KR20200053233A - Polyester protective film for flexible display device - Google Patents

Abstract

Since a protective film according to an embodiment of the present invention has a small modulus difference at a room temperature in two directions perpendicular to in-plane, whitening and cracks can be significantly suppressed despite frequent bending and folding. Due to the properties, the protective film can be applied to a flexible display device.

Description

Polyester protective film for flexible display device {POLYESTER PROTECTIVE FILM FOR FLEXIBLE DISPLAY DEVICE}

The following embodiments relate to a polyester protective film for a flexible display device. More specifically, embodiments relate to a protective film having a modulus controlled at room temperature, a laminate including the same, and a flexible display device.

2. Description of the Related Art Display technologies are evolving again and again due to demands due to the development of IT devices, and technologies such as curved displays and bended displays have already been commercialized. 2. Description of the Related Art Recently, in the field of mobile devices that require large screens and portability at the same time, flexible display devices that can be flexibly bent or folded according to external force are preferred. In particular, it is a great advantage that a foldable display device can be folded to be small when not in use to increase portability, and to be widely spread when used to realize a large screen.

1A and 1B, such a foldable display device includes an in-folding type (1) in which a screen is positioned inside a folding direction, and an out in which a screen is positioned outside a folding direction. It is being developed as an out-folding type (2). As the transparent substrate 200 applied as a cover window of these foldable display devices, for example, a polyimide-based film is used in the in-folding type, and ultra-thin glass (UTG) is used in the out-folding type. In addition, a protective film 100 is applied to the surface of the transparent substrate 200 for shock mitigation, scattering prevention, scratch prevention, and the like. Recently, attempts have been made to manufacture a protective film for a flexible display device using a polyester resin. .

Korean Patent Publication No. 2017-0109746

An important property of the material applied to the flexible display device is as important as the flexibility, so that there is no deterioration of the original property despite frequent bending or folding. Since the general material is completely folded and unfolded, it is almost impossible to return to its original shape, so the development of the material applied to the flexible display device must be accompanied by research to overcome this limitation.

Specifically, referring to FIG. 1A, in the case of the infolding type 1, the outfolding type 2 is caused by deformation due to compressive stress applied to a small folding point (a) inward, and also referring to FIG. 1B. In the case of whitening or cracks in the protective film 100, etc., due to deformation due to tensile stress occurring at the point (b) that is greatly folded outward, characteristics tend to deteriorate. Such whitening and cracks can generally be solved when the modulus of the protective film is low at room temperature, but polyester resins such as polyethylene terephthalate (PET) generally have a large modulus at room temperature, so whitening and cracking when applied to flexible display devices There is this easily occurring problem.

As a result of the research conducted by the present inventors to solve this problem, it was found that by reducing the modulus difference at room temperature in the longitudinal direction and the width direction of the polyester film, whitening and cracking can be significantly suppressed despite frequent warping or folding. .

Accordingly, it is intended to provide a protective film having a modulus controlled at room temperature through the following embodiments, a laminate and a display device including the same.

According to one embodiment, it comprises a polyester resin, applied to the cover of the flexible display device, strain until the whitening occurs (strain) is at least 10%, the first direction and the second perpendicular to each other in the plane of the film When defining the direction, a protective film is provided that satisfies the following formula (1) at 25 ° C:

0.7 ≤ m2 / m1 ≤ 1.3 (1)

In the above formula, m1 and m2 are modulus of the protective film for the first direction and the second direction, respectively.

According to another embodiment, the protective film comprising a polyester resin; And a transparent substrate disposed on the protective film and satisfying the following formulas (1) to (3) at 25 ° C. when defining the first and second directions perpendicular to each other in the plane of the film. Is provided:

0.7 ≤ m2 / m1 ≤ 1.3 (1)

0.5 ≤ m1 / m3 ≤ 0.9 (2)

0.4 ≤ m2 / m4 ≤ 0.9 (3)

In the above formula, m1 and m2 are the modulus of the protective film for the first direction and the second direction, respectively, and m3 and m4 are the modulus of the transparent substrate for the first direction and the second direction, respectively.

In the protective film according to the embodiment, since the difference in modulus at room temperature for two directions perpendicular to the plane is small, whitening and cracking can be significantly suppressed even during frequent bending or folding. Accordingly, even if the protective film includes a polyester resin having a relatively large modulus at room temperature, whitening and cracks can be suppressed. In addition, since the protective film is excellent in other mechanical properties, thermal properties, and durability, it can serve as a protective film for shock absorption, scattering prevention, and scratch prevention.

Due to these characteristics, the protective film may be applied to a flexible display device, and not only the deformation occurring at the point of small inward folding in the in-folding type, but also the deformation occurring at the point of large folding outward in the out-folding type, The original characteristics may not deteriorate.

1A and 1B are cross-sectional views of an in-folding and out-folding type flexible display device, respectively.
2 shows a cross-sectional view of a multilayer protective film according to one embodiment.
3 shows a cross-sectional view of a laminate according to one embodiment.

In the case where each film, panel, or layer described in the following embodiments is described as being formed “on” or “under” of each film, panel, or layer, the “phase ( "on" "and" under "include both directly or indirectly formed through other components.

In addition, the size of each component in the drawings may be exaggerated for explanation, and does not mean the size actually applied.

In addition, it should be understood that all numerical ranges representing physical property values, dimensions, and the like of the components described in the present specification are modified in all cases to the term "about" unless otherwise specified.

[Protective film]

The protective film according to an embodiment includes a polyester resin, is applied to the cover of the flexible display device, and has a strain of up to 10% or more until whitening occurs, and the first direction perpendicular to each other in the plane of the film And when defining the second direction, it is applied to the cover of the flexible display device, which satisfies the following formula (1) at 25 ° C.

0.7 ≤ m2 / m1 ≤ 1.3 (1)

In the above formula, m1 and m2 are modulus of the protective film for the first direction and the second direction, respectively.

Film composition

The protective film includes a polyester resin.

The polyester resin may be a homopolymer resin or a copolymer resin in which dicarboxylic acid and diol are polycondensed. In addition, the polyester resin may be a blend resin in which the homopolymer resin or copolymer resin is mixed.

Examples of the dicarboxylic acid are terephthalic acid, isophthalic acid, orthophthalic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5- Naphthalenedicarboxylic acid, diphenylcarboxylic acid, diphenoxyethanedicarboxylic acid, diphenylsulfonecarboxylic acid, anthracenedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,3-cyclo Hexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2 And 2-dimethylglutaric acid, adipic acid, 2-methyladipic acid trimethyladipic acid, pimelic acid, azelaic acid, sebacic acid, suberic acid, dodecadicarboxylic acid, and the like.

In addition, examples of the diol include ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexane dimethanol, 1,4-cyclohexane dimethanol, decamethylene glycol, 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.

Preferably, the polyester resin may be an aromatic polyester resin having excellent crystallinity, for example, polyethylene terephthalate (PET) resin as a main component.

As an example, the protective film may include at least about 85% by weight of a polyester resin, specifically a PET resin, and more specifically, at least 90% by weight, at least 95% by weight, or at least 99% by weight. As another example, the protective film may further include other polyester resin in addition to the PET resin. Specifically, the protective film may further include a polyethylene naphthalate (PEN) resin of about 15% by weight or less. More specifically, the protective film may further include about 0.1% to 10% by weight, or about 0.1% to 5% by weight of PEN resin.

The protective film is preferably a stretched film from the viewpoint of high crystallinity and excellent mechanical properties. Specifically, the protective film may be a biaxially stretched polyester film, and may be, for example, a film stretched at a stretching ratio of 2.0 to 5.0 in the longitudinal direction (MD) and the transverse direction (TD), respectively.

In addition, the protective film by using a polyester as a main component, the crystallinity increases during the manufacturing process through heating, stretching, etc., mechanical properties such as tensile strength can be improved.

Modulus

The protective film has low levels of modulus (m1, m2) in the first and second directions perpendicular to each other in the plane at 25 ° C.

In particular, the protective film has a small difference (| m1-m2 |) between the m1 and the m2 of the film at 25 ° C. That is, the m2 / m1 ratio may be close to 1 at 25 ° C.

Accordingly, the protective film is advantageous in application to a flexible display device since whitening does not occur even during frequent bending or folding.

The m1 and the 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 at 25 ° C. Specifically, m1 and m2 may be 2 GPa to 5 GPa, 2.5 GPa to 5 GPa, 2 GPa to 4.5 GPa, 2 GPa to 4.5 GPa, or 2 GPa to 4 GPa at 25 ° C.

In addition, the difference (| m1-m2 |) between m1 and m2 may be 1.0 GPa or less at 25 ° C. Specifically, the difference (| m1-m2 |) between m1 and 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 at 25 ° C.

In addition, the m2 / m1 ratio may be 0.7 to 1.3, 0.8 to 1.2, or 0.9 to 1.1 at 25 ° C.

As an example, the first direction may be the width direction (TD) of the film, and the second direction may be the longitudinal direction (MD) of the film.

In this case, the m2 / m1 ratio 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 at 25 ° C.

Alternatively, the m2 / m1 ratio 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 at 25 ° C.

Due to these characteristics, the protective film may be applied to a cover of a flexible display device, particularly a foldable display device, and not only a deformation occurring at a small folding point inward from the in-folding type, but also a large folding outward from the out-folding type Even by the deformation occurring at the point, the characteristics may not deteriorate.

In addition, the 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 at 85 ° C.

In addition, the m1 may have a difference of 1 GPa to 4 GPa, a difference of 2 GPa to 4 GPa, or a difference of 1 GPa to 3 GPa from each other at 25 ° C and 85 ° C.

In addition, the m2 may have a difference between 1 GPa and 4 GPa, a difference between 2 GPa and 4 GPa, or a difference between 1 GPa and 3 GPa at 25 ° C and 85 ° C.

Properties

The thickness of the protective film may be 10 μm to 100 μm, 20 μm to 60 μm, or 40 μm to 60 μm. As an example, the protective film may have a thickness of 20 μm to 60 μm.

The protective film may have a strain of 10% or more, 12% or more, 15% or more, or 20% or more until whitening occurs. When it is within the above range, whitening does not occur even in the deformation of the film due to frequent folding, and thus it is advantageous to apply to the flexible display device. The strain (strain) means the ratio of the changed dimension to the initial dimension of the film, wherein the changed dimension may be an increased dimension or a reduced dimension.

The protective film may be 100 or more times, 1000 or more times, 10,000 or more times, 10,000 or more times, 50,000 or more times, 100,000 or more times, 150,000 or more times, or 200,000 times or more of folding times at a 135 ° angle until breakage occurs. When it is within the above range, breakage does not occur even with frequent folding, so it is advantageous to apply to the flexible display device.

In addition, the protective film has a moisture permeability of 10 g / m 2 · day to 100 g / m 2 · day, 10 g / m 2 · day to 50 g / m 2 · day or 10 g / m 2 · day to 30 g / m 2 · day. Can be.

In addition, the protective film may have a transmittance of 10% or less, 5% or less, or 3% or less at a wavelength of 380 nm.

As an example, the protective film may have a moisture permeability of 10 g / m 2 · day to 50 g / m 2 · day, and a transmittance of 5% or less at a wavelength of 380 nm.

The protective film may have a crystallinity of 35% to 55%. When within the above range, mechanical properties such as tensile strength are excellent, but excessive crystallization can be prevented.

In addition, the protective film 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 during hard coating treatment of 2 to 4 μm in thickness.

The protective film may have a ratio (s2 / s1) of the heat shrinkage ratio (s2) in the second direction to the heat shrinkage ratio (s1) in the first direction at 85 ° C. and 24 hours. 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.

In addition, the 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%.

In addition, the 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, the s2 may be 0.2% to 3%, 0.3% to 2%, 0.4% to 1%, or 0.5% to 0.8%.

Since the protective film has the above-described heat-shrinking properties, lifting by shrinkage may not occur under high temperature conditions when applied as a protective film. Accordingly, it is possible to prevent wavy patterns, glitter phenomenon, interlayer peeling, and cracks.

Phase difference

The protective film may have an in-plane retardation (Ro) of 600 nm or less, 500 nm or less, 400 nm or less, 300 nm or less, or 200 nm or less. When within the above range, the occurrence of rainbow stains can be minimized.

In addition, the protective film may have a minimum in-plane retardation (Ro _min ) of 200 nm or less or 150 nm or less. Specifically, the minimum in-plane retardation of the protective film may be 120 nm or less, 100 nm or less, 85 nm or less, 75 nm or less, or 65 nm or less.

Meanwhile, the lower limit of the in-plane retardation of the protective film may be 0 nm, or the lower limit of the in-plane retardation (Ro) may be 10 nm or more, 30 nm or more, or 50 nm or more for balancing optical properties and mechanical properties. have.

In addition, the protective film may have a thickness direction retardation (Rth) of 4,000 or more, 5,000 nm or more, or 5,500 nm or more.

In addition, the protective film has a maximum thickness direction retardation (Rth _max ) of 6,000 nm or more, for example, 6,500 nm or more, for example, 7,500 nm or more, for example, 8,000 nm or more, for example, 8,500 nm or more Can be.

The thickness direction retardation may be a measurement value based on a thickness of 40 μm to 50 μm. When within the above range, the orientation degree of the molecule is large, and crystallization is promoted, which is preferable in terms of mechanical properties. In addition, since the ratio (Rth / Ro) of the thickness direction phase difference (Rth) to the in-plane phase difference (Ro) increases as the thickness direction retardation (Rth) increases, rainbow unevenness can be effectively suppressed.

On the other hand, considering the thickness limit and cost for removing rainbow stains from the protective film, 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.

Here, the in-plane retardation (Ro) is the product of the anisotropy (Δnxy = ｜ nx-ny ｜) of the refractive indexes of two orthogonal axes in the plane of the film and the film thickness (d) (△ nxy × d) As a parameter defined by, it is a measure showing optical isotropy or anisotropy. In addition, the minimum in-plane retardation (Ro _min ) means the lowest measured value when the in-plane retardation (Ro) is measured at multiple points in the plane of the film.

In addition, the thickness direction retardation (Rth) is a film of two birefringences Δnxz (= | nx-nz |) and △ nyz (= | ny-nz |), respectively, when viewed from a cross section in the film thickness direction. This parameter is defined as the average of the phase differences obtained by multiplying the thickness (d). In addition, the maximum thickness direction retardation (Rth _max ) means the highest measured value when the thickness direction retardation (Rth) is measured at multiple points in the plane of the film, respectively.

According to the film manufacturing process according to an embodiment, a minimum in-plane retardation and a maximum thickness direction retardation may appear in the center of the width of the film. Therefore, the minimum in-plane retardation of the film and the retardation in the maximum thickness direction may be values measured at the center of the width of the film. In this specification, the 'width center' may be defined as an intermediate point of the width of the film after stretching in the width direction (TD) and the length direction (MD). The film does not have only one center of width, and can be set to infinity depending on the measurement point. On the other hand, the width center of the final film cut into various forms after manufacture may not coincide with the width center of the first film (the film before cutting), and in this case, the minimum in-plane retardation and maximum thickness direction retardation from the width center of the film May not appear.

In addition, when the film is applied to an optical component for large-screen use, it is preferable that the deviation of the in-plane phase difference within the effective width (that is, the difference between the maximum value and the minimum value) is small. Here, the effective width refers to the distance between points that have moved a certain distance from the center of the width toward both ends along the width direction, for example, may be defined as ± 1,500 mm from the center of the width, that is, about 3,000 mm. On the other hand, as described above, the width center of the final film cut into various shapes after manufacture may not coincide with the width center of the initial film (the film before the foundation). In this case, the effective width may refer to the distance between the point where the minimum in-plane retardation appears in the film and moves a certain distance toward both ends along the width direction.

The protective film has little variation in in-plane retardation within the effective width. Specifically, the amount of change (| ΔRo | / | Δx |) of the in-plane retardation with respect to the displacement in the width direction of the protective film may be 0 to 100 nm / mm. Specifically, | ΔRo | / | Δx | Values can be 0-50 nm / mm, 0-35 nm / mm, 0-25 nm / mm, or 0-20 nm / mm. Here, the displacement in the width direction (Δx) means a distance (x ₂ -x ₁ ) between predetermined points on the width direction (x-axis), and the amount of change in the in-plane phase difference (ΔRo) is the difference in the in-plane phase difference at each constant point ( Ro ₂ -Ro ₁ ). When within the above range, even if the width of the film is wide, since the in-plane retardation (Ro) does not increase significantly, it is possible to effectively prevent the occurrence of rainbow stains.

In addition, the optical film has little variation in retardation in the thickness direction within the effective width. Specifically, in the optical film, the amount of change in the retardation in the thickness direction (| ΔRth | / | Δx |) with respect to displacement in the width direction may be less than 1000 nm / 3 m, less than 700 nm / 3 m, or less than 500 nm / 3 m. have. Here, the displacement (Δx) in the width direction means a distance (x ₂ -x ₁ ) between predetermined points on the width direction (x-axis), and a change amount (ΔRth) of the phase difference in the thickness direction phase difference is the thickness direction phase difference at each constant point. Means the difference (Rth ₂ -Rth ₁ ).

In addition, the protective film may have a ratio (Rth / Ro) of a thickness direction phase difference (Rth) to an in-plane phase difference (Ro) of 10 or more, 15 or more, or 20 or more. The smaller the in-plane retardation (Ro) and the larger the thickness direction retardation (Rth), the better it is to prevent the occurrence of rainbow stains, so it is preferable to keep the ratio (Rth / Ro) of both numerical values large. In particular, the protective film may have a ratio (Rth _max / Ro _min ) of a maximum thickness direction retardation (Rth _max ) to a minimum in-plane retardation (Ro _min ) of 30 or more, 40 or more, 50 or more, or 60 or more.

[Method of manufacturing protective film]

A method of manufacturing a protective film according to one embodiment includes: (1) extruding a composition comprising a polyester resin to obtain an unstretched film; (2) stretching the unstretched film in the length direction and the width direction; And (3) heat-setting the stretched film.

In the above manufacturing method, the protective film is prepared by extruding the raw material resin and preheating, stretching and heat setting.

At this time, the composition of the polyester resin used as a raw material for the protective film is as exemplified above.

In addition, the extrusion may be carried out at a temperature condition of 230 ℃ to 300 ℃, or 250 ℃ to 280 ℃.

The protective film is preheated at a constant temperature before stretching. The range of the preheating temperature satisfies the range of Tg + 5 ° C to Tg + 50 ° C based on the glass transition temperature (Tg) of the polyester resin, and at the same time, it is determined to be a range that satisfies the range of 70 ° C to 90 ° C. You can. When within the above range, it is possible to secure the flexibility that is easy for the protective film to be stretched, and at the same time, it is possible to effectively prevent the phenomenon of breaking during stretching.

The stretching is performed by biaxial stretching, and may be stretched biaxially in the width direction (tenter direction, TD) and in the longitudinal direction (machine direction, MD), for example, through simultaneous biaxial stretching or sequential biaxial stretching. Preferably, a sequential biaxial stretching method may be performed, first stretching in one direction and then stretching in a direction perpendicular to the direction.

The lengthwise stretching ratio is in the range of 2.0 to 5.0, and more specifically, may be in the range of 2.8 to 3.5. In addition, the stretching ratio in the width direction may be in the range of 2.0 to 5.0, and more specifically, in the range of 2.9 to 3.7. Preferably, the lengthwise stretching ratio d1 and the widthwise stretching ratio d2 are similar, and specifically, the ratio (d2 / d1) of the lengthwise stretching ratio d2 to the widthwise stretching ratio d1 is 0.5 to 1.0. , 0.7 to 1.0, or 0.9 to 1.0.

The stretching ratios d1 and d2 are ratios showing the length after stretching when the length before stretching is 1.0.

Further, the speed of the stretching may be 6.5 m / min to 8.5 m / min, but is not particularly limited.

The stretched sheet may be heat-set at 150 ° C to 250 ° C, more specifically 160 ° C to 230 ° C. The heat setting may be performed for 5 seconds to 1 minute, and more specifically, for 10 seconds to 45 minutes.

After starting the heat setting, the film may be relaxed in the longitudinal direction and / or the width direction, and the temperature range at this time may be 150 ° C to 250 ° C.

[Multi-layer protective film]

Referring to FIG. 2, the multilayer protective film 10 according to the embodiment includes the protective film 100 according to the embodiment, and the coating layer 110 disposed on the protective film 100.

In the multilayer protective film, the protective film has substantially the same configuration and properties as the protective film according to one embodiment described above.

The coating layer may be one or more, and may include a functional coating layer for improving hardness, antistatic, scattering prevention, refractive index control, surface protection, and the like.

In addition, the coating layer may have a composition suitable for each function, for example, may include an acrylic resin.

The protective film and the coating layer may prevent color development or transparency degradation due to optical interference by controlling the refractive index.

Specifically, the ratio (r2 / r1) of the refractive index (r2) of the coating layer to the refractive index (r1) of the protective film is 0.7 to 1.0, 0.7 to 0.9, 0.7 to 0.8, 0.8 to 1.0, 0.9 to 1.0, or 0.8 To 0.9.

Primer coating layer

As an example, the coating layer may include a primer coating layer. The primer coating layer serves to improve adhesion between the protective film and another functional coating layer.

The thickness of the primer layer may be 10 nm to 200 nm, 50 nm to 120 nm, 80 nm to 95 nm, 80 nm to 90 nm or 80 nm to 85 nm.

The primer coating layer may be formed from a coating composition comprising a thermosetting resin. For example, the coating composition may include a polyester-based resin, a polyurethane-based resin, or mixtures thereof.

The primer coating layer may be formed using a composition in which a raw material resin and a photopolymerization initiator and other additives are mixed and dispersed in a solvent, if necessary. Water may be preferably used as the solvent, and accordingly, the composition for the primer coating layer may be prepared in the form of an aqueous coating solution such as an aqueous solution, an aqueous dispersion or an emulsion. Moreover, you may use some organic solvent.

For example, the composition for the primer coating layer, the solid content in the composition is preferably 3% to 20% by weight, or 4% to 10% by weight.

Hard coating layer

As another example, the coating layer may include a hard coating layer. The hard coating layer serves to improve the hardness of the surface of the multilayer protective film.

The thickness of the hard coating layer may be 0.5 μm to 20 μm, 1 μm to 10 μm, 1 μm to 8 μm, 1 μm to 5 μm, or 1.5 μm to 3.5 μm.

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 multilayer protective film may have a surface hardness of 2H or more.

The hard coating layer may include a photo-curable resin. Examples of the photocurable resin include compounds having one or more unsaturated bonds, such as acrylate-based compounds.

In addition, the composition for the hard coating layer may include a thermosetting resin. Examples of the thermosetting resin, phenol resin, urea resin, diallyl phthalate resin, melamine resin, guanamine resin, unsaturated polyester resin, polyurethane resin, epoxy resin, amino alkyd resin, melamine-urea condensation resin, silicon resin, And polysiloxane resins.

[Laminate]

Referring to FIG. 3, the laminate 20 according to one embodiment includes a protective film 100 including a polyester resin, and a transparent substrate 200 disposed on the protective film 100, When defining the first and second directions perpendicular to each other in the plane of the film, the following equations (1) to (3) are satisfied at 25 ° C:

0.7 ≤ m2 / m1 ≤ 1.3 (1)

0.5 ≤ m1 / m3 ≤ 0.9 (2)

0.4 ≤ m2 / m4 ≤ 0.9 (3)

In the above formula, m1 and m2 are the modulus of the protective film for the first direction and the second direction, respectively, and m3 and m4 are the modulus of the transparent substrate for the first direction and the second direction, respectively.

Protective film

In the laminate, the protective film has substantially the same composition and physical properties as the protective film according to one embodiment described above.

For example, the modulus (m1, m2) for the first direction and the second direction of the protective film may be the same as the modulus value specifically exemplified in the protective film according to the embodiment.

Transparent substrate

The transparent substrate may be a cover window of the display device.

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

As an example, the transparent substrate may include a polyimide resin.

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

The transparent substrate may have a strain of 10% or more, 12% or more, 15% or more, or 20% or more until whitening occurs. When it is within the above range, whitening does not occur despite deformation due to frequent folding, and thus it is advantageous to apply to the flexible display device. The strain refers to a ratio of a changed dimension to an initial dimension of the film, where the changed dimension may be an increased dimension or a reduced dimension. In particular, the protective film and the transparent substrate may all have a strain of up to 10% until whitening occurs.

The transparent substrate may be 100 times or more, 1000 times or more, 10,000 times or more, 50,000 times or more, 100,000 times or more, 150,000 times or more, or 200,000 times or more of repeated folding at a 135 ° angle until breakage occurs. When it is within the above range, breakage does not occur even with frequent folding, so it is advantageous to apply to the flexible display device.

In addition, the transparent substrate has low levels of modulus (m3, m4) in the first and second directions perpendicular to each other in the plane at 25 ° C.

In particular, the transparent substrate has a small difference (| m3-m4 |) between m3 and m4 at 25 ° C. That is, the m4 / m3 ratio may be close to 1 at 25 ° C.

Accordingly, the transparent substrate is advantageous in application to a flexible display device since whitening and cracking do not occur even during frequent bending or folding.

The m3 and the 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 at 25 ° C. Specifically, 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, all at 25 ° C.

Also, the difference (| m3-m4 |) between m3 and m4 may be 2.0 GPa or less at 25 ° C. Specifically, the difference (| m3-m4 |) between m3 and 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.

In addition, the m4 / m3 ratio may be 0.7 to 1.3, 0.8 to 1.2, or 0.9 to 1.1 at 25 ° C.

As an example, the first direction may be the width direction (TD) of the film, and the second direction may be the longitudinal direction (MD) of the film.

The m4 / m3 ratio 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 at 25 ° C.

Alternatively, the m4 / m3 ratio 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 at 25 ° C.

In addition, the modulus (m1, m2) for the first direction and the second direction of the protective film and the modulus (m3, m4) for the first direction and the second direction of the transparent substrate are the equations (2) and (3) Satisfies

Specifically, the m1 / m3 ratio 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.

In addition, the m2 / m4 ratio 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.

Further, m1 and m2 may be 2.5 Gpa to 5 Gpa at 25 ° C, and m3 and m4 may be 5 Gpa to 10 Gpa at 25 ° C.

Due to these characteristics, the laminate can be applied to a cover of a flexible display device, particularly a foldable display device, and is not only a deformation that occurs at a small folding point inward from the in-folding type, but also a large folding outward from the folding type. Even by the deformation occurring at the point, the characteristics may not deteriorate.

Coating layer

Referring to FIG. 3, the laminate 20 may further include one or more coating layers 110 disposed on the protective film 100.

The coating layer 110 may be disposed between the protective film 100 and the transparent substrate 200 or may be disposed on an outer surface of the protective film 100.

The coating layer may be a functional coating layer for improving hardness, antistatic, scattering prevention, refractive index control, surface protection, and the like, and detailed description thereof is as described above.

[Display device]

A display device according to an embodiment includes: a display panel; And a protective film according to the embodiment, which is disposed on one surface of the display panel.

A display device according to another embodiment includes a display panel; And a laminate according to the embodiment, which is disposed on one surface of the display panel.

The display device may be a flexible display device, in particular, a foldable display device.

Specifically, the foldable display device may be an in-folding type or an out-folding type according to a folding direction.

In the display device, the protective film or the laminate has substantially the same configuration and properties as the protective film or the laminate according to one embodiment described above.

Specifically, the protective film or the laminate has a small modulus difference in two directions perpendicular to the in-plane, so that whitening may be significantly suppressed despite frequent folding. In addition, since the protective film or the laminate is excellent in other mechanical properties, thermal properties, and durability, it may serve as a protective film for shock relief, scattering prevention, and scratch prevention.

[Example]

It will be described in more detail through the following examples, but is not limited to these ranges.

Preparation of polyester films of Examples 1 to 4 and Comparative Examples 1 and 2:

Polyethylene terephthalate (PET) resin (SKC) was extruded in an extruder and cast in a casting roll to prepare an unstretched sheet. The unstretched sheet was stretched in the longitudinal direction (MD) and in the width direction (TD). Thereafter, the stretched sheet was heat-set, and annealed and cooled to prepare a polyester film. At this time, the relaxation was performed in two stages while lowering the temperature in the annealing step.

Test Example 1: Modulus measurement

Modulus (Young's modulus) was measured at 25 ° C and 85 ° C for each direction of MD and TD of the film sample, and the results are shown in the following table. Specifically, the film sample was cut to a width of 1.5 cm, mounted on a universal tester (UTM, 5566A, Instron), and increased in length to measure stress accordingly to calculate modulus.

Test Example 2: Evaluation of folding (MIT folding test)

The MIT folding test according to ASTM D 2176 and TAPPI T 511 was performed on a film sample by a folding endurance tester (MIT-DA, Toyoseiki). Specifically, the MIT folding test was performed under conditions of a folding angle of 135 °, a sample size of 10 mm in width, a load of 500 gf, a radius of curvature of 0.38R, and a folding speed of 175 times / minute. The results are classified according to the following criteria and are shown in the table below.

-O: More than 200,000 repetitive foldings until cracks or breaks occur.

-X: Less than 100,000 folding cycles until cracks or breaks occur.

Test Example 3: Evaluation of whitening (Crack strain test)

For film samples, using a universal tester (UTM, 5566A, Instron), the minimum elongation until whitening occurs in 1% increments was measured, and the results were classified according to the following criteria and shown in the table below.

-O: Whitening does not occur even at an elongation of 10% or more.

-X: Whitening occurs at an elongation of less than 10%.

division thickness
(㎛) MD Modulus (GPa)
/ TD modulus (GPa)
(Measured at 25 ℃) MD Modulus (GPa)
/ TD modulus (GPa)
(Measurement at 85 ℃) Folding
evaluation colloquial Chinese
evaluation Example 1 40 3.7 / 3.8 1.5 / 1.9 O O Example 2 40 3.7 / 4.0 1.5 / 1.6 O O Example 3 50 3.8 / 3.9 1.9 / 2.1 O O Example 4 30 4.0 / 4.1 1.6 / 1.6 O O Comparative Example 1 50 3.8 / 4.6 2.3 / 2.8 X X Comparative Example 2 80 2.2 / 5.8 1.5 / 3.2 X X

As shown in the above table, the polyester films of Examples 1 to 4 in which the MD modulus and the TD modulus were adjusted to a preferred range were passed both the folding evaluation and the whitening evaluation, and were suitable as a protective film for the flexible display cover.

On the other hand, the polyester films of Comparative Examples 1 and 2, in which MD modulus and TD modulus were outside the preferred range, did not pass folding evaluation and whitening evaluation, and were unsuitable for use as a protective film for a flexible display cover.

Example 5: Preparation of a multilayer protective film

The procedure of Example 1 was repeated to prepare a polyester film having a thickness of 40 μm and a refractive index of 1.64.

In the process of preparing the polyester film, before stretching in the longitudinal direction (MD) and then stretching in the width direction (TD), a primer layer was formed on the polyester film. At this time, a thermosetting polyurethane-based resin composition was applied on the polyester film using a Mayer bar and dried to form a primer layer having a thickness of 85 nm and a refractive index of 1.54.

Prepare a hard coating layer composition in which pentaerythritol triacrylate (PETA) was dissolved in a methyl isobutyl ketone (MIBK) solvent at a concentration of 30% by weight, and 5% by weight of a photopolymerization initiator (Irg184, BASF) was added to the solid content. Did. The hard coating layer composition was coated on a primer layer so that the film thickness after drying by a bar coater was 5 μm to form a coating film. The formed coating film was dried at 80 ° C. for 1 minute to remove the solvent and fixed by ultraviolet irradiation at about 300 mJ / cm 2 to form a hard coating layer having a thickness of 2.5 μm and a refractive index of 1.50.

Example 6: Preparation of laminate

The polyester films of Examples 1 to 4 and Comparative Examples 1 and 2 were laminated with polyimide films (TPI, SKC) through an optically transparent adhesive (OCA 8146-x, 3M) to prepare laminates, respectively. At this time, the modulus of the polyimide film was about 6.5 GPa in the MD direction and the TD direction, respectively, at about 25 ° C.

In addition, folding evaluation and whitening evaluation were performed on the laminates prepared as above, and as a result, the laminate including the polyester films of Examples 1 to 4 passed folding evaluation and whitening evaluation, but Comparative Example 1 And the laminate comprising the polyester film of 2 did not pass the folding evaluation and whitening evaluation.

1: In-folding type flexible display device,
2: Outfolding type flexible display device,
10: multilayer protective film, 20: laminate,
100: protective film, 110: coating layer,
200: transparent substrate (cover window),
300: flexible display device body,
a, b: The folding point.

Claims (15)

Contains a polyester resin,
It is applied to the cover of a flexible display device,
The strain until whitening occurs (strain) is more than 10%,
When defining the first and second directions perpendicular to each other in the plane of the film,
A protective film satisfying the following formula (1) at 25 ° C:
0.7 ≤ m2 / m1 ≤ 1.3 (1)
In the above formula, m1 and m2 are modulus of the protective film for the first direction and the second direction, respectively.
According to claim 1,
The protective film, wherein m1 and m2 are both 4.5 GPa or less at 25 ° C.
According to claim 2,
The protective film in which the difference (| m1-m2 |) between m1 and m2 is 1.0 GPa or less at 25 ° C.
According to claim 1,
The first direction is the width direction (TD) of the film,
A protective film in which the second direction is a longitudinal direction (MD) of the film.
The method of claim 4,
The m2 / m1 ratio of 0.8 to 1.0 at 25 ℃, protective film.
According to claim 2,
The protective film, wherein m1 and m2 are both 2.5 GPa or less at 85 ° C.
According to claim 2,
The protective film, wherein m1 has a difference of 1 GPa to 4 GPa with each other at 25 ° C and 85 ° C.
According to claim 1,
The number of repetitive foldings at an angle of 135 ° until the protective film breaks is 100,000 or more, a protective film.
According to claim 1,
A protective film, wherein the protective film has a thickness of 20 μm to 60 μm.
According to claim 1,
The protective film
In-plane retardation (Ro) of 600 nm or less and
A protective film having a thickness direction retardation (Rth) of 4,000 nm or more.
According to claim 1,
The protective film
A protective film in which the ratio (s2 / s1) of the heat shrinkage ratio (s2) in the second direction to the heat shrinkage ratio (s1) in the first direction at 85 ° C. and 24 hours is 1 to 10.
A protective film comprising a polyester resin; And
A transparent substrate disposed on the protective film,
When defining the first and second directions perpendicular to each other in the plane of the film,
A laminate, which satisfies the following formulas (1) to (3) at 25 ° C:
0.7 ≤ m2 / m1 ≤ 1.3 (1)
0.5 ≤ m1 / m3 ≤ 0.9 (2)
0.4 ≤ m2 / m4 ≤ 0.9 (3)
In the above formula, m1 and m2 are the modulus of the protective film for the first direction and the second direction, respectively, and m3 and m4 are the modulus of the transparent substrate for the first direction and the second direction, respectively.
The method of claim 12,
The m1 and m2 is 2.5 Gpa to 5 Gpa at 25 ° C,
The laminate, wherein m3 and m4 are 5 Gpa to 10 Gpa at 25 ° C.
The method of claim 12,
The protective film and the transparent substrate have a strain until all whitening occurs (strain) is 10% or more, a laminate.
The method of claim 12,
The transparent substrate includes a polyimide-based resin,
The laminate is applied to the cover of the foldable display device.

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