KR20210058787A - Polyester protective film for flexible display device - Google Patents

Abstract

Since a protective film according to the embodiment has a small difference in modulus at room temperature with respect to two directions perpendicular to the plane, even with frequent bending or folding, whitening and cracking can be dramatically suppressed. Due to the above characteristics, the protective film may be applied to a flexible display device.

Description

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

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

Display technology is constantly evolving owing to the demand according to the development of IT devices, and technologies such as curved displays and bent displays have already been commercialized. Recently, in the field of mobile devices in which a large screen and portability are required at the same time, a flexible display device that can flexibly bend or fold according to an external force is preferred. Particularly, a foldable display device is a big advantage in that it can be folded to make it small when not in use to increase portability, and when it is in use, it can be spread out to realize a large screen.

Such a foldable display device includes an in-folding type (1) in which a screen is located inside the folding direction, and an out-of-folding display device in which the screen is located outside the folding direction, with reference to FIGS. 1A and 1B. 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 in an in-folding type, and an ultra-thin glass (UTG) in an out-folding type are mainly used. In addition, the protective film 100 is applied to the surface of the transparent substrate 200 to relieve impact, prevent scattering, and prevent scratches. Recently, there have been attempts to manufacture a protective film for a flexible display device using a polyester resin. .

Korean Patent Application Publication No. 2017-0109746

A characteristic as important as flexibility in a material applied to a flexible display device is that the original characteristic does not deteriorate even with frequent bending or folding. Since general materials are almost impossible to return to their original appearance due to the marks when they are completely folded and unfolded, research to overcome these limitations must be accompanied in the development of materials applied to flexible display devices.

Specifically, referring to FIG. 1A, in the case of the in-folding type (1), the out-folding type (2) by deformation due to compressive stress applied to a point (a) that is folded inwardly small, and with reference to FIG. 1B. In the case of, due to the deformation due to the tensile stress occurring at the point (b) that is largely folded outward, whitening or cracking occurs in the protective film 100, etc., and the characteristics are liable to be deteriorated. Such whitening and cracking 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, and thus whitening and cracking when applied to a flexible display device. There is a problem with this easily occurring.

As a result of research by the present inventors to solve this problem, it was found that whitening and cracking can be dramatically suppressed even in frequent bending or folding by reducing the difference in modulus at room temperature for the length and width directions of the polyester film. .

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

According to one embodiment, it includes a polyester resin, is applied to a cover of a flexible display device, and has a strain of 10% or more until whitening occurs, and repeated folding at a 135° angle until fracture occurs. The number of times is 100,000 or more, and when defining the first direction and the second direction perpendicular to each other in the plane of the film, the first direction is the width direction (TD) of the film, and the second direction is the length direction of the film (MD ), the draw ratio in the width direction (d1) is 2.9 to 3.7, the draw ratio in the longitudinal direction (d2) is 2.8 to 3.5, and the draw ratio in the longitudinal direction to the draw ratio in the width direction (d1) ( A protective film is provided in which the ratio of d2) (d2/d1) is 0.9 to 1.0 and 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 with respect to the first and second directions, respectively.

According to another embodiment, a protective film comprising a polyester resin; And a transparent substrate disposed on the protective film, and when defining a first direction and a second direction perpendicular to each other in the plane of the film, a laminate satisfying the following formulas (1) to (3) at 25° C. 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 modulus of the protective film with respect to the first and second directions, respectively, and m3 and m4 denote modulus of the transparent substrate with respect to the first and second directions, respectively.

The protective film according to the embodiment has a small difference in modulus at room temperature for two vertical directions in the plane, so that whitening and cracking can be significantly suppressed even in frequent bending or folding. Accordingly, even if the protective film includes a polyester resin having a relatively large modulus at room temperature, whitening and cracking may be suppressed. In addition, since the protective film is excellent in other mechanical properties, thermal properties, and durability, it can play a role of mitigating impact, preventing scattering, and preventing scratches as a protective film.

Due to this characteristic, the protective film can be applied to a flexible display device, and not only by the deformation occurring at the point where it is folded small inward in the in-folding type, but also due to the deformation occurring at the point where it is largely folded outward in the out-folding type, The original properties may not be degraded.

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

In the case where each film, panel, or layer described in the following embodiments is described as being formed in “on” or “under” of each film, panel, or layer, “upper ( "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 the sake of explanation, and does not mean a size that is actually applied.

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

[Protective film]

The protective film according to an embodiment includes a polyester resin, is applied to a cover of a flexible display device, a strain until whitening occurs is 10% or more, and a 135° angle until fracture occurs The number of repetitive folding of is over 100,000 times. When the protective film defines a first direction and a second direction perpendicular to each other in the plane of the film, the first direction is the width direction (TD) of the film, the second direction is the length direction (MD) of the film, The draw ratio (d1) in the width direction is 2.9 to 3.7, the draw ratio (d2) in the longitudinal direction is 2.8 to 3.5, and the draw ratio (d2) in the length direction to the draw ratio (d1) in the width direction is The ratio (d2/d1) is 0.9 to 1.0, and the following formula (1) is satisfied at 25°C.

0.7 ≤ m2/m1 ≤ 1.3 (1)

In the above formula, m1 and m2 are modulus of the protective film with respect to the first and second directions, respectively.

Film composition

The protective film includes a polyester resin.

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

Examples of the dicarboxylic acid include 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 ,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-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, 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, and for example, a polyethylene terephthalate (PET) resin may be used as a main component.

As an example, the protective film may include about 85% by weight or more of a polyester resin, specifically, a PET resin, and more specifically, 90% by weight or more, 95% by weight or more, or 99% by weight or more. As another example, the protective film may further include a polyester resin other than PET resin. Specifically, the protective film may further include about 15% by weight or less of polyethylene naphthalate (PEN) resin. 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 in that it has high crystallinity and excellent mechanical properties. Specifically, the protective film may be a biaxially stretched polyester film, and for example, may be a film stretched at a draw ratio of 2.0 to 5.0 with respect to the length direction (MD) and the width direction (TD), respectively.

In addition, since the protective film is made of polyester as a main component, crystallinity may increase during the manufacturing process through heating and stretching, and mechanical properties such as tensile strength may be improved.

Modulus

The protective film has a low level of modulus (m1, m2) in the first direction and the second direction perpendicular to each other 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, since the protective film does not cause whitening even in frequent bending or folding, it is advantageous to be applied to a flexible display device.

Both 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 at 25°C. Specifically, both 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 between m1 and m2 (|m1-m2|) may be 1.0 GPa or less at 25°C. Specifically, 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 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 length 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 this characteristic, the protective film can be applied to a cover of a flexible display device, especially a foldable display device, and not only deformation occurring at a point where it is folded inward in the in-folding type, but also largely folded outward in the out-folding type. Even by the deformation occurring at the point, the properties may not be deteriorated.

In addition, both 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, 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 at 25°C and 85°C.

In addition, m2 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 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 when the film is deformed due to frequent folding, so it is advantageous to be applied to a flexible display device. The strain refers to a ratio of a changed dimension to an initial dimension of the film, and the changed dimension may be an increased dimension or a decreased dimension.

The protective film may have a number of repetitive folding at a 135° angle until fracture occurs 100 or more, 1,000 or more, 10,000 or more, 50,000 or more, 100,000 or more, 150,000 or more, or 200,000 or more. When within the above range, fracture does not occur even in frequent folding, so it is advantageous to be applied to a flexible display device.

In addition, the protective film has a moisture permeability of 10 g/m²ㆍday to 100 g/m²ㆍday, 10 g/m²ㆍday to 50 g/m²ㆍday or 10 g/m²ㆍday to 30 g/m²ㆍday I can.

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 it is within the above range, it is possible to prevent excessive crystallization while having excellent mechanical properties such as tensile strength.

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 of 2 to 4 μm in a hard coating treatment.

The protective film may have a ratio (s2/s1) of the thermal contraction rate (s2) in the second direction to the thermal contraction rate (s1) in the first direction (s2/s1) at 85° C. and 24 hours. Specifically, the ratio of the thermal contraction rate (s2/s1) may be 1.4 to 9, 1.5 to 8, or 1.6 to 7.

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%.

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%.

Since the protective film has the above-described heat shrinkage properties, when applied as a protective film, lifting due to shrinkage may not occur under high temperature conditions. Accordingly, wave patterns, glitter phenomenon, delamination, cracks, etc. can be prevented in advance.

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 it is within the above range, it is possible to minimize the occurrence of rainbow spots.

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 to balance optical properties and mechanical properties. have.

In addition, the protective film may have a retardation (Rth) in the thickness direction 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. I can.

The retardation in the thickness direction may be a measured value based on a thickness of 40 μm to 50 μm. When it is within the above range, crystallization is promoted due to a large molecular orientation, which is preferable in terms of mechanical properties. In addition, as the thickness direction retardation Rth increases, the ratio (Rth/Ro) of the thickness direction retardation Rth to the in-plane retardation Ro increases, so that rainbow spots can be effectively suppressed.

Meanwhile, in consideration of 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 a parameter defined as the product of the anisotropy of the refractive indices of two orthogonal axes in the plane of the film (β=|nx-ny|) and the film thickness (d) (βХd) As, it is a measure representing 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 a plurality of points in the plane of the film, respectively.

In addition, the thickness direction retardation (Rth) refers to the film thickness (d) for the two birefringences β=|nx-nz|) and β=|ny-nz|) as seen from the cross section in the film thickness direction. It is a parameter defined as the average of the phase difference obtained by multiplying by. In addition, the maximum thickness direction retardation Rth _max means the highest measured value when the thickness direction retardation Rth is measured at a plurality of points in the plane of the film.

According to the film manufacturing process according to an embodiment, a minimum in-plane retardation and a maximum thickness direction retardation may appear at the center of the width of the film. Accordingly, the minimum in-plane retardation and the maximum thickness direction retardation of the film may be values measured at the center of the width of the film. In the present 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, but can be set infinitely depending on the measurement point. On the other hand, the width center of the final film cut in various forms after manufacturing may not coincide with the width center of the first film (film before cutting), and in this case, the minimum in-plane retardation and the 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 use with a large screen, it is preferable that the deviation of the in-plane retardation (that is, the difference between the maximum and minimum values) within the effective width is small. Here, the effective width refers to a distance between points that have moved a certain distance from the center of the width toward both ends along the width direction, and may be defined as, for example, ±1,500 mm from the center of the width, that is, about 3,000 mm. Meanwhile, 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 first film (film before cutting). In this case, the effective width may refer to the distance between the point at which the minimum in-plane retardation appears in the film and the point moved by a certain distance toward both ends along the width direction.

The protective film has little variation in the in-plane retardation within the effective width. Specifically, the protective film may have an in-plane retardation change amount (|ΔRo|/|Δx|) of 0 to 100 nm/mm with respect to the displacement in the width direction. Specifically, |ΔRo|/|Δx| of the protective film The value may be 0 to 50 nm/mm, 0 to 35 nm/mm, 0 to 25 nm/mm, or 0 to 20 nm/mm. Here, the displacement in the width direction (Δx _{) means the distance (x 2 -x 1} ) between certain points in the width direction (x-axis), and the amount of change in the in-plane phase difference (ΔRo) is the in-plane phase difference at each of the certain points. It means the difference (Ro ₂ -Ro ₁ ). When within the above range, since the in-plane retardation Ro does not increase significantly even if the width of the film is widened, it is possible to effectively prevent the occurrence of rainbow spots.

In addition, the optical film has little variation in the retardation in the thickness direction within the effective width. Specifically, the optical film has a thickness direction retardation change amount (|△Rth|/|△x|) with respect to the displacement in the width direction is less than 1,000 nm/3 m, less than 700 nm/3 m, or 500 nm/3 m May be less than. Here, the displacement in the width direction (△x) means the distance (x ₂ -x ₁ ) between certain points in the width direction (x-axis), and the amount of change in the retardation phase difference in the thickness direction (△Rth) is the thickness at each certain point. It means the difference in the direction phase difference (Rth ₂ -Rth ₁ ).

In addition, the protective film may have a ratio (Rth/Ro) of a retardation (Rth) in a thickness direction to an in-plane retardation (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 is, the more advantageous it is to prevent the occurrence of rainbow spots, so it is preferable that the ratio of both values (Rth/Ro) is kept 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.

[Production method of protective film]

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

In the above manufacturing method, the protective film is prepared by extruding the raw 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 performed at a temperature condition of 230°C to 300°C or 250°C to 280°C.

The protective film is preheated at a certain temperature before stretching. The range of the preheating temperature is determined to satisfy 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, satisfy the range of 70°C to 90°C. I can. When it is within the above range, the protective film can be easily stretched, and at the same time, it is possible to effectively prevent the phenomenon that the protective film is broken during stretching.

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

The lengthwise stretch ratio may be in the range of 2.0 to 5.0, and more specifically 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 draw ratio (d1) and the widthwise draw ratio (d2) are similar, and specifically, the ratio (d2/d1) of the draw ratio (d2) in the longitudinal direction to the draw ratio (d1) in the width direction (d2/d1) is 0.5 to 1.0 , 0.7 to 1.0, or 0.9 to 1.0 may be.

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

In addition, the stretching speed 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, may be performed for 10 seconds to 45 minutes.

After starting the heat setting, the film may be relaxed in the length 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, a multilayer protective film 10 according to an exemplary embodiment includes a protective film 100 according to the exemplary embodiment and a coating layer 110 disposed on the protective film 100.

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

The coating layer may be one or more, and may include a functional coating layer for improving hardness, preventing static electricity, preventing scattering, controlling a refractive index, and protecting a surface.

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

By controlling the refractive index of the protective film and the coating layer, it is possible to prevent color expression or decrease in transparency due to optical interference.

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 other functional coating layers.

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 containing a thermosetting resin. For example, the coating composition may include a polyester-based resin, a polyurethane-based resin, or a mixture thereof.

The primer coating layer may be formed using a composition obtained by mixing and dispersing a raw material resin and, if necessary, a photopolymerization initiator and other additives in a solvent. 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. In addition, some organic solvents may be used.

For example, the composition for the primer coating layer, it is preferable that the solid content in the composition is 3% by weight to 20% by weight, or 4% by weight 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 at least one coating layer 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 photocurable resin. Examples of the photocurable resin include compounds having one or two or more unsaturated bonds, such as an acrylate compound.

In addition, the composition for the hard coating layer may include a thermosetting resin. Examples of the thermosetting resin include phenol resin, urea resin, diallylphthalate resin, melamine resin, guanamine resin, unsaturated polyester resin, polyurethane resin, epoxy resin, amino alkyd resin, melamine-urea cocondensation resin, silicon resin, Polysiloxane resin, etc. are mentioned.

[Laminate]

Referring to FIG. 3, the laminate 20 according to an 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 direction and the second direction 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 modulus of the protective film with respect to the first and second directions, respectively, and m3 and m4 denote modulus of the transparent substrate with respect to the first and second directions, respectively.

Protective film

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

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

Transparent substrate

The transparent substrate may be a cover window of a 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-based resin.

The transparent substrate 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 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 even with deformation due to frequent folding, so it is advantageous to be applied to a flexible display device. The strain refers to a ratio of a changed dimension to an initial dimension of the film, and the changed dimension may be an increased dimension or a decreased dimension. In particular, the protective film and the transparent substrate may all have a strain of 10% or more until whitening occurs.

The transparent substrate may have 100 or more, 1,000 or more, 10,000 or more, 50,000 or more, 100,000 or more, 150,000 or more, or 200,000 or more times of repetitive folding at a 135° angle until fracture occurs. When within the above range, fracture does not occur even in frequent folding, so it is advantageous to be applied to a flexible display device.

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

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

Accordingly, the transparent substrate is advantageously applied to a flexible display device because whitening and cracking do not occur even in frequent bending or folding.

Both 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 at 25°C. Specifically, both 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 at 25°C.

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.

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 length direction (MD) of the film.

In this case, 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 of the protective film in the first and second directions (m1, m2) and the modulus in the first and second directions of the transparent substrate (m3, m4) are expressed in the above formulas (2) and (3). Is satisfied.

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.

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

Due to this characteristic, the stacked body can be applied to a cover of a flexible display device, particularly a foldable display device. Even by the deformation occurring at the point, the properties may not be deteriorated.

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 the outer surface of the protective film 100.

The coating layer may be a functional coating layer for improving hardness, preventing static electricity, preventing scattering, controlling a refractive index, protecting a surface, and the like, and a 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 exemplary embodiment 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 exemplary embodiment disposed on one surface of the display panel.

The display device may be a flexible display device, particularly a foldable display device.

Specifically, the foldable display device may be of 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 characteristics as the protective film or the laminate according to the embodiment described above.

Specifically, since the protective film or the laminate has a small difference in modulus in two vertical directions in a plane, the whitening phenomenon can be remarkably suppressed even in frequent folding. In addition, since the protective film or the laminate is excellent in other mechanical properties, thermal properties, and durability, it can play a role of reducing impact, preventing scattering, and preventing scratches as a protective film.

[Example]

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

Examples 1 to 4: Preparation of polyester film

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

Test Example 1: Modulus measurement

The 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 and mounted on a universal testing machine (UTM, 5566A, Instron), and the resulting stress was measured while increasing the length to calculate the modulus.

Test Example 2: MIT folding test

The MIT folding test based on ASTM D 2176 and TAPPI T 511 for the film sample was performed 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/min. The results are classified according to the following criteria and shown in the table below.

-O: The number of repetitive folding until crack or fracture occurs is 200,000 or more

-X: The number of repetitive folding until crack or fracture occurs is less than 100,000 times.

Test Example 3: Whitening evaluation (Crack strain test)

For the film sample, the minimum elongation until whitening was measured by increasing by 1% using a universal tester (UTM, 5566A, Instron), 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 when the elongation is less than 10%.

division thickness
(㎛) MD modulus (GPa)
/TD modulus (GPa)
(Measured at 25℃) MD modulus (GPa)
/TD modulus (GPa)
(Measured at 85℃) Folding
evaluation all sorts of flowers
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

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

Example 5: Preparation of multilayer protective film

By repeating the procedure of Example 1, a polyester film having a thickness of 40 µm and a refractive index of 1.64 was prepared.

In the process of manufacturing a polyester film previously, a primer layer was formed on the polyester film after stretching in the longitudinal direction (MD) and before stretching in the width direction (TD). 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.

To 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 a photopolymerization initiator (Irg184, BASF) was added 5% by weight based on the solid content. I did. The hard coating layer composition was coated on the primer layer so that the film thickness after drying by a bar coater became 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 irradiation with ultraviolet rays of 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 were laminated with a polyimide film (TPI, SKC) through an optically transparent adhesive (OCA 8146-x, 3M) to prepare a laminate. At this time, the modulus of the polyimide film was about 6.5 GPa in the MD direction and the TD direction at about 25°C.

In addition, the laminates thus prepared were subjected to folding evaluation and whitening evaluation in the same manner as before, and as a result, the laminates including the polyester films of Examples 1 to 4 passed the folding evaluation and whitening evaluation.

1: In-folding type flexible display device,
2: an out-folding 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: folding point.

Claims (10)

Including polyester resin,
It is applied to the cover of the flexible display device,
The strain until whitening occurs is 10% or more,
The number of repeated folding at a 135° angle until fracture occurs is more than 100,000 times,
When defining a first direction and a second direction perpendicular to each other in the plane of the film,
The first direction is the width direction (TD) of the film, the second direction is the length direction (MD) of the film,
The draw ratio (d1) in the width direction is 2.9 to 3.7, the draw ratio (d2) in the length direction is 2.8 to 3.5,
The ratio (d2/d1) of the stretching ratio in the longitudinal direction (d2) to the stretching ratio in the width direction (d1) is 0.9 to 1.0,
A protective film 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 with respect to the first and second directions, respectively. The method of claim 1,
The protective film in which both m1 and m2 are 4.5 GPa or less at 25°C. The method of claim 2,
A protective film in which the difference between m1 and m2 (|m1-m2|) is 1.0 GPa or less at 25°C. The method of claim 1,
The m2 / m1 ratio is 0.8 to 1.0 at 25 ℃, the protective film. The method of claim 2,
The m1 and the m2 are both 2.5 GPa or less at 85°C, a protective film. The method of claim 2,
The m1 has a difference of 1 GPa to 4 GPa from each other at 25 ℃ and 85 ℃, the protective film. The method of claim 2,
The m2 has a difference of 1 GPa to 4 GPa from each other at 25 ℃ and 85 ℃, the protective film. The method of claim 1,
The protective film has a thickness of 20 μm to 60 μm, a protective film. The method of claim 1,
The protective film
The protective film in which the ratio (s2/s1) of the heat contraction rate (s2) in the second direction to the thermal contraction rate (s1) in the first direction (s2/s1) is 1 to 10 under conditions of 85° C. and 24 hours. The method of claim 9,
The protective film, wherein the thermal contraction rate (s1) in the first direction is 1% or less, and the thermal contraction rate (s2) in the second direction is 3% or less.

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