be an increased size or a decreased size. Specifically, the strains of the protective film and the transparent cover before whitening occur may be 10% or more, respectively.

[0158] The transparent cover can withstand repeated folding at an angle of 135° for 100 times or more, 1000 times or more, 10000 times or more, 50000 times or more, 100000 times or more, 150000 times or more, or 200000 times or more until it breaks. Within the above range, it can be advantageously applied to a flexible display device because it does not break even when folded frequently.

[0159] When a first direction and a second direction perpendicular to each other within a plane are defined, the laminate may satisfy the following relations (11) to (13) at 25°C.

[0160] 0.7≤m2 / m1≤1.3(11)

[0161] 0.5≤m1 / m3≤0.9(12)

[0162] 0.4≤m2 / m4≤0.9(13)

[0163] In the above relationship, m1 and m2 are the moduli of the base layer in the first direction and the second direction respectively, and m3 and m4 are the moduli of the transparent cover in the first direction and the second direction respectively.

[0164] Furthermore, in a plane at 25° C., the transparent cover has a low level of modulus (m3, m4) in first and second directions perpendicular to each other.

[0165] In particular, the transparent cover has a small difference (|m3–m4|) between m3 and m4 of the film at 25°C. That is, the m4 / m3 ratio may be close to 1 at 25°C.

[0166] Therefore, the transparent cover can be advantageously applied to a flexible display device because whitening or cracks do not occur even if it is frequently bent or folded.

[0167] At 25° C., m3 and m4 may be 10 GPa or less, 9.5 GPa or less, 9 GPa or less, 8.5 GPa or less, or 8 GPa or less. Specifically, at 25° C., m3 and m4 may be 5 GPa to 10 GPa, 5 GPa to 9.5 GPa, 5 GPa to 8.5 GPa, or 5 GPa to 8 GPa.

[0168] In addition, the difference between m3 and m4 (|m3–m4|) may be 2.0 GPa or less at 25° C. Specifically, the difference between m3 and m4 (|m3–m4|) may be 1.5 GPa or less, 1.0 GPa or less, 0.5 GPa or less, 0.2 GPa or less, or 0.1 GPa or less at 25° C.

[0169] Furthermore, at 25°C, the ratio of m4 / m3 may be 0.7 to 1.3, 0.8 to 1.2, or 0.9 to 1.1.

[0170] For example, the first direction may be the transverse direction (TD) of the film, and the second direction may be the machine direction (MD) of the film.

[0171] In this case, the ratio of m4 / m3 at 25°C may be 0.7 to 1.0, 0.8 to 1.0, 0.9 to 1.0, 0.7 to 0.9, 0.7 to 0.8 or 0.8 to 0.9.

[0172] Alternatively, at 25°C, the ratio of m4 / m3 may be 1.0 to 1.3, 1.0 to 1.2, 1.0 to 1.1, 1.1 to 1.3, 1.2 to 1.3, or 1.1 to 1.2.

[0173] Furthermore, the moduli (m1, m2) of the protective film in the first direction and the second direction and the moduli (m3, m4) of the transparent cover in the first direction and the second direction satisfy the above-mentioned relations (12) and (13).

[0174] Specifically, the ratio of m1 / m3 may be 0.5 to 0.9, 0.5 to 0.8, 0.5 to 0.7, 0.6 to 0.9, 0.7 to 0.9 or 0.6 to 0.8.

[0175] Furthermore, the ratio of m2 / m4 may be 0.4 to 0.9, 0.4 to 0.8, 0.4 to 0.7, 0.5 to 0.9, 0.6 to 0.9, or 0.5 to 0.8.

[0176] In addition, m1 and m2 may be 2.5 GPa to 5 GPa at 25° C., respectively, and m3 and m4 may be 5 GPa to 10 GPa at 25° C., respectively.

[0177] Due to these characteristics, the laminate can be applied to flexible display devices, especially covers of foldable display devices, and can prevent the deterioration of characteristics caused by deformation at a small inward folding point in the inward folding type and deformation at a large outward folding point in the outward folding type.

[0178] Additional functional layers

[0179] The laminate (20) may further include one or more functional coating layers formed between the protective film (100) and the transparent cover (200).

[0180] For example, the one or more functional coatings may be a layer for enhancing adhesion between the protective film and the transparent cover.

[0181]

Display device

[0182] A display device according to an embodiment includes a display panel; and a protection film disposed at one side of the display panel according to an embodiment.

[0183] A display apparatus according to another embodiment includes a display panel; and the laminate according to the embodiment disposed on one side of the display panel.

[0184] The display device may be a flexible displacement device, in particular a foldable display device.

[0185] Specifically, the foldable display device may be an inward-folding type or an outward-folding type according to a folding direction.

[0186] The protective film or laminate in the displacement device has substantially the same configuration and properties as those of the protective film or laminate according to the above-described embodiment.

[0187] Specifically, since the protective film or laminate has a small difference between the moduli in two directions perpendicular to each other in a plane, the whitening phenomenon can be significantly suppressed even when folded frequently. In addition, since the protective film or laminate is excellent in other mechanical properties, thermal properties and durability, it can be used as a protective film to mitigate impact, prevent scattering and prevent scratches.

[0188] Mode for Carrying Out the Invention

[0189] Hereinafter, the present invention will be described in detail with reference to Examples. However, the following Examples are intended to illustrate the present invention, and the scope of the present invention is not limited thereto.

[0190] Preparation and Evaluation of Polyester Films (Examples A1 to A4, Comparative Examples A1 and A2)

[0191] Polyethylene terephthalate (PET) resin (SKC) is extruded by an extruder and cast onto a casting roll to prepare an unstretched sheet. The unstretched sheet is stretched in the machine direction (MD) and the transverse direction (TD). Thereafter, the stretched sheet is heat-set, annealed, and cooled to prepare a polyester film. Here, relaxation is performed in two stages while lowering the temperature in the annealing step.

[0192] The specific process conditions are summarized in Table 1 below.

[0193]

Table 1

[0194]

[0195] The polyester film obtained above was subjected to the following tests. The results are shown in Table 2 below.

[0196] (1) Modulus measurement

[0197] Young's modulus was measured in the longitudinal and transverse directions of the film sample at 25° C. and 85° C., respectively. The results are shown in the following Table 2. Specifically, the film sample was cut into 1.5 cm width, mounted on a universal testing machine (UTM, 5566A, Instron), and the stress was measured while increasing the length to obtain the modulus.

[0198] (2) MIT folding test

[0199] According to ASTM D2176 and TAPPIT 511, the film samples were subjected to MIT folding test using a folding resistance tester (MIT-DA, Toyoseiki). Specifically, the MIT folding test was carried out under the conditions of a folding angle of 135°, a sample width of 10 mm, a load of 500 gf, a curvature radius of 0.38R, and a folding speed of 175 times / min. The results were classified according to the following criteria and are shown in Table 2 below.

[0200] -○: The number of repeated foldings is 100,000 times or more until cracks or breaks occur.

[0201] -×: Folding is repeated less than 100,000 times until cracks or breaks occur.

[0202] (3) Crack strain test

[0203] The minimum elongation of each film sample was measured using a universal testing machine (UTM, 5566A, Instron) at 1% stretch until whitening occurred. The results were classified according to the following criteria and are shown in Table 2 below.

[0204] -○: Even at an elongation of 10% or more, whitening does not occur.

[0205] -×: Whitening occurs when the elongation is less than 10%.

[0206]

Table 2

[0207]

[0208] As shown in Table 2 above, since the polyester films of Examples A1 to A4, in which the longitudinal modulus and the transverse modulus were adjusted to the preferred ranges, passed the folding test and the whitening test, they were suitable as protective films for covers of flexible display devices.

[0209] In contrast, since the polyester films of Comparative Examples A1 and A2, in which the longitudinal modulus and the transverse modulus were outside the preferred ranges, failed the folding test and the whitening test, they were not suitable as protective films for covers of flexible display devices.

[0210] Preparation and evaluation of protective films (Examples B1 to B6, Comparative Examples B1 and B2)

[0211] Polyethylene terephthalate (PET) resin (SKC) is extruded by an extruder and cast onto a casting roll to prepare an unstretched sheet. The unstretched sheet is stretched in the machine direction (MD), and a primer layer is formed on the unstretched sheet. Here, a thermosetting polyurethane resin composition is applied using a Meyer rod and dried to form a primer layer.

[0212] Thereafter, it is stretched in the transverse direction (TD), and the stretched sheet is heat-set, annealed, and cooled to prepare a polyester film. Here, relaxation is performed in two stages while lowering the temperature in the annealing step.

[0213] The specific process conditions are summarized in Table 3 below.

[0214]

Table 3

[0215]

[0216] The protective film thus prepared was tested as follows.

[0217] (1) Refractive index and in-plane phase difference

[0218] The refractive index ((nx+ny) / 2), in-plane phase difference (Ro), thickness direction phase difference (Rth) and in-plane phase difference (|ΔRo| / |Δx|) of the film samples within the effective width range were measured.

[0219] First, the orientation axis direction of the film sample is obtained with two polarizers. The sample is cut into 4 cm, cm rectangles to make the orientation axis direction orthogonal. The in-plane phase difference (Ro) and thickness direction phase difference (Rth) are measured at the center of the width using a phase difference meter (axis scanning, axonometric, measurement wavelength: 550nm). In addition, the refractive index, which is the basic data of the phase difference meter, is measured at room temperature by an Abbe refractometer (NAR-4T, Atago Co., Ltd., measurement wavelength: 589.3nm); and the thickness d (μm) is measured with an electronic micrometer (Millitron 1245D, Feinpruf GmbH).

[0220] (2) Total transmittance

[0221] The total transmittance and haze were measured using a haze meter (NDH-5000W, Nippon Denkoku Kogyo).

[0222] (3) Interference speckles

[0223] A black tape was attached to the back of the film sample, and then observed with the naked eye to check whether interference spots appeared.

[0224] -None: No rainbow spots and colors were observed in any direction.

[0225] - Slightly Visible: Slight interference spots are observed in certain directions.

[0226] - Strongly visible: Strong interference spots are observed.

[0227]

Table 4

[0228]

[0229] As shown in the above Table 4, the protective film according to the example has excellent transmittance, and no interference spots are observed. In contrast, the protective film according to the comparative example shows interference spots.

[0230] Preparation and evaluation of protective film (Example C1)

[0231] Step (1): Formation of primer layer

[0232] Under the process conditions of Example B3, a primer layer was formed on the polyester base layer of Example A1.

[0233] Step (2): Formation of hard coating

[0234] Pentaerythritol triacrylate (PETA) was dissolved in a methyl isobutyl ketone (MIBK) solvent at a concentration of 30% by weight, and then a photoinitiator (Irg184, BASF) was added thereto at a concentration of 5% by weight based on the solid content, thereby preparing a composition for a hard coating layer. The composition for a hard coating layer was coated on the primer layer by a rod coater to form a coating layer so that the layer thickness after drying was 5 μm. The coating film thus formed was dried at 80° C. for 1 minute to remove the solvent, and was subjected to a 300 mJ / cm 2 The ultraviolet rays are irradiated to cure to form a hard coating.

[0235] Step (3): Formation of the first refractive layer

[0236] A first refractive layer is formed on the hard coating layer from a composition comprising a multifunctional acrylate resin and titanium dioxide (TiO2). After the composition is coated on the hard coating layer, it is dried at 80°C for 1 minute and irradiated with about 650 mJ / cm 2 The composition has a solid content of about 40% by weight and a viscosity of about 6.5 mPa·s at 25° C.

[0237] Step (4): Formation of the second refractive layer

[0238] The second refractive layer is formed on the first refractive layer from a resin containing silica particles.

[0239] The refractive index and thickness of each protective film (Example C1) thus prepared are summarized in Table 5 below.

[0240]

Table 5

[0241] distinguish Refractive Index thickness Grassroots 1.66 40μm Primer layer 1.60 85nm Hard coating 1.52 2.5μm First refractive layer 1.75 60nm Second refractive layer 1.29 100nm

[0242] The protective film thus prepared was tested as follows.

[0243] (1) Reflectivity measurement

[0244] The reflectivity of the protective film prepared in Example C1 was measured.

[0245] A black tape (Nitto) was attached to glass having almost no light absorption, and the optical multilayer film sample was attached thereto. The reflectance was measured using a spectrophotometer (U-4100, Hitachi, Ltd.).

[0246] As a result, the reflectance of light having a wavelength of 550 nm was measured to be 5% or less.

[0247] Preparation and evaluation of laminates

[0248] Primer layers were formed on the polyester films of Examples A1 to A4 and Comparative Examples A1 and A2 in the same manner as described above to obtain protective films, respectively.

[0249] The protective film thus prepared was laminated with a polyimide film (TPI, SKC) via an optically clear adhesive (OCA8146-x, 3M) to prepare a laminate, wherein the modulus of the polyimide film in the longitudinal and transverse directions at about 25° C. was about 6.5 GPa, respectively.

[0250] In addition, each laminate was subjected to folding evaluation and whitening evaluation in the same manner as above. As a result, the laminate comprising each polyester film of Examples A1 to A4 passed the folding test and the whitening test. On the contrary, the laminate comprising the polyester film of Comparative Examples A1 and A2 did not pass the folding test and the whitening test.

Claims

1. A protective film comprising a base layer comprising a polyester resin; and a primer layer disposed on the base layer, It is characterized in that The protective film has a reflectivity of 5% or less for light having a wavelength of 550 nm, a strain until whitening occurs in the base layer is 10% or more, and satisfies the following relationship (1): 0.7≤n2 / n1≤1.0(1) In the above relationship, n1 and n2 are the refractive indices of the base layer and the primer layer, respectively.

2. The protective film according to claim 1, characterized in that: When a first direction and a second direction perpendicular to each other in a plane are defined, when the base layer is repeatedly folded 100 times or more at an angle of 135° at 25°C until it breaks, the ratio (m2 / m1) of the modulus (m2) in the second direction to the modulus (m1) in the first direction is 0.7 to 1.3, and the base layer has an in-plane phase difference (Ro) of 600nm or less and a thickness direction phase difference (Rth) of 4000nm or more.

3. The protective film according to claim 2, characterized in that: Both the m1 and the m2 are 4.5 GPa or less at 25°C.

4. The protective film according to claim 1, characterized in that: The protective film also satisfies the following relationship (2): 0≤n1–n2≤0.10(2) In the above relationship, n1 and n2 are the refractive indices of the base layer and the primer layer, respectively.

5. The protective film according to claim 1, characterized in that: The protective film further includes a hard coating layer disposed on the primer layer, and further satisfies the following relations (3) to (5): n3 <n2<n1 (3) 0.05≤n1–n3≤0.20 (4) 0 <n2–n3≤0.13 (5) In the above relationship, n1, n2, and n3 are the refractive indices of the base layer, the primer layer, and the hard coating layer, respectively.

6. The protective film according to claim 5, characterized in that: The refractive index (n1) of the base layer is 1.61 to 1.69 and the thickness is 20 μm to 60 μm, the refractive index (n2) of the primer layer is 1.54 to 1.63 and the thickness is 20 nm to 120 nm, and the refractive index (n3) of the hard coating layer is 1.50 to 1.53 and the thickness is 1 μm to 5 μm.

7. The protective film according to claim 5, characterized in that: The protective film further includes a first refractive layer disposed on the hard coating layer and a second refractive layer disposed on the first refractive layer, and further satisfies the following relations (6) and (7): n5 <n3<n2<n1<n4 (6) 0.25≤n4–n5≤0.60 (7) In the above relationship, n1, n2, n3, n4, and n5 are the refractive indices of the base layer, the primer layer, the hard coating layer, the first refractive layer, and the second refractive layer, respectively.

8. The protective film according to claim 7, characterized in that: The refractive index (n1) of the base layer is 1.61 to 1.69, and the thickness is 20 μm to 60 μm, the refractive index (n2) of the primer layer is 1.54 to 1.63, and the thickness is 20 nm to 120 nm, the refractive index (n3) of the hard coating layer is 1.50 to 1.53, and the thickness is 1 μm to 5 μm, the refractive index (n4) of the first refractive layer is 1.60 to 1.75, and the thickness is 60 nm to 800 nm, and the second refractive layer has a refractive index (n5) of 1.25 to 1.35 and a thickness of 80 nm to 120 nm.

9. A laminate comprising a transparent cover for a flexible display device; and a protective film comprising a base layer disposed on the transparent cover and comprising a polyester resin; and a primer layer disposed on the base layer, It is characterized in that The reflectivity of the protective film is 5% or less for light having a wavelength of 550 nm, the strain until whitening occurs in the transparent cover and the base layer is 10% or more, and the following relationship (1) is satisfied: 0.7≤n2 / n1≤1.0(1) In the above relationship, n1 and n2 are the refractive indices of the base layer and the primer layer, respectively.

10. The laminate according to claim 9, characterized in that When a first direction and a second direction perpendicular to each other in a plane are defined, the following relations (11) to (13) are satisfied at 25°C: 0.7≤m2 / m1≤1.3(11) 0.5≤m1 / m3≤0.9(12) 0.4≤m2 / m4≤0.9(13) In the above relationship, m1 and m2 are the moduli of the base layer in the first direction and the second direction, respectively, and m3 and m4 are the moduli of the transparent cover in the first direction and the second direction, respectively.

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