KR102275644B1 - Polyester protective film for flexible display device - Google Patents

Abstract

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

Description

Polyester protective film for flexible display devices

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.

The display technology is being developed again and again thanks to the demand according to the development of IT devices, and technologies such as a curved display and a bent display have already been commercialized. Recently, in the field of mobile devices that require both a large screen and portability, a flexible display device that can be flexibly bent or folded according to an external force is preferred. In particular, a foldable display device can be folded down when not in use to increase portability, and when not in use, it can be spread widely to realize a large screen.

Such a foldable display device, with reference to FIGS. 1A and 1B , includes an in-folding type (1) in which a screen is positioned inside the folding direction, and an out-of-folding display in which the screen is positioned outside in the 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 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 for shock relief, scattering prevention, scratch prevention, etc., and recent attempts have been made to manufacture a protective film for a flexible display device using a polyester resin. .

Korean Patent 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 when frequently bent or folded. Since it is almost impossible to return to an original shape due to the fact that a general material is completely folded and unfolded, it is almost impossible to return to its original shape. Therefore, research to overcome this limitation should be accompanied in the development of a material applied to a flexible display device.

Specifically, with reference to FIG. 1A , in the case of an in-folding type (1), due to a deformation due to a compressive stress applied to a point (a) that is folded small inward, also referring to FIG. 1B, an out-folding type (2) In the case of , whitening or cracks occur in the protective film 100 due to deformation due to tensile stress occurring at the point (b), which is largely folded outward, and thus the properties are easily deteriorated. Such whitening and cracking can be generally 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 a flexible display device This is an easily occurring problem.

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

Accordingly, an object of the present invention is to provide a protective film having a modulus controlled at room temperature through the following embodiments, a laminate including the same, and a display device.

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

0.7 ≤ m2/m1 ≤ 1.3 (1)

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

According to another embodiment, a protective film comprising a polyester resin; and a transparent substrate disposed on the protective film, wherein when defining a first direction and a second direction perpendicular to each other within the plane of the film, the laminate satisfies the following formulas (1) to (3) at 25°C Offered:

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 with respect to the first direction and the second direction, respectively, and m3 and m4 are the modulus of the transparent substrate with respect to the first direction and the second direction, respectively.

Since the protective film according to the embodiment has a small difference in modulus at room temperature with respect to two directions perpendicular to the plane, whitening and cracking can be remarkably suppressed even with frequent bending or folding. Accordingly, even if the protective film includes a polyester resin having a relatively high modulus at room temperature, whitening and cracking may be suppressed. In addition, since the protective film has excellent other mechanical properties, thermal properties, and durability, it can serve as a protective film to mitigate impact, prevent scattering, and prevent scratches.

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

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

Where each film, panel, or layer, etc. described in the embodiments below is described as being formed "on" or "under" each film, panel, or layer, etc., "on ( “on” and “under” include both those formed directly or indirectly through other elements.

Also, the size of each component in the drawings may be exaggerated for explanation, and does not mean a size that is actually applied.

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

[protective film]

The protective film according to an exemplary embodiment includes a polyester resin, is applied to a cover of a flexible display device, has a strain of 10% or more until whitening occurs, and has a 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, satisfying the following equation (1) at 25 ℃.

0.7 ≤ m2/m1 ≤ 1.3 (1)

In the above formula, m1 and m2 are the modulus of the protective film with respect to 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 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, for example, a polyethylene terephthalate (PET) resin may be 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 another polyester resin in addition to the PET resin. Specifically, the protective film may further include a polyethylene naphthalate (PEN) resin in an amount of about 15% by weight or less. More specifically, the protective film may further include about 0.1 wt% to 10 wt%, or about 0.1 wt% to 5 wt% of the PEN resin.

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

In addition, since the protective film has polyester as a main component, the degree of crystallinity may be increased in the manufacturing process through heating, stretching, etc., and mechanical properties such as tensile strength may be improved.

modulus

In the protective film, the modulus (m1, m2) in the first direction and the second direction perpendicular to each other in plane are both low at 25°C.

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

Accordingly, the protective film is advantageously applied to a flexible display device because whitening does not occur even when it is frequently bent or folded.

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.

Also, the difference (|m1-m2|) between m1 and 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 a width direction (TD) of the film, and the second direction may be a 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 these characteristics, the protective film can be applied to a cover of a flexible display device, particularly a foldable display device, and is not only deformed at a small folding point in the in-folding type, but also greatly folded outward in the out-folding type. Even with 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.

Also, 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.

Also, 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 from each other at 25°C and 85°C.

Properties

The protective film may have a thickness of 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 advantageously applied to a flexible display device. The strain refers to the ratio of the changed dimension to the original dimension of the film, wherein the changed dimension may be an increased dimension or a decreased dimension.

The protective film may have a number of repeated folding at an angle of 135° until fracture occurs 100 or more, 1000 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, breakage does not occur even with frequent folding, so it is advantageous to apply to a flexible display device.

In addition, the protective film has a moisture permeability of 10 g/m2·day to 100 g/m2·day, 10 g/m2·day to 50 g/m2·day, or 10 g/m2·day to 30 g/m2·day. 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 water vapor transmission rate 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 thickness during hard coating treatment.

In the protective film, a ratio (s2/s1) of a thermal contraction rate s2 in the second direction to a thermal contraction rate s1 in the first direction at 85° C. and 24 hours may be 1 to 10. 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%.

Also, 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 characteristics, lifting due to shrinkage may not occur under high-temperature conditions when applied as a protective film. Accordingly, it is possible to prevent wave patterns, glitter, delamination, cracks, and the like 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, the occurrence of rainbow spots 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 in order to balance 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

The thickness direction retardation may be a measured value based on a thickness of 40 μm to 50 μm. When within the above range, the degree of orientation of the molecules 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 retardation Rth to the in-plane retardation Ro increases as the thickness direction retardation Rth increases, rainbow blotch can be effectively suppressed.

Meanwhile, 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 in consideration of the thickness limit and cost for removing rainbow spots from the protective film.

Here, the in-plane retardation (Ro) is the product of the anisotropy of the refractive indices of two axes orthogonal in the plane of the film (Δnxy=|nx-ny|) and the film thickness (d) (Δnxy×d) As a parameter defined as , it is a measure of 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 respectively measured at a plurality of points in the plane of the film.

In addition, thickness direction retardation (Rth) refers to two birefringences Δnxz (= | nx-nz |) and Δnyz (= | ny-nz |) when viewed from a cross section in the thickness direction of the film, respectively. It is a parameter defined as the average of the retardation 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 respectively measured at a plurality of points in the plane of the film.

According to the film manufacturing process according to the exemplary 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 a midpoint of the width of the film after stretching in the width direction (TD) and the length direction (MD). There is not only one center of width in the film, but it can be set to infinity depending on the measurement point. On the other hand, the width center of the final film cut into various shapes after manufacturing may not coincide with the width center of the original film (film before cutting), and in this case, the minimum in-plane retardation and the maximum thickness direction retardation at the width center of the film may not appear.

In addition, when the film is applied to an optical component for a large screen, it is preferable that the deviation of the in-plane retardation (ie, the difference between the maximum value and the minimum value) is small within the effective width. Here, the effective width refers to a distance between points moved a certain distance from the center of the width toward both ends in the width direction, and may be defined, for example, as ±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 manufacturing may not coincide with the width center of the initial film (film before cutting). In this case, the effective width may refer to a distance between points that have moved a certain distance toward both ends along the width direction from the point where the minimum in-plane retardation appears in the film.

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

In addition, the optical film has less variation in retardation in the thickness direction within the effective width. Specifically, in the optical film, the amount of change (|ΔRth|/|Δx|) of the thickness direction retardation with respect to the 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 certain points in the width direction (x-axis), and the amount of change (ΔRth) of the phase difference in the thickness direction is the thickness direction retardation at each predetermined point means the difference (Rth ₂ -Rth ₁ ).

In addition, the protective film may have a ratio (Rth/Ro) of a thickness direction retardation (Rth) 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) are, the more advantageous it is to prevent rainbow spots. In particular, the protective film may have a ratio (Rth _max /Ro _min ) of the maximum thickness direction retardation (Rth _{max ) to the minimum in-plane retardation (Ro min} ) of 30 or more, 40 or more, 50 or more, or 60 or more.

[Method for producing protective film]

A method for producing a protective film according to an embodiment includes the steps of (1) extruding a composition including a polyester resin to obtain an unstretched film; (2) stretching the unstretched film in the longitudinal direction and the transverse direction; and (3) heat-setting the stretched film.

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

At this time, the composition of the polyester resin used as a raw material of 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 predetermined 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 in a range that satisfies the range of 70 ° C to 90 ° C. can When it is within the above range, it is possible to effectively prevent the protective film from breaking during stretching while ensuring flexibility for stretching.

The stretching is performed by biaxial stretching, and for example, it may be stretched in two axes in the width direction (tenter direction, TD) and longitudinal direction (machine direction, MD) through a simultaneous biaxial stretching method or a sequential biaxial stretching method. Preferably, the sequential biaxial stretching method of 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 stretch ratio in the width direction may be in the range of 2.0 to 5.0, more specifically, in the range of 2.9 to 3.7. Preferably, the draw ratio in the longitudinal direction (d1) and the draw ratio in the width direction (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 is 0.5 to 1.0 , 0.7 to 1.0, or 0.9 to 1.0.

The said draw ratios d1 and d2 are ratios which show the length after extending|stretching, when the length before extending|stretching is made into 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 longitudinal direction and/or the width direction, and the temperature range at this time may be 150°C to 250°C.

[Multilayer Protective Film]

Referring to FIG. 2 , the multilayer protective film 10 according to an embodiment includes the protective film 100 according to the 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 properties as the protective film according to the exemplary embodiment described above.

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

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

The protective film and the coating layer can prevent color expression or transparency deterioration 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 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 including 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 resin, a photopolymerization initiator, and other additives 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. It is also possible to use some organic solvents.

For example, the composition for the primer coating layer preferably has a solid content of 3 wt% to 20 wt%, or 4 wt% to 10 wt% in the composition.

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 photocurable resin. The photocurable resin may include, for example, a compound having one or more unsaturated bonds, such as an acrylate-based 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, diallyl phthalate 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]

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 and second directions perpendicular to each other in the plane of the film, the following formulas (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 with respect to the first direction and the second direction, respectively, and m3 and m4 are the modulus of the transparent substrate with respect to 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 the exemplary embodiment described above.

For example, the modulus m1 and m2 of the protective film in the first direction and the second direction may be the same as the modulus values specifically exemplified in the protective film according to the exemplary 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-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 when deformed due to frequent folding, so that it is advantageously applied to a flexible display device. The strain refers to the ratio of the changed dimension to the original dimension of the film, wherein the changed dimension may be an increased dimension or a decreased dimension. In particular, both the protective film and the transparent substrate may have a strain of 10% or more until whitening occurs.

The transparent substrate may have a number of repeated folding at an angle of 135° until fracture occurs 100 or more, 1000 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, breakage does not occur even with frequent folding, so it is advantageous to apply to a flexible display device.

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

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

Accordingly, the transparent substrate is advantageously applied to a flexible display device because whitening and cracks do not occur despite 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.

Also, 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 a width direction (TD) of the film, and the second direction may be a 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 (m1, m2) of the protective film with respect to the first direction and the second direction and the modulus (m3, m4) of the transparent substrate with respect to the first direction and the second direction are the formulas (2) and (3) is satisfied with

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.

Also, 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 deformed at the point of small folding inward in the in-folding type, but also greatly folded outward in the out-folding type. Even with 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 an outer surface of the protective film 100 .

The coating layer may be a functional coating layer for hardness improvement, antistatic, scattering prevention, refractive index control, surface protection, etc.

[display device]

A display device according to an embodiment includes a display panel; and a protective film according to the embodiment disposed on one surface of the display panel.

A display device according to another embodiment includes a display panel; and the laminate according to the 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 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 exemplary embodiment described above.

Specifically, since the protective film or the laminate has a small difference in modulus in two directions perpendicular to the plane, the whitening phenomenon can be remarkably suppressed even by frequent folding. In addition, since the protective film or the laminate has excellent other mechanical properties, thermal properties, and durability, it may serve as a protective film to mitigate impact, prevent scattering, and prevent scratches.

[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, the 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, respectively, in the MD and TD directions of the film sample, and the results are shown in the table below. Specifically, the film sample was cut to be 1.5 cm wide and mounted on a universal testing machine (UTM, 5566A, Instron), and the modulus was calculated by measuring the resulting stress while increasing the length.

Test Example 2: folding evaluation (MIT folding test)

MIT folding test (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 Corporation). Specifically, the MIT folding test was performed under the 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 were classified according to the criteria below and shown in the table below.

- O: The number of repeated folding until cracks or fractures occur more than 200,000 times.

- X: The number of repeated folding until cracks or fractures occur less than 100,000 times.

Test Example 3: Whitening evaluation (Crack strain test)

Using a universal tester (UTM, 5566A, Instron, Inc.) for the film sample, the minimum elongation rate was measured until whitening occurred while increasing by 1%, 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 rate is less than 10%.

division thickness
(μm) 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 preferred range, passed both the folding evaluation and the whitening evaluation, and were suitable as a protective film for the flexible display cover.

delete

Example 5: Preparation of 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 previously, after stretching in the longitudinal direction (MD) and before 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.

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 Corporation) was added with 5% by weight based on the solid content to prepare a hard coating layer composition did. The hard coating layer composition was coated on the primer layer to have a film thickness of 5 μm after drying by a bar coater to form a coating film. The formed coating film was dried at 80° C. for 1 minute to remove the solvent and fixed by UV irradiation 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 laminates

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, respectively. 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, respectively.

In addition, folding evaluation and whitening evaluation were performed on the laminates prepared in this way, and as a result, the laminates including the polyester films of Examples 1 to 4 passed the folding evaluation and the 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 (15)

delete delete delete delete delete delete delete delete delete delete delete a protective film comprising a polyester resin; and
a transparent substrate disposed on the protective film;
When defining a first direction and a second direction perpendicular to each other in the plane of the film,
A laminate that 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 with respect to the first direction and the second direction, respectively, and m3 and m4 are the modulus of the transparent substrate with respect to the first direction and the second direction, respectively.
13. The method of claim 12,
wherein m1 and m2 are 2.5 Gpa to 5 Gpa at 25°C,
wherein m3 and m4 are 5 Gpa to 10 Gpa at 25°C.
13. The method of claim 12,
The protective film and the transparent substrate have a strain of 10% or more until whitening occurs.
13. The method of claim 12,
The transparent substrate comprises a polyimide-based resin,
The laminate is applied to a cover of a foldable display device.

Images