KR20240040017A - Display device - Google Patents

Abstract

A display device of one embodiment includes a display module, a window disposed on the display module, and a protective layer disposed on the window, wherein the protective layer includes a first base layer, a second base layer disposed on the first base layer, and a substrate adhesive layer disposed between the first base layer and the second base layer and comprising a silicone-based adhesive, a surface disposed on the second base layer, having a single-layer structure, and comprising a hard coating material and an anti-fingerprint material. By including a coating layer, it can have excellent impact resistance and folding characteristics.

Description

Display device {DISPLAY DEVICE}

The present invention relates to a display device, and more particularly, to a display device including a protective layer.

Various display devices used in multimedia devices such as televisions, mobile phones, tablet computers, game consoles, etc. are being developed. A display device may include various functional layers to provide a high-quality color image to a user.

Meanwhile, research on thin display devices has recently been conducted in order to implement various types of display devices, such as display devices including curved surfaces, rollable display devices, or foldable display devices. Meanwhile, a thin display device can be implemented by reducing the number of functional layers and including functional layers containing various functions.

Meanwhile, when implementing a thin display device, it is necessary to ensure the impact resistance of the display device at the same time. Accordingly, research is being conducted on the structure of display devices with excellent impact resistance and optical properties.

The purpose of the present invention is to provide a display device with improved impact resistance and visibility.

One embodiment includes a display module, a window disposed on the display module, and a protective layer disposed on the window, wherein the protective layer includes a first base layer and a second base disposed on the first base layer. layer, and a substrate adhesive layer disposed between the first base layer and the second base layer, comprising a silicone-based adhesive, disposed on the second base layer, having a single-layer structure, a hard coating material, and an anti-fingerprint material. A display device including a surface coating layer containing a material is provided.

The substrate adhesive layer may have a tensile modulus of 0.1 GPa or more and 4.5 GPa or less.

The thickness of the substrate adhesive layer may be 3 ㎛ or more and 4.5 ㎛ or less.

The edge portion of the substrate adhesive layer may be disposed inside each of the edge portions of the first base layer and the edge portion of the second base layer.

The edge portion of the substrate adhesive layer is disposed inside the edge portion of the first base layer by a first width, and is disposed inside the edge portion of the second base layer by a second width, and the first width and the second width The width may be independently 450 ㎛ or more and 600 ㎛ or less.

When viewed in plan, the area of the substrate adhesive layer may be smaller than each of the areas of the first base layer and the second base layer.

The modulus of the first base layer and the modulus of the second base layer may each independently be 3.5 GPa or more and 5.0 GPa or less.

The thickness of the first base layer and the thickness of the second base layer may each independently be 35 ㎛ or more and 50 ㎛ or less.

The first base layer may include polyethylene terephthalate.

The second base layer may be made of a different material from the first base layer.

The protective layer further includes an adhesive layer disposed between the first base layer and the window, and the thickness of the adhesive layer may be 25 μm or more and 70 μm or less.

The protective layer, the window, and the display module may be folded and unfolded based on a folding axis.

The window may include thin film glass.

The thickness of the window may be 80 μm or less.

The thickness of the surface coating layer may be 3㎛ or more and 10㎛ or less.

The surface contact angle of the surface coating layer may be 110 degrees or more and 120 degrees or less.

Each of the first base layer and the second base layer is a stretched film with a controlled optical axis, and the tensile modulus on the first stretched axis and the tensile modulus on the second stretched axis of each of the first base layer and the second base layer are The differences may each independently be between 0 and 1.1 Gpa.

The Vickers hardness of the surface coating layer may be 40 or more.

The surface coating layer may include 0 wt% to 10 wt% of the anti-fingerprint material based on the total weight of the surface coating layer.

It may further include a hard coating layer disposed on at least one of the upper and lower surfaces of the first base layer and the upper and lower surfaces of the second base layer.

The display device of one embodiment includes two base layers and a protective layer including an adhesive layer, thereby providing a display device with excellent impact resistance.

Additionally, the display device of one embodiment may have improved visibility by including a protective layer including a surface coating layer including a hard coating material and an anti-fingerprint material.

1 is a perspective view of a display device according to an embodiment of the present invention.
FIG. 2A is a diagram showing the display device shown in FIG. 1 in an in-folding state along the first folding axis.
FIG. 2B is a diagram showing the display device shown in FIG. 1 in an out-folded state along the first folding axis.
FIG. 3A is a diagram showing the display device shown in FIG. 1 in an in-folded state along the second folding axis.
FIG. 3B is a diagram illustrating the display device shown in FIG. 1 in an out-folded state along the second folding axis.
Figure 4 is an exploded perspective view of a display device according to an embodiment.
Figure 5 is a cross-sectional view taken along the cutting line II′ shown in Figure 4.
Figure 6 is an enlarged view of a portion of a display module according to one embodiment.
Figure 7 is a cross-sectional view showing a protective layer according to one embodiment.
8 is an enlarged view of a portion of a protective layer according to one embodiment.
Figure 9a is a perspective view showing a portion of a protective layer according to one embodiment.
Figure 9b is a perspective view showing a portion of a protective layer according to one embodiment.
Figure 10a is a cross-sectional view showing a protective layer according to one embodiment.
Figure 10b is a cross-sectional view showing a protective layer according to one embodiment.
Figure 10c is a cross-sectional view showing a protective layer according to one embodiment.
Figure 10d is a cross-sectional view showing a protective layer according to one embodiment.
Figure 10e is a cross-sectional view showing a protective layer according to one embodiment.
Figure 10f is a cross-sectional view showing a protective layer according to one embodiment.

Since the present invention can be subject to various changes and have various forms, specific embodiments will be illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to a specific disclosed form, and should be understood to include all changes, equivalents, and substitutes included in the spirit and technical scope of the present invention.

In this specification, when a component (or region, layer, portion, etc.) is referred to as being “on,” “connected to,” or “coupled to” another component, it is directly placed/on the other component. This means that they can be connected/combined or a third component can be placed between them.

Meanwhile, in the present application, “directly disposed” may mean that there is no layer, film, region, plate, etc. added between a part of the layer, film, region, plate, etc. and another part. For example, “directly placed” may mean placed without using an additional member, such as an adhesive member, between two layers or two members.

Like reference numerals refer to like elements. Additionally, in the drawings, the thickness, proportions, and dimensions of components are exaggerated for effective explanation of technical content.

“And/or” includes all combinations of one or more that the associated configurations may define.

Terms such as first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. For example, a first component may be named a second component, and similarly, the second component may also be named a first component without departing from the scope of the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.

Additionally, terms such as “below,” “on the lower side,” “above,” and “on the upper side” are used to describe the relationship between the components shown in the drawings. The above terms are relative concepts and are explained based on the direction indicated in the drawings. In this specification, “disposed on” may refer to not only the upper part of a member but also the lower part.

Unless otherwise defined, all terms (including technical terms and scientific terms) used in this specification have the same meaning as commonly understood by a person skilled in the art to which the present invention pertains. Additionally, terms such as those defined in commonly used dictionaries should be interpreted as having meanings consistent with their meanings in the context of the relevant technology, and unless explicitly defined herein, should not be interpreted as having too idealistic or overly formal meanings. is not allowed.

Terms such as “include” or “have” are intended to designate the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, but do not include one or more other features, numbers, or steps. , it should be understood that it does not exclude in advance the possibility of the existence or addition of operations, components, parts, or combinations thereof.

Hereinafter, a display device according to an embodiment will be described with reference to the drawings.

1 is a perspective view of a display device according to an embodiment of the present invention.

Referring to FIG. 1 , the display device DD may have a rectangular shape with a short side parallel to the first direction DR1 and a long side parallel to the second direction DR2 that intersects the first direction DR1. . However, the shape of the display device DD is not limited to this, and the display device DD may be provided in various shapes. For example, it has a rectangular shape with a long side parallel to the first direction DR1 and a short side parallel to the second direction DR2, or a rectangular shape with a side parallel to the first direction DR1 and a side parallel to the second direction DR2. It can have a square shape with the same length.

The display device DD may be a foldable electronic device. Specifically, the display device DD according to an embodiment of the present invention may be folded based on the folding axes FX1 and FX2 extending in a predetermined direction. Hereinafter, a state that is folded based on the folding axes (FX1, FX2) is defined as a folded state, and a state that is not folded is defined as a non-folded state. The folding axes FX1 and FX2 are rotation axes that are virtual lines that occur when the display device DD is folded. For example, the folding axes FX1 and FX2 may be formed by mechanical structures included in the display device DD.

The folding axes FX1 and FX2 may extend in the first direction DR1 or the second direction DR2. In one embodiment, the folding axis extending in the second direction DR2 is defined as the first folding axis FX1, and the folding axis extending in the first direction DR1 is defined as the second folding axis FX2. The display device DD may be folded based on at least one of the first and second folding axes FX1 and FX2.

The display device DD in one embodiment may be a large display device such as a television or monitor, as well as a small or medium-sized display device such as a mobile phone, tablet, car navigation system, or game console. This is only an example, and can be applied to other electronic devices as long as it does not deviate from the concept of the present invention.

As shown in FIG. 1, the display device DD displays an image IM in the third direction DR3 on the display surface IS parallel to each of the first and second directions DR1 and DR2. It can be displayed. The display surface IS on which the image IM is displayed may correspond to the front surface of the display device DD.

The display surface IS of the display device DD may be divided into a plurality of areas. The display surface IS of the display device DD may have a display area DA and a non-display area NDA defined. The display area (DA) may be an area where the image (IM) is displayed, and the user recognizes the image (IM) through the display area (DA). The display area DA may have a rectangular shape. The non-display area (NDA) may surround the display area (DA). Accordingly, the shape of the display area DA may be substantially defined by the non-display area NDA. However, this is an exemplary illustration, and the non-display area NDA may be disposed adjacent to only one side of the display area DA or may be omitted. The display device DD according to an embodiment may include various embodiments and is not limited to any one embodiment.

The non-display area NDA is an area adjacent to the display area DA and is an area where the image IM is not displayed. The bezel area of the display device DD may be defined by the non-display area NDA.

The display device (DD) according to the present invention can detect a user's input (TC) applied from the outside. The user's input (TC) includes various types of external inputs, such as parts of the user's body, light, heat, or pressure. In this embodiment, the user's input (TC) is shown as the user's hand being applied to the front. However, this is shown as an example, and as described above, the user's input TC may be provided in various forms, and the display device DD may be provided in various forms depending on the structure of the display device DD. ) may detect a user's input (TC) applied to the side or back of the device, and is not limited to any one embodiment.

The display device DD can activate the display surface IS to display the image IM and at the same time detect the user's input TC. In this embodiment, the area that detects the user's input (TC) is shown as being provided in the display area (DA) where the image (IM) is displayed. However, this is shown as an example, and the area for detecting the user's input (TC) may be provided in the non-display area (NDA) or may be provided in all areas of the display surface (IS).

FIG. 2A is a diagram showing the display device DD shown in FIG. 1 in an in-folding state along the first folding axis FX1, and FIG. 2B is a diagram showing the display device DD shown in FIG. 1 (In-Folding). This is a diagram showing a state in which DD) is out-folded along the first folding axis FX1.

Referring to FIGS. 1 and 2A , the display device DD may be a foldable display device. The display device DD may be folded based on a folding axis (eg, a first folding axis FX1 and a second folding axis FX2) extending in a predetermined direction. FIG. 2A exemplarily shows the display device DD folded with respect to the first folding axis FX1 extending in the second direction DR2.

A plurality of areas may be defined in the display device DD according to the operation type. The plurality of areas may be divided into a folding area (FA1) and at least one non-folding area (NFA1, NFA2). A folding area (FA1) is defined between two non-folding areas (NFA1, NFA2).

The folding area FA1 is an area that is folded based on the first folding axis FX1 and is an area that substantially forms a curvature. Here, the first folding axis FX1 may extend along the second direction DR2, that is, the long axis direction of the display device DD. The folding area FA1 is defined as an area folded along the first folding axis FX1 and extending in the second direction DR2.

In one embodiment, the unfolded areas NFA1 and NFA2 may include a first unfolded area NFA1 and a second unfolded area NFA2. The first non-folding area NFA1 is adjacent to one side of the folding area FA1 in the first direction DR1, and the second non-folding area NFA2 is adjacent to the folding area FA1 in the first direction DR1. adjacent to the other side.

The display device DD may be in-folded or out-folded. In-folding refers to folding the display surfaces IS so that they face each other, and out-folding refers to folding the display devices DD so that the rear surfaces face each other. Specifically, folding so that the display surfaces of different non-folding areas (NFA1, NFA2) are folded to face each other is defined as in-folding, and folding so that the display surfaces of different non-folding areas (NFA1, NFA2) face outward is defined as in-folding. It is defined as out-folding.

The display device DD shown in FIG. 2A may be in-folded so that the display surface IS of the first non-folding area NFA1 and the display surface IS of the second non-folding area NFA2 face each other. As the first non-folding area NFA1 is rotated clockwise along the first folding axis FX1, the display device DD may be in-folded. In order to in-fold the display device DD so that the first non-folding area NFA1 and the second non-folding area NFA2 are aligned, the first folding axis FX1 is aligned with the display device DD in the first direction DR1. It can be defined at the center of (DD).

Referring to FIG. 2B , the display device DD may be out-folded based on the first folding axis FX1. The display device DD may display the image IM when the display surface IS of the first non-folding area NFA1 and the display surface IS of the second non-folding area NFA2 are exposed to the outside. . In addition, the display surface (IS) of the folding area (FA1) exposed to the outside can also display an image (IM). As shown in FIG. 1, the display device DD can display an image IM in an unfolded state. The first non-folding area (NFA1), the second non-folding area (NFA2), and the folding area (FA1) each display images that provide independent information or portions of one image that provide one piece of information. Each can be displayed.

The display device DD may be manufactured to have both an in-folding state and an out-folding state, or may be manufactured to have either an in-folding state or an out-folding state.

Meanwhile, in FIGS. 2A and 2B, one folding area FA1 is defined in the display device DD, but the present invention is not limited to this. For example, a plurality of folding areas may be defined in the display device DD of one embodiment.

FIG. 3A is a diagram showing the display device shown in FIG. 1 in an in-folded state along the second folding axis, and FIG. 3B is a diagram showing the display device shown in FIG. 1 in an out-folded state along the second folding axis. This is a drawing showing.

Referring to FIGS. 3A and 3B , the display device DD may be in-folded or out-folded based on the second folding axis FX2. The second folding axis FX2 may extend along the first direction DR1, that is, the minor axis direction of the display device DD.

A plurality of areas may be defined in the display device DD according to the operation type. The plurality of areas may be divided into a folding area (FA2) and at least one non-folding area (NFA3, NFA4). The folding area (FA2) is defined between the two non-folding areas (NFA3 and NFA4).

The folding area FA2 is an area that is folded based on the second folding axis FX2 and is an area that substantially forms a curvature. The folding area FA2 is defined as an area folded along the second folding axis FX2 and extending in the first direction DR1.

In one embodiment, the unfolded areas NFA3 and NFA3 may include a third unfolded area NFA3 and a fourth unfolded area NFA3. The third non-folding area NFA3 is adjacent to one side of the folding area FA2 in the second direction DR2, and the fourth non-folding area NFA4 is adjacent to the folding area FA2 in the second direction DR2. It can be adjacent to the other side.

3A and 3B illustrate that one folding area FA2 is defined in the display device DD, but the embodiment is not limited thereto. For example, a plurality of folding areas may be defined in the display device DD of one embodiment.

Figure 4 is an exploded perspective view of a display device according to an embodiment. Figure 5 is a cross-sectional view of a display device according to an embodiment. 6 is a cross-sectional view of a portion of a display device according to an embodiment. FIG. 5 may be a cross-sectional view taken along the cutting line II′ shown in FIG. 4.

4 to 6, the display device DD according to one embodiment includes a display module DM that displays an image, a window WM disposed on the display module DM, and a window WM on the window WM. It may include a protective layer (PL) disposed. The display module DM may constitute a part of the display device DD (see FIG. 1).

The display module (DM) may include a display panel (DP) and an input detection unit (ISP). The display panel DP according to one embodiment may be an emissive display panel and is not particularly limited. For example, the display panel DP may be an organic light emitting display panel or a quantum dot light emitting display panel. The light emitting layer of the organic light emitting display panel may include an organic light emitting material. The light emitting layer of the quantum dot light emitting display panel may include quantum dots, quantum rods, etc. Hereinafter, the display panel DP will be described as an organic light emitting display panel.

The display panel DP may be a flexible display panel. Accordingly, the display panel DP may be rolled as a whole, or may be folded or unfolded around the folding axis FX2.

The input detection unit (ISP) may be placed directly on the display panel (DP). According to one embodiment, the input detection unit (ISP) may be formed on the display panel (DP) through a continuous process. That is, when the input sensing unit (ISP) is placed directly on the display panel (DP), the adhesive film is not disposed between the input sensing unit (ISP) and the display panel (DP). However, the present invention is not limited to this. An adhesive film may be disposed between the input sensing unit (ISP) and the display panel (DP). In this case, the input sensing unit (ISP) is not manufactured through a continuous process with the display panel (DP), but is manufactured through a separate process from the display panel (DP) and then applied to the upper surface of the display panel (DP) with an adhesive film. can be fixed to

The display panel (DP) generates an image, and the input detection unit (ISP) acquires coordinate information about the user's input (eg, touch event).

The window WM may be placed on the display module DM. The window WM may include an optically transparent insulating material. Accordingly, the image generated in the display module DM can penetrate the window WM and be easily recognized by the user.

For example, the window WM may include a glass substrate or a synthetic resin film. When the window WM is thin film glass, the thickness of the window WM may be 80 μm or less, for example, 30 μm, but the thickness of the window WM is not limited thereto.

When the window WM is a synthetic resin film, the window WM may include a polyimide (Pl) film or a polyethylene terephthalate (PET) film.

The window WM may have a multi-layer structure or a single-layer structure. For example, the window WM may include a plurality of synthetic resin films bonded with an adhesive, or may include a glass substrate and a synthetic resin film bonded with an adhesive. The window WM may be made of a flexible material. Accordingly, the window WM may be folded or unfolded around the folding axis FX2. That is, the shape of the window WM may be changed when the shape of the display module DM is changed.

The window WM transmits images from the display module DM and alleviates external shocks, thereby preventing the display module DM from being damaged or malfunctioning due to external shocks. Shock from the outside refers to a force from the outside that can be expressed as pressure, stress, etc. that causes a defect in the display module (DM).

A protective layer (PL) is disposed on the window (WM). The protective layer PL may be a layer to protect the window WM from external shock and prevent scattering when the window WM is damaged. The structure of the protective layer PL will be described in detail later with reference to FIGS. 7 and 8.

One or more functional layers may be disposed between the display module (DM) and the window (WM). For example, the display device DD of one embodiment may include an anti-reflection layer (RPL) between the display module (DM) and the window (WM) that blocks reflection of external light. The anti-reflection layer (RPL) can prevent the problem of elements constituting the display module (DM) being visible from the outside due to external light incident through the front of the display device (DD). The anti-reflection layer (RPL) may include a phase retarder and a polarizer. The phase retarder may be a film type or a liquid crystal coating type, and may include a _/2 phase retarder and/or a _/4 phase retarder. The polarizer may also be a film type or a liquid crystal coating type. The film type may include a stretched synthetic resin film, and the liquid crystal coating type may include liquid crystals arranged in a predetermined arrangement. The phase retarder and polarizer can be implemented as one polarizing film. Meanwhile, the display device DD of one embodiment may further include a protective film disposed on or below the anti-reflection layer RPL.

The anti-reflection layer (RPL) may be disposed on the input sensing unit (ISP). That is, the anti-reflection layer (RPL) may be disposed between the input sensing unit (ISP) and the window (WM). The anti-reflection layer (RPL) and window (WM) may be coupled to each other through an adhesive film (ADL). Although not shown in the drawing, an adhesive film may be further disposed between the input sensing unit (ISP) and the anti-reflection layer (RPL) to secure the anti-reflection layer (RPL) to the input sensing unit (ISP).

As an example, the adhesive film (ADL) may include an optically clear adhesive film (OCA, Optically Clear Adhesive film). However, the adhesive film (ADL) is not limited to this and may include conventional adhesives or adhesives. For example, the adhesive film (ADL) may include optically clear adhesive resin (OCR) or pressure sensitive adhesive film (PSA).

The display module (DM) can display images according to electrical signals and transmit/receive information about external input. The display module (DM) may be defined by an active area (AA) and a peripheral area (NAA). The active area (AA) can be defined as an area that emits an image provided from the display module (DM).

The peripheral area (NAA) is adjacent to the active area (AA). For example, the surrounding area (NAA) may surround the active area (AA). However, this is shown as an example, and the peripheral area (NAA) may be defined in various shapes and is not limited to any one embodiment. According to one embodiment, the active area AA of the display module DM may correspond to at least a portion of the display area DA.

As shown in FIGS. 4 and 5 , the display device DD further includes a support plate SP disposed on the rear surface of the display module DM to support the display module DM. The support plate SP may be a metal plate. The support plate (SP) may be a stainless steel plate. The strength of the support plate SP may be greater than that of the display module DM.

The support plate SP may include a number of support plates SP1 and SP2 corresponding to the non-folding areas NFA3 and NFA4. For example, the support plate SP may include a first support plate SP1 and a second support plate SP2 spaced apart from the first support plate SP1. The first and second support plates SP1 and SP2 may be disposed to correspond to the non-folding areas NFA3 and NFA4, respectively. That is, the first support plate SP1 is disposed corresponding to the third non-folding area NFA3 of the display module DM, and the second support plate SP2 is disposed in the fourth non-folding area NFA3 of the display module DM. It is deployed in response to NFA4). When the display module DM is folded based on the second folding axis FX2, the first and second support plates SP1 and SP2 may be arranged to be spaced apart from each other in the second direction DR2.

The first and second support plates SP1 and SP2 may be spaced apart from each other corresponding to the folding area FA2. The first and second support plates SP1 and SP2 may partially overlap the folding area FA2. That is, the separation distance between the first and second support plates SP1 and SP2 in the second direction DR2 may be smaller than the width of the folding area FA2.

The support plate SP may further include a connection module for connecting the first and second support plates SP1 and SP2. The connection module may include a hinge module or an articulated module.

Although the case where the support plate SP includes two support plates SP1 and SP2 is shown, the present invention is not limited to this. That is, when the number of folding axes FX2 increases, the support plate SP may include a plurality of support plates separated based on the folding axis FX2.

Although FIG. 4 shows a structure in which the support plate SP is divided into first and second support plates SP1 and SP2, the present invention is not limited to this. That is, the support plate SP may be formed in the form of a single plate disposed to correspond to the first and second non-folding areas NFA3 and NFA4 and the folding area FA2. Here, the support plate SP may include a plurality of holes formed through the support plate SP formed in the folding area FA2.

A buffer film may be further disposed between the display module DM and the support plate SP. The buffer film may include a polymer material. The buffer film may be a layer for absorbing shock applied from the outside. The buffer film may be respectively coupled to the display module (DM) and the support plate (SP) through an adhesive film.

As shown in FIGS. 4 to 6 , the sidewall WS1 of the window WM may be disposed inside the sidewall WS2 of the protective layer PL. Being placed medially may mean that it is closer to the active area (AA) than other comparison objects. That is, the sidewall WS1 of the window WM may not protrude more than the sidewall WS2 of the protective layer PL. Accordingly, the protective layer PL can block external shocks from being transmitted to the side wall WS1 of the window WM, and as a result, the probability of cracks occurring in the window WM can be reduced.

Here, the sidewall WS1 of the window WM may be disposed inside the sidewall WS2 of the protective layer PL by the first width W1. Here, the first width W1 may mean a width in a direction parallel to the second direction DR2. Additionally, the first width W1 may correspond to the distance between the sidewall WS1 of the window WM and the sidewall WS2 of the protective layer PL when viewed in a plan view.

As the first width W1 increases, the side wall WS2 of the protective layer PL protrudes more than the side wall WS1 of the window WM, which further reduces the probability of cracks occurring in the window WM. You can.

Additionally, an adhesive film (ADL) is disposed on the back of the window (WM). The side wall WS3 of the adhesive film ADL may be disposed inside the side wall WS2 of the protective layer PL. Being placed medially may mean that it is closer to the active area (AA) than other comparison objects. In particular, the side wall WS3 of the adhesive film ADL may not protrude more than the side wall WS1 of the window WM.

Here, the side wall WS3 of the adhesive film ADL may be disposed inside the side wall WS2 of the protective layer PL by the second width W2. The second width W2 may correspond to the distance between the sidewall WS2 of the protective layer PL and the sidewall WS3 of the adhesive film ADL when viewed in plan. Here, the second width W2 may mean a width in a direction parallel to the second direction DR2.

Additionally, the second width W2 may be greater than or equal to the first width W1. That is, when the second width W2 is greater than the first width W1, the side wall WS3 of the adhesive film ADL may be disposed inside the side wall WS1 of the window WM. When the second width W2 is the same as the first width W1, the side wall WS3 of the adhesive film ADL and the side wall WS1 of the window WM may be disposed on the same line.

Figure 7 is a cross-sectional view showing a protective layer according to one embodiment. Figure 8 is an enlarged cross-sectional view showing the surface coating layer and base layer shown in Figure 7. Figure 9a is a perspective view showing a portion of a protective layer according to one embodiment. Figure 9b is a perspective view showing a portion of a protective layer according to one embodiment.

Referring to Figures 7 and 8, the protective layer (PL) may include a first base layer (BS1), a substrate adhesive layer (P-ADL), a second base layer (BS2), and a surface coating layer (SCL). there is. The protective layer (PL) in one embodiment may further include an adhesive layer (AL).

The first base layer BS1 and the second base layer BS2 may each include a base material BL. Although only the second base layer BS2 is shown in FIG. 8 as an example, the description of the second base layer BS2 may be equally applied to the first base layer BS1. The base material (BL) is phenylene, polyethyleneterephthalate (PET), polyimide (PI), polyamide (PAI), polyethylene naphthalate (PEN), and polycarbonate ( It may include at least one of polycarbonate (PC), and thermoplastic polyurethane (TPU). Meanwhile, the base material BL included in the first base layer BS1 and the base material BL included in the second base layer BS2 may be different from each other. For example, the base material BL of the first base layer BS1 may be polyethylene terephthalate, and the base material BL of the second base layer BS2 may be thermoplastic polyurethane or polyimide. However, this is only an example and the embodiment is not limited thereto, and the base material BL included in the first base layer BS1 and the second base layer BS2 may be the same as polyethylene terephthalate.

The base layers (BS1, BS2) may further include an ultraviolet ray blocking material (UVCL) added to the base material (BL). In one embodiment, the ultraviolet ray blocking material (UVCL) may include inorganic chemicals such as zinc oxide (ZnO) or titanium dioxide (TiO2). Ultraviolet ray blocking material (UVCL) can perform the function of reflecting or scattering ultraviolet rays. In this way, by including the ultraviolet ray blocking material (UVCL) in the base layers BS1 and BS2, the protective layer PL can have an ultraviolet ray blocking function. For example, the protective layer PL may transmit less than 20% of ultraviolet rays with a wavelength of 380 nm or less. In particular, when the window WM includes thin film glass, the ultraviolet ray blocking function of the window may be weak. In this case, the ultraviolet ray blocking material is included in the base layers BS1 and BS2, so that the ultraviolet ray blocking function can be performed through the protective layer PL.

The thickness T1 of the first base layer BS1 and the thickness T3 of the second base layer BS2 may each independently be 35 μm or more and 50 μm or less. The thickness T1 of the first base layer BS1 and the thickness T3 of the second base layer BS2 may be the same or different from each other. If the thickness T1 of the first base layer BS1 or the thickness T3 of the second base layer BS2 is less than 35 μm, the protective layer PL may not have sufficient impact resistance. In addition, when the thickness T1 of the first base layer BS1 or the thickness T3 of the second base layer BS2 exceeds 50㎛, the thickness of the protective layer PL increases and the protective layer PL becomes thicker. The folding characteristics of the display device (DD, see FIG. 1) including a may be deteriorated.

The tensile modulus of the first base layer (BS1) and the tensile modulus of the second base layer (BS2) may each independently be 3.5 GPa or more and 5.0 GPa or less. The tensile modulus of the first base layer BS1 and the tensile modulus of the second base layer BS2 may be the same or different from each other. If the tensile modulus of the first base layer (BS1) or the tensile modulus of the second base layer (BS2) is less than 3.5 GPa, the protective layer (PL) does not have sufficient impact resistance. When the tensile modulus of the first base layer BS1 or the tensile modulus of the second base layer BS2 exceeds 5.0 GPa, the folding characteristics of the display device DD (see FIG. 1) including the protective layer PL are may deteriorate.

The first base layer (BS1) and the second base layer (BS2) may each be a stretched film with a controlled optical axis. The first base layer (BS1) and the second base layer (BS2) may each have a difference between the elastic modulus of the first stretching axis and the elastic modulus of the second stretching axis of 1.1 GPa or less. When the difference between the elastic modulus on each first stretching axis and the elastic modulus on the second stretching axis is greater than 1.1 Gpa, the first base layer (BS1) and the second base layer (BS2) are The shrinkage strain of the second base layer BS2 may increase.

The substrate adhesive layer (P-ADL) may be disposed between the first base layer (BS1) and the second base layer (BS2). The substrate adhesive layer (P-ADL) may adhere the first base layer (BS1) and the second base layer (BS2). The substrate adhesive layer (P-ADL) may include a silicone-based adhesive. However, this is only an example, and the material of the substrate adhesive layer (P-ADL) is not limited thereto.

The substrate adhesive layer (P-ADL) may have a tensile modulus of 0.1 GPa or more and 4.5 GPa or less. When the tensile modulus of the substrate adhesive layer (P-ADL) is 0.1 GPa or less, the substrate adhesive layer (P-ADL) cannot adhere the first base layer (BS1) and the second base layer (BS2). If the tensile modulus of the substrate adhesive layer (P-ADL) exceeds 4.5 GPa, the folding characteristics of the protective layer (PL) may be deteriorated.

Substrate adhesive layer thickness (㎛) One 2 3 4 4.5 5 Adhesion (gf/inch) 33 400 570 648 722 397

The thickness (T2) of the substrate adhesive layer (P-ADL) may be 3 μm or more and 4.5 μm or less. The thickness T2 of the substrate adhesive layer (P-ADL) may be determined by considering the adhesive strength of the substrate adhesive layer (P-ADL), the impact resistance of the protective layer (PL), and the folding characteristics, etc. as follows. Referring to Table 1, the adhesive strength of the substrate adhesive layer (P-ADL) is when the substrate adhesive layer (P-ADL) thickness (T2) is 2 μm or more and the substrate adhesive layer (P-ADL) thickness (T2) is 1 μm. It has a remarkable value of more than 10 times that of , and it can be seen that the adhesive strength tends to become stronger as the substrate adhesive layer (P-ADL) becomes thicker. However, it can be seen that when the thickness (T2) of the substrate adhesive layer (P-ADL) exceeds 4.5 ㎛, the adhesive strength actually decreases. In other words, it can be seen that the adhesive strength of the substrate adhesive layer (P-ADL) is significant when the thickness (T2) of the substrate adhesive layer (P-ADL) is in the range of 2 ㎛ to 5 ㎛, but rather decreases in the range of 4.5 ㎛ or more. Therefore, the appropriate range of the thickness (T2) of the substrate adhesive layer (P-ADL) considering the adhesive strength of the substrate adhesive layer (P-ADL) is 2 ㎛ or more and 4.5 ㎛ or less.

When the thickness (T2) of the substrate adhesive layer (P-ADL) is less than 3 μm, the protective layer (PL) does not have sufficient impact resistance. If the thickness (T2) of the substrate adhesive layer (P-ADL) exceeds 5 μm, the folding characteristics of the protective layer (PL) may deteriorate.

Therefore, if the thickness (T2) of the substrate adhesive layer (P-ADL) is 3㎛ or more and 4.5㎛ or less, there is a significant effect in terms of the adhesive strength of the substrate adhesive layer (P-ADL) and the impact resistance and folding characteristics of the protective layer (PL). can be seen.

The edge portion (E2) of the substrate adhesive layer (P-ADL) may be disposed inside compared to the edge portion (E1) of the first base layer (BS1) and the edge portion (E2) of the second base layer (BS2). . Here, being placed inside means being placed relatively adjacent to the active area (AA, see FIG. 1). The edge portion E2 of the substrate adhesive layer (P-ADL) may be disposed inside the edge portion E1 of the first base layer BS1 by the first width W4. The edge portion E2 of the substrate adhesive layer (P-ADL) may be disposed inside the edge portion E3 of the second base layer BS2 by the second width W5.

The edge portion of the edge portion E2 of the substrate adhesive layer (P-ADL) is disposed inside the edge portion E1 of the first base layer BS1 and the edge portion E3 of the second base layer BS2, thereby providing protection. The impact resistance of the layer (PL) may increase. The first base layer (BS1) and the second base layer (BS2) may each include a portion that does not overlap with the substrate adhesive layer (P-ADL). The portions of each of the first base layer (BS1) and the second base layer (BS2) that do not overlap with the substrate adhesive layer (P-ADL) may function to absorb external shock. Specifically, the portion where each of the first base layer (BS1) and the second base layer (BS2) does not overlap with the substrate adhesive layer (P-ADL) consumes energy generated by external impact as vibration, which is transmitted to other layers. The amount of shock can be reduced.

The first width W4 and the second width W5 may each independently be 450 μm or more and 600 μm or less. The first width W4 and the second width W5 may be the same or different from each other. If the first width W4 or the second width W5 is less than 450㎛, there is a problem that the substrate adhesive layer P-ADL may flow out during the folding operation of the display device DD (see FIG. 1). If the first width (W4) or the second width (W5) exceeds 600㎛, foreign substances may enter the space between the first base layer (BS1), the second base layer (BS2), and the substrate adhesive layer (P-ADL). This may adversely affect the reliability of the display device (DD, see FIG. 1).

The protective layer (PL) of one embodiment includes a first base layer (BS1) and a second base layer (BS2) bonded by a substrate adhesive layer (P-ADL), thereby controlling the thickness compared to the case where it includes one base layer. This is easy and can have improved impact resistance. Specifically, the shock transmitted from the outside is transmitted in the order of the second base layer (BS2), the first base layer (BS1), and the window (WM), and the two base layers (BS1, BS2) of the protective layer (PL) As it passes through, the shock is dispersed. Accordingly, the shock transmitted from the outside to the window WM may be reduced.

Meanwhile, referring to FIGS. 9A and 9B, when viewed in plan view, the area of the substrate adhesive layer (P-ADL) may be smaller than the area of the first base layer (BS1) and smaller than the area of the second base layer (BS2). .

The surface coating layer (SCL) may include a hard coating material (HC) and an anti-fingerprint material (AF) added to the hard coating material (HC). The surface coating layer (SCL) has a single-layer structure. That is, the surface coating layer (SCL) includes one layer including a hard coating material (HC) and an anti-fingerprint material (AF). The hard coating material (HC) may include a high strength material of pencil hardness F or higher. As an example, the hard coating material (HC) may include at least one of a siloxane resin, an epoxy resin, or an acrylic resin. Meanwhile, the Vickers hardness of the surface coating layer (SCL) may be 40 or more.

Silonic acid resin may include silsesquioxane, siloxane compounds, and inorganic particles surface-treated with silane. Silsesquioxane may be a ladder type silsesquioxane. Silsesquioxane may be included in an amount of 30 wt% or more and 60 wt% or less based on the total weight of the hard coating material (HC). If silsesquioxane is contained in less than 30 wt%, the flexibility of the surface coating layer (SCL) of one embodiment may not be improved, and if silsesquioxane is contained in more than 60 wt%, the flexibility of the surface coating layer (SCL) of one embodiment may not be improved. Surface hardness may decrease.

The siloxane compound may have at least one acrylate functional group. For example, the siloxane compound may have an acrylate group as a terminal group. For example, the siloxane compound may be a siloxane polymer or siloxane oligomer having an acrylate group as a terminal group. In one embodiment, the siloxane compound may be provided as a polymer formed integrally with silsesquioxane.

The siloxane compound may be included in an amount of 10 wt% or more and 40 wt% or less based on the total weight of the hard coating material (HC). If the content of the siloxane compound is less than 10wt%, the surface hardness and strength of the surface coating layer (SCL) may decrease, and if the content of the siloxane compound exceeds 40wt%, the brittleness of the surface coating layer (SCL) increases, reducing flexibility. This may decrease and the occurrence of cracks may increase during bending.

The hard coating material (HC) of one embodiment may include surface-treated inorganic particles. The surface-treated inorganic particles may be inorganic particles surface-treated with silane. In one embodiment, the inorganic particles surface-treated with silane may be inorganic particles surface-treated with a silane coupling agent. For example, the average size of the inorganic particles may be 10 nm or more and 50 nm or less. The average size of the inorganic particles may represent the average diameter of the inorganic particles. For example, the average diameter of the inorganic particles may be 10 nm or more and 30 nm or less.

In one embodiment, when the average size of the inorganic particles exceeds 50 nm, the optical transparency of the surface coating layer (SCL) may be reduced. Additionally, if the average size of the inorganic particles is less than 10 nm, the effect of improving surface hardness in the surface coating layer (SCL) may be reduced.

The inorganic particles may be SiO2, TiO2, Al2O3, ZrO2, ZnO, AlN, Si3N4, or a combination thereof. That is, the inorganic particles may include at least one of SiO2, TiO2, Al2O3, ZrO2, ZnO, AlN, and Si3N4. The hard coating material (HC) of one embodiment includes SiO2 surface treated with silane, TiO2 surface treated with silane, Al2O3 surface treated with silane, ZrO2 surface treated with silane, ZnO surface treated with silane, It may include AlN, Si3N4 surface-treated with silane, or mixtures thereof.

The hard coating material (HC) of one embodiment may include 10 wt% or more and 30 wt% or less of inorganic particles surface-treated with silane based on the total weight. If the content of the inorganic particles is less than 10 wt%, the surface hardness and strength of the surface coating layer (SCL) may decrease, and if the content of the inorganic particles exceeds 30 wt%, the compatibility of the inorganic particles may decrease.

Epoxy resin or acrylic resin is a monomer or oligomer containing at least one epoxy group, oxetane group, acrylate group, methacrylate group, urethane acrylate group, and ethylene oxide (EO) addition type acrylate group, and may have flexibility. there is. More specifically, the epoxy resin may be at least one selected from the group consisting of glycidyl-type epoxy resin, alicyclic epoxy resin, and oxetane-based resin.

Glycidyl-type epoxy resins include bisphenol A-type epoxy resins, bisphenol F-type epoxy resins, bisphenol S-type epoxy resins, naphthalene-type epoxy resins or hydrogenated products thereof, epoxy resins with a dicyclopentadiene skeleton, and triglycidyl isocyanurate. It may be an epoxy resin having a late skeleton, an epoxy resin having a cardo skeleton, and an epoxy resin having a polysiloxane structure.

The alicyclic epoxy resin is 3,4-epoxycyclohexylmethyl-3', 4'-epoxycyclohexanecarboxylate, 1,2,8,9-diepoxylimonene, and ε-caprolactone oligomer, respectively, at both ends of 3, It may be an epoxy resin made by ester bonding 4-epoxycyclohexylmethanol and 3,4-epoxycyclohexanecarboxylic acid and having a hydrogenated bisphenol A skeleton.

The oxetane-based resin may be an oxetane resin having a hydroxy structure and an oxetane resin having a methoxy methyl benzene structure.

Acrylic resin is a monomer or oligomer containing at least one member selected from the group consisting of an acrylate group, methacrylate group, urethane acrylate group, ethylene oxide (EO), and addition-type acrylate group.

For example, acrylic resins include bisphenol-A ethylene oxide diacrylate, bisphenol-A ethylene oxide dimethacrylate, bisphenol-A ethoxylate diacrylate, bisphenol-A ethoxylate diacrylate, and bisphenol-A poly. Ethoxylate diacrylate, bisphenol-A diacrylate, bisphenol-S diacrylate, dicyclopentadienyl diacrylate, pentaerythritol triacrylate, tris(2-hydroxyethyl)isocyanurate tri. Acrylate, pentaerythritol tetraacrylate, bisphenol-A dimethacrylate, bisphenol-S dimethacrylate, dicyclopentadienyl dimethacrylate, pentaerythritol trimethacrylate, tris (2-hydroxy These include ethyl)isocyanurate trimethacrylate and pentaerythritol tetramethacrylate.

The anti-fingerprint material (AF) may include a water-repellent material or an oil-repellent material to improve the contamination resistance of the surface coating layer (SCL). As an example, the anti-fingerprint material (AF) may be an inorganic particle containing a fluorine-based material. The fluorine-based material may include one of polytetra fluoroethylene (PTFE), polyvinylidene phoride (PVDF), and amorphous fluorine (Teflon AF, Cytop).

The surface coating layer (SCL) using the anti-fingerprint material (AF) may have a surface contact angle of 110 degrees to 120 degrees. The surface contact angle may vary depending on the weight ratio of the anti-fingerprint material (AF) to the surface coating layer (SCL). As an example, the anti-fingerprint material (AF) may have a weight ratio greater than 0 wt% and less than or equal to 10 wt% based on the total weight of the surface coating layer (SCL). If the anti-fingerprint material (AF) has a weight ratio greater than 10 wt%, the surface hardness of the surface coating layer (SCL) may decrease.

The surface coating layer (SCL) may have a thickness of 3㎛ to 10㎛. In particular, the surface coating layer (SCL) may have a thickness of 4㎛ to 6㎛, but is not limited thereto. If the thickness of the surface coating layer (SCL) is greater than 10㎛, flexibility may be reduced, and if the thickness of the surface coating layer (SCL) is less than 3㎛, the surface hardness of the surface coating layer (SCL) may be reduced.

The adhesive layer (AL) may be disposed below the first base layer (BS1). The adhesive layer AL may be disposed between the first base layer BS1 and the window WM (see FIG. 5). The adhesive layer (AL) may include a pressure-sensitive adhesive material. However, the present invention is not limited to this. The adhesive layer (AL) may include an optically transparent adhesive resin. Meanwhile, the thickness T5 of the adhesive layer (AL) may be 25 μm or more and 70 μm or less. If the thickness (T5) of the adhesive layer (AL) is less than 25㎛, the protective layer (PL) may not have sufficient impact resistance. If the thickness T5 of the adhesive layer AL exceeds 70 μm, the folding characteristics of the display device DD (see FIG. 1) may deteriorate.

Additionally, the total light transmittance of the protective layer PL may be 90% or more and may have a haze value of less than or equal to 1%. If the haze value is greater than 1%, the light transmittance of the protective layer (PL) may decrease.

10A to 10F are cross-sectional views of a protective layer according to one embodiment, respectively. Content that overlaps with the description of the protective layer with reference to FIGS. 1 to 9B will not be described again, and the differences will be mainly explained.

Referring to FIGS. 10A to 10F , the protective layers PL-1 to PL-6 according to one embodiment may include at least one hard coating layer HC1 to HC4. Specifically, the hard coating layers (HC1 to HC4) of the protective layers (PL-1 to PL-6) according to one embodiment may increase the impact resistance of the protective layers (PL-1 to PL-6). At least one hard coating layer (HC1 to HC4) may include a hard coating material. The hard coating material may include a high strength material of pencil hardness F or higher. As an example, the hard coating material may include at least one of a siloxane resin, an epoxy resin, or an acrylic resin. The description of the hard coating material may be the same as the description of the hard coating material (HC, see FIG. 8) included in the surface coating layer (SCL, see FIG. 8).

Referring to FIG. 10A, the protective layer (PL-1) of one embodiment may further include a first hard coating layer (HC1) disposed between the second base layer (BS2) and the substrate adhesive layer (P-ADL). . Referring to FIG. 10B, the protective layer PL-2 of one embodiment may further include a second hard coating layer HC2 disposed between the second base layer BS2 and the surface coating layer SCL. Referring to FIG. 10C, the protective layer (PL-3) of one embodiment includes a first hard coating layer (HC1) and a second base layer (BS2) disposed between the second base layer (BS2) and the substrate adhesive layer (P-ADL). ) and a second hard coating layer (HC2) disposed between the surface coating layer (SCL).

Referring to FIG. 10D, the protective layer PL-4 of one embodiment may further include a third hard coating layer HC3 disposed below the first base layer BS1. Referring to FIG. 10E, the protective layer (PL-5) of one embodiment may further include a fourth hard coating layer (HC4) disposed between the first base layer (BS1) and the substrate adhesive layer (P-ADL). . Referring to FIG. 10F, the protective layer (PL-6) of one embodiment includes a third hard coating layer (HC3) disposed below the first base layer (BS1), the first base layer (BS1), and the substrate adhesive layer (P-ADL). ) may further include a fourth hard coating layer (HC4) disposed between them.

Meanwhile, although not shown, a hard coating layer is included on at least one of the upper and lower surfaces of the first base layer (BS1), and a hard coating layer is included on at least one of the upper and lower surfaces of the second base layer (BS2). It may be.

The display device of one embodiment is disposed on a first base layer, a second base layer disposed on the first base layer, a substrate adhesive layer disposed between the first base layer and the second base layer, and the second base layer; It has a single-layer structure and includes a protective layer including a hard coating material and a surface coating layer containing an anti-fingerprint material. Accordingly, the display device of one embodiment may have improved impact resistance and folding characteristics. The display device of one embodiment includes a protective layer including two base layers, so that thickness can be easily adjusted. Additionally, the display device of one embodiment may have improved visibility by including a protective layer including a surface coating layer including a hard coating material and an anti-fingerprint material.

Although the present invention has been described above with reference to preferred embodiments, those skilled in the art or have ordinary knowledge in the relevant technical field should not deviate from the spirit and technical scope of the present invention as set forth in the claims to be described later. It will be understood that the present invention can be modified and changed in various ways within the scope of the present invention.

Therefore, the technical scope of the present invention should not be limited to what is described in the detailed description of the specification, but should be defined by the scope of the patent claims.

DM: Display module WM: Windows
PL: Protective layer SCL: Surface coating layer
BS1, BS2: 1st, 2nd base layer
P-ADL: Base material adhesive layer AL: Adhesive layer
HC: Hard coating material AF: Anti-fingerprint material
BL: Base material UVCL: UV blocking material
DD: display device

Claims (20)

display module;
a window disposed on the display module; and
Comprising a protective layer disposed on the window,
The protective layer is,
first base layer;
a second base layer disposed on the first base layer; and
a substrate adhesive layer disposed between the first base layer and the second base layer and including a silicone-based adhesive;
A display device disposed on the second base layer, having a single-layer structure, and including a surface coating layer including a hard coating material and an anti-fingerprint material. According to paragraph 1,
The display device wherein the substrate adhesive layer has a tensile modulus of 0.1 GPa or more and 4.5 GPa or less. According to paragraph 1,
A display device wherein the substrate adhesive layer has a thickness of 3 ㎛ or more and 4.5 ㎛ or less. According to paragraph 1,
The display device wherein the edge portion of the substrate adhesive layer is disposed inside each of the edge portions of the first base layer and the edge portion of the second base layer. According to paragraph 4,
The edge portion of the substrate adhesive layer is disposed inside the edge portion of the first base layer by a first width, and is disposed inside the edge portion of the second base layer by a second width,
The first width and the second width are each independently 450 μm or more and 600 μm or less. In paragraph 1
When viewed from a plane,
The display device in which the area of the substrate adhesive layer is smaller than each of the areas of the first base layer and the second base layer. According to paragraph 1,
The modulus of the first base layer and the modulus of the second base layer are each independently 3.5 GPa or more and 5.0 GPa or less. According to paragraph 1,
The display device wherein the thickness of the first base layer and the thickness of the second base layer are each independently 35 ㎛ or more and 50 ㎛ or less. According to paragraph 1,
A display device wherein the first base layer includes polyethylene terephthalate. According to clause 9,
The display device wherein the second base layer is made of a material different from the first base layer. According to paragraph 1,
The protective layer further includes an adhesive layer disposed between the first base layer and the window,
A display device wherein the adhesive layer has a thickness of 25 ㎛ or more and 70 ㎛ or less. According to paragraph 1,
A display device in which the protective layer, the window, and the display module are folded and unfolded based on a folding axis. According to clause 12,
A display device wherein the window includes thin film glass. According to clause 13,
A display device wherein the window has a thickness of 80 μm or less. According to clause 14,
A display device wherein the surface coating layer has a thickness of 3 ㎛ or more and 10 ㎛ or less. According to clause 15,
A display device wherein the surface contact angle of the surface coating layer is 110 degrees or more and 120 degrees or less. According to paragraph 1,
Each of the first base layer and the second base layer is a stretched film with a controlled optical axis,
The difference between the tensile modulus on the first stretching axis and the tensile modulus on the second stretching axis of each of the first base layer and the second base layer is independently 0 or more and 1.1 Gpa or less. According to paragraph 1,
A display device wherein the surface coating layer has a Vickers hardness of 40 or more. According to paragraph 1,
The surface coating layer includes 0 wt% to 10 wt% of the anti-fingerprint material based on the total weight of the surface coating layer. According to paragraph 1,
The display device further includes a hard coating layer disposed on at least one of the upper and lower surfaces of the first base layer and the upper and lower surfaces of the second base layer.