Flexible display device

A flexible display device includes: a display panel which display an image, and includes a first folding region folded or unfolded around a first folding axis, and first and second non-folding regions respectively located at both sides of the first folding region; and a cover window which is on the display panel, and includes a first cover portion having a first hardness and a second cover portion having a second hardness less than the first hardness, wherein the first cover portion is located corresponding to an outer folding portion a display surface of which is exposed to an outside.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the priority benefit of Republic of Korea Patent Application No. 10-2018-0097913 filed in Republic of Korea on Aug. 22, 2018, which is hereby incorporated by reference in its entirety.

BACKGROUND

Field of Technology

The present disclosure relates to a flexible display device, and particularly, relates to a flexible display device which is extremely foldable and can improve durability.

Discussion of the Related Art

Recently, with the advent of an information-oriented society, a field of displays for processing and displaying a massive amount of information has rapidly advanced. Thus, various flat panel display devices have been developed and highlighted.

As examples of the flat panel display devices, there are a liquid crystal display (LCD) device, a plasma display panel (PDP) device, a field emission display (FED) device, an electroluminescence display (ELD) device, an organic light emitting diode (OLED) display device, and the like. The flat panel display devices exhibit excellent characteristics in terms of thin profile, light weight, and low power consumption thereof and thus have rapidly replaced the conventional cathode ray tube (CRT) displays.

Since such the flat panel display devices use a glass substrate to endure a high heat generated in production processes, there are limits to light weight, thin profile and flexibility.

Thus, a flexible display device, which is manufactured to maintain a display performance intactly using a substrate of a flexible material such as a plastic instead of a conventional glass substrate of non-flexibility even when it is curved or bent like paper, is rising as a next generation flat panel display device.

The flexible display device uses a thin film transistor (TFT) substrate which is made of a plastic material not a glass material, and may be categorized into an unbreakable display of a high durability, a bendable display to be bent without being broken, a rollable display, a foldable display, and the like. The flexible display device has advantages in use of space, interior decoration and design, and may be applied in various fields.

Recently, in order to realize ultra thin profile, light weight and large-sized display area, a bendable or foldable display device which is portable in a folded state and displays images in an unfolded state, has been researched actively.

The bendable or foldable display device may be applied to various devices, for example, a mobile device, such as mobile phone, ultra mobile PC, electronic book, or electronic newspaper, TV, monitor and the like.

A flexible display device, including the bendable or foldable display device, may include a display panel to display images, and a window member located in front of the display panel. The window member may include a polarization layer which serves to prevent a reflection of an external light, a touch film which supplies an input of a user's information through a touch on its screen, and a cover glass which protects the display panel.

In the flexible display device, since the window member is formed thicker than the display panel, for an outer folding with the window member being located outermost in the flexible display device, the window member is difficult to form at a folding region which is subjected to a compressive force and a contractile force a lot in folding and unfolding.

By the folding region where the window member is not formed, the flexible display device is very weak to an external impact.

Further, an image is difficult to display at the folding region, or even though an image is displayed, the image displayed at the folding region is different in brightness and the like from an image displayed at a non-folding region. Thus, the folding region is perceived and this results in reduction of continuity of image, or a touch is not recognized at the folding region and thus a user is inconvenienced.

SUMMARY

Accordingly, the present disclosure is directed to a flexible display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

An advantage of the present invention is to provide a flexible display device which can improve durability. Further, another advantage of the present disclosure is to provide a flexible display device which can solve reduction of continuity of image and no touch recognition due to a folding region. Further, another advantage of the present disclosure is to provide a flexible display device which can have a thin profile. Further, another advantage of the present invention is to provide a flexible display device which can is extremely foldable.

To achieve these and other advantages and in accordance with the purpose of the present disclosure, as embodied and broadly described herein, a flexible display device includes: a display panel which display an image, and includes a first folding region folded or unfolded around a first folding axis, and first and second non-folding regions respectively located at both sides of the first folding region; and a cover window which is on the display panel, and includes a first cover portion having a first hardness and a second cover portion having a second hardness less than the first hardness, wherein the first cover portion is located corresponding to an outer folding portion a display surface of which is exposed to an outside.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The same reference numbers may be used throughout the drawings to refer to the same or like parts.

FIGS. 1A and 1Bare perspective views schematically illustrating an unfolded state and a folded state of a flexible display device according to a first embodiment of the present disclosure.

In this embodiment, a foldable display device is described as a flexible display device100by way of example. This embodiment may be applied to other type display devices, for example, a curved display device, a bendable display device, a rollable display device, a stretchable display device, and the like.

The flexible display device100may be used for a large-sized electronic device, such as TV, an exterior advertising board, or the like, and be used for a small- and medium-sized electronic device, such as a mobile phone, a personal computer, a laptop computer, a personal digital terminal, a vehicle navigator, a game console, a portable electronic device, a wristwatch-style electronic device, a camera, or the like.

Referring toFIG. 1A, the flexible display device100may include a plurality of regions which are defined on a display surface and separated from each other. The flexible display device100may be divided into a display region EA1and EA2and a non-display region NEA. The display region EA1and EA2is a region where an image is displayed, and the non-display region NEA is a region, adjacent to the display region EA1and EA2, where an image is not displayed.

The display region EA1and EA2may be divided into first and second display regions EA1and EA2with respect to a folding axis FX, and the non-display region NEA may be located around the display region EA1and EA2.

The flexible display device100may include a folding region FA which is folded along the folding axis FX, a first non-folding region NFA1, including the first display region EA1, which is not folded, and a second non-folding region NFA2, including the second display region EA2, which is not folded.

Accordingly, referring toFIG. 1B, as the second non-folding region NFA2may be rotated clockwise around the folding axis FX, the flexible display device100may be in an outer folding state such that the first and second display regions EA1and EA2face away from each other i.e., face in opposite outer directions.

The flexible display device100of this embodiment may be extremely folded outwardly, and may have a window member (200ofFIG. 2A) at the folding region FA and the non-folding regions NFA1and NFA2as well. Thus, durability may be improved.

Accordingly, reduction of continuity of image and no touch recognition by a folding region may be solved. Further, the flexible display device may be configured to have a thin profile. This is described in more detail further with reference toFIGS. 2A and 2B.

FIGS. 2A and 2Bare cross-sectional views enlarging a flexible display device according to a first embodiment of the present disclosure.FIGS. 2A and 2Bshow folded state and unfolded state of a flexible display device according to a first embodiment of the present disclosure.

Referring toFIGS. 2A and 2B, the flexible display device100may be in an outer folding state such that the first display region EA1of the first non-folding region NFA1and the second display region EA2of the second non-folding region NFA2face away from each other i.e., face in opposite outer directions.

The flexible display device100may include a display panel110to display an image, and a window member200in front of a display surface of the display panel110. The window member200may include a multi panel140including a touch layer120and a polarization layer130, and a cover window300which serves to protect the display panel110.

For the purpose of explanations, directions in the drawings may be defined such that assuming that a display surface of the display panel110is assumed to face front, the cover window300is located in front of the display panel110, and the multi panel140is located between the display panel110and the cover window300.

The display panel110may use an organic light emitting diode (OLED) panel as a leader of a flexible display panel which can maintain a display performance intactly even when it is curved or bent like paper. In the display panel110configured as the OLED panel, a substrate where a driving thin film transistor (TFT) and a light emitting diode are formed may be encapsulated by a protection film.

The multi panel140may be located on the display panel110. In the multi panel140, the touch layer120may obtain coordinate information of a touch input position.

In the touch layer120, a first touch electrode121and a second touch electrode123may be spaced apart from each other with a dielectric layer125to form a touch sensor.

The polarization layer130may be located on the touch layer120. As the display panel110may use the OLED panel, the polarization layer130may serve to prevent reduction of contrast which is caused by reflection of an external light toward the display panel110.

Since the multi panel140of this embodiment may be configured with the touch layer120and the polarization layer130which are formed integrally, an additional optical adhesive layer may not be required to be formed between the touch layer120and the polarization layer130. Thus, a thickness of the multi panel140may be 40 μm or less. This may be explained in detail later.

The cover window300may be located on the multi panel140including the touch layer120and the polarization layer130to protect the display panel110. The cover window300may protect the display panel110and the multi panel140against an external light, and may transmit a light emitted from the display panel110so that an image produced by the display panel110is seen from the outside.

The cover window300may be divided into a first cover portion310having a first hardness and a second cover portion320having a second hardness less than the first hardness. A surface hardness (i.e., pencil hardness) of the first hardness may be 9H or greater, and a surface hardness of the second hardness may be 7H or less.

The first cover portion310of the first hardness may be formed of a glass material, and particularly, may include a non-alkali glass, and more particularly, may include aluminosilicate glass. The aluminosilicate glass has excellent scratch resistance and damage resistance.

The first cover portion310may have a thickness of 30 μm or less. With a surface hardness of 9H or greater, the first cover portion310may have a prescribed elastic modulus and thus have a flexible property.

In other words, while the first cover portion310may have a high hardness, the first cover portion310may has a flexibility not to form cracks even when it is fully folded.

In case that the first cover portion310may have a thickness of 31 μm or greater, it may be difficult for the first cover portion310to have a flexibility, and thus a folding of the flexible display device may be difficult.

The first cover portion310may have an elastic modulus of 20 GPa to 100 GPa.

The second cover portion320may be formed of one of polymethylmethacrylate (PMMA), polycarbonate (PC), cycloolefin polymer (COP), polyethylene terephthalate (PET), polyimide (PI), or polyaramid (PA).

The second cover portion320may be formed to have a thickness of 30 μm or less in view of a step with the first cover portion310. In order for the thin second cover portion320to have a surface hardness of 7H or greater, the second cover portion320may further include a hard coating layer.

Even though not shown in the drawings, the second cover portion320may be formed with a plurality of layers that include a first layer made of PI and a second layer as a hard coating layer. Alternatively, the second cover portion320may be formed with a single layer that is made of PI and hard coating ingredients contained in the PI base.

The second cover portion320may have an elastic modulus of 20 GPa to 100 GPa. As an elastic modulus may be in inverse proportion to a hardness, the second cover portion320may have a hardness less than that of the first cover portion310so that the second cover portion320may have an elastic modulus greater than that of the first cover portion310.

In order to prevent a surface step produced by the first and second cover portions310and320that are made of different materials, a surface treatment layer330for a surface planarization is further formed on the first and second cover portions310and320.

With prevention of the surface step of the first and second cover portions310and320, the surface treatment layer330may have at least one of an anti-reflection function, an anti-glare function, and an anti-finger function.

The surface treatment layer330may be formed to have a thickness of about 5 μm. In case that the surface treatment layer330may have a thickness over 5 μm, a total thickness of a cover window300including the surface treatment layer330may be great. Thus, in one embodiment the surface treatment layer330has a thickness of about 5 μm that is a minimum to achieve an object and effect as desired.

As the first cover portion310may have a high surface hardness, the first cover portion310might be located over an entire display surface of the flexible display device100to improve durability of the flexible display device100. However, since the first cover portion310of a glass material is more expensive than the second cover portion320, in one embodiment the first cover portion310is formed corresponding to the first non-folding region NFA1and the folding region FA of an outer folding portion of the flexible display device100which an external impact is applied to when folded.

In other words, when an outer folding happens, the outer-folded flexible display device100may be configured to be divided into a non-emission portion, which meets a support portion, such as a table or user's hand, that supports a surface of the flexible display device100, and a main emission portion which provides a user with an image. By a user's touch and the like, the main emission portion gets an external impact more than the non-emission portion.

Accordingly, in the flexible display device100of this embodiment, the first cover portion310of the cover window300may be located at the first non-folding region NFA1and the folding region FA of the outer folding portion of the flexible display device100which corresponds to the main emission portion, and the second cover portion320may be located at the second non-folding region NFA2that may serve as the non-emission region which an external impact is relatively less applied to.

Particularly, regarding the folding region FA, instead of the first cover portion310entirely covering the folding region FA, the second cover portion320being located at a region, out of the folding region FA, where a stress is produced relatively less when folded, may be preferable.

In the flexible display device100, when folded and unfolded, a stress is produced at the folding region FA according to an external force. Ideally, in case that the folding region FA is folded to have a uniform folding radius with respect to the folding axis FX, a stress is most at a center portion O of the folding region FA, and an intensity of a stress decreases in a direction from the folding region FA to the non-folding region NFA1and NFA2.

In the flexible display device100of this embodiment, the first cover portion310may be formed corresponding to the first non-folding region NFA1and the folding region FA as an outer folding portion of the flexible display device100which an external impact is relatively more applied to when folded. Accordingly, the first cover portion310may be located at a region from the first non-folding region NFA1to the center portion O of the folding region FA, and an optimum position of the first cover portion310in a region A from the center portion O of the folding region FA to the second non-folding region NFA2may preferably meet following formulas 1 and 2.

In other words, when a region A may be defined to be divided into 8 equal parts having the same width, since a stress applied to the region A may relatively decrease from a fourth part, a region, in the region A, where the first cover portion310may located, may be defined according to a radius of curvature of the folding region FA by the following formulas 1 and 2.
2πR*¼=1.57R(mm).  Formula 1:

In the formula 1, R is a radius of curvature and is defined in millimeters, and ¼ is a distance from the center portion O of the folding region FA to the second non-folding region NFA2and is a ½ of the folding region FA.
1.57R(mm)*⅝=0.98R(mm).  Formula 2:

In the formula 2, when dividing the region A into 8 equal parts, ⅝ indicates a distance to a ⅝ point from the center portion O of the folding region FA.

By the formulas 1 and 2, the first cover portion310may be located at the first non-folding region NFA1, from the first non-folding region NFA1to the center portion O of the folding region FA, and to 0.98R (mm) from the center portion O of the folding region FA, and other region may be occupied by the second cover portion320.

Accordingly, in the flexible display device100of this embodiment, since the first cover portion310having an elastic modulus and a surface hardness of 9H or greater as well may be located at the first non-folding region NFA1and the folding region FA as the outer folding portion which an external impact is relatively more applied to, the extreme outer folding may be possible, and durability may be improved.

Particularly, the cover window300may be located at the folding region FA and the first and second non-folding regions NFA1and NFA2as well, and thus durability of the flexible display device100may be further improved. Accordingly, reduction of continuity of image and no touch recognition by the folding region FA may be solved.

Further, since the cover window300may be formed with the separated first and second cover portions310and320, production cost may be reduced along the above technical advantages, and thus production efficiency may be improved.

The display panel110and the multi panel140may be attached to each other using the optical adhesive layer150, and the cover window300having the first and second cover portions310and320may be configured integrally with the multi panel140by a roll to roll method or roll to sheet method, and thus the flexible display device100may be modulized integrally.

The optical adhesive layer150may be formed of an optically cleared adhesive (OCA) and at a thickness of 10 μm to 25 μm. In this regard, in case that a thickness of the optical adhesive layer150may be 10 μm or less, an adhesive strength is weak and thus it is difficult to integrally modulize the display panel110and the multi panel140. In case that a thickness of the optical adhesive layer150may be 25 μm or greater, a folding of the flexible display device100is difficult.

Thus, the flexible display device100may have a thin profile.

FIG. 3Ais a perspective view schematically illustrating an unfolded state of a flexible display device according to a second embodiment of the present invention, andFIGS. 3B and 3Care cross-sectional views enlarging a flexible display device according to a second embodiment of the present invention.FIGS. 3B and 3Cshow folded state and unfolded state of a flexible display device according to a second embodiment of the present invention.

FIG. 3Dis a view illustrating a simulation result of measuring stress applied to a folding region (a region A).

Explanations of parts similar to parts of the first embodiment may be omitted.

Referring toFIG. 3A, the flexible display device100may include a plurality of regions which are defined on a display surface and separated from each other. The flexible display device100may be divided into a display region EA1, EA2and EA3and a non-display region NEA. The display region EA1, EA2and EA3is a region where an image is displayed, and the non-display region NEA is a region, adjacent to the display region EA1, EA2and EA3, where an image is not displayed.

The display region EA1, EA2and EA3may be divided into first, second and third display regions EA1, EA2and EA3with respect to first and second folding axes FX1and FX2, and the non-display region NEA may be located around the display region EA1, EA2and EA3.

The flexible display device100may include first and second folding regions FA1and FA2which are folded along the first and second folding axes FX1and FX2respectively, a first non-folding region NFA1, including the first display region EA1, which is not folded, a second non-folding region NFA2, including the second display region EA2, which is not folded, and a third non-folding region NFA3, including the third display region EA3, which is not folded.

Accordingly, referring toFIG. 3B, in the flexible display device100, as the second non-folding region NFA2may be rotated clockwise around the first folding axis FX1, the first and second display regions EA1and EA2face away from each other so that the first and second non-folding regions NFA1and NFA2may be in an outer folding state. Further, as the third non-folding region NFA3may be rotated counterclockwise around the second folding axis FX2, the second and third display regions EA2and EA3face each other so that the second and third non-folding regions NFA2and NFA3may be in an inner folding state.

The flexible display device100of this embodiment may be extremely folded outwardly and inwardly, and may have a window member200at the folding regions FA1and FA2and the non-folding regions NFA1, NFA2and NFA3as well. Thus, durability may be improved.

In other words, the flexible display device100may be folded outwardly around the first folding axis FX1such that the first display region EA1of the first non-folding region NFA1and the second display region EA2of the second non-folding region NFA2face away from each other, and may also be folded inwardly around the second folding axis FX2such that the second display region EA2of the second non-folding region NFA2and the third display region EA3of the third non-folding region NFA3face each other.

A region where a display surface of the flexible display device100is exposed to the outside when folded may be defined as an outer folding portion, and a region where a display surface of the flexible display device100is located at an inner side when folded may be defined as an inner folding portion.

The flexible display device100may include a display panel110to display an image, and a window member200in front of a display surface of the display panel110. The window member200may include a multi panel140including a touch layer120and a polarization layer130, and a cover window300which serves to protect the display panel110.

For the purpose of explanations, directions in the drawings may be defined such that assuming that a display surface of the display panel110is assumed to face front, the cover window300is located in front of the display panel110, and the multi panel140is located between the display panel110and the cover window300.

The display panel110may use a liquid crystal display (LCD) panel, a plasma display panel (PDP), a field emission display (FED) panel, an electroluminescence display (ELD) panel, or an organic light emitting diode (OLED) panel. In one embodiment, the display panel110may use an organic light emitting diode (OLED) panel as a leader of a flexible display panel which can maintain a display performance intactly even when it is curved or bent like paper.

The OLED panel is a self-luminescent type display panel and does not require a backlight unit used for an LCD device which is not a non-self luminescent type display device. Thus, the OLED panel may have a light weight and a thin profile.

In addition, the OLED panel may have advantages in viewing angle, contrast ratio, and power consumption as compared with the LCD panel. Furthermore, the OLED panel may be driven with a low direct current (DC) voltage and have rapid response speed. Moreover, since inner elements of the OLED panel have a solid phase, the OLED panel may have high durability against an external impact and have a wide available temperature range.

Specifically, since the OLED panel is manufactured through a simple process, manufacturing costs may be reduced as compared with the LCD panel.

In the display panel110configured as the OLED panel, a substrate where a driving thin film transistor (TFT) and a light emitting diode are formed may be encapsulated by a protection film.

Even though not shown in the drawings, the driving TFT may be formed in each pixel region on the substrate, a first electrode may be formed on the driving TFT and be connected to each driving TFT, an organic light emitting layer emitting its specific color light may be formed on the first electrode, and a second electrode may be formed on the organic light emitting layer.

The first and second electrode and the organic light emitting layer between the first and second electrodes form an organic light emitting diode. The first electrode may serve as an anode, and the second electrode may serve as a cathode.

The multi panel140may be located on the display panel110. In the multi panel140, the touch layer120may obtain a coordinate information of a touch input position.

In the touch layer120, a first touch electrode121and a second touch electrode123may be spaced apart from each other with a dielectric layer125to form a touch sensor. When an input means such as a finger or pen contacts a position of the touch sensor, the first and second touch electrodes may be electrically connected to each other, a voltage changed by a resistance of the touched position may be read out, and a control device may find a coordinate of the position according to the change of voltage.

The first and second touch electrodes121and123may include a transparent conductive oxide material, for example, ITO, IZO, ZnO or ITZO. Alternatively, the first and second touch electrodes121and123may include a metal material, for example, Mo, Ag, Ti, Cu, Al or an alloy thereof.

The polarization layer130may be located on the touch layer120. As the display panel110may use the OLED panel, the polarization layer130may serve to prevent reduction of contrast which is caused by reflection of an external light toward the display panel110.

In other words, by locating the polarization layer130, which may block an external light, in a transmission direction of a light which is emitted from the organic light emitting layer in a driving mode of the display panel110using the OLED panel to display an image, a contrast may be improved.

The polarization layer130may be formed using a circular polarization plate, which may include a phase retardation layer and a linear polarization layer. The phase retardation layer and the linear polarization layer may be stacked in such an order that the linear polarization layer is located close to an incidence side of an external light and the phase retardation layer is located at an inner side of the linear polarization layer.

The phase retardation layer may be configured using a quarter wave plate (QWP) having a λ/4 retardation value. The linear polarization layer has a polarizing axis to linearly polarize a light in a direction of the polarizing axis.

In detail, the linear polarization layer transmits a light parallel with the polarizing axis, and absorbs a light not parallel with the polarizing axis. Accordingly, when a light passes through the linear polarization layer, a light is linearly polarized in a direction of the polarizing axis.

Since the multi panel140of this embodiment may be configured with the touch layer120and the polarization layer130which are formed integrally, an additional optical adhesive layer may not be required to be formed between the touch layer120and the polarization layer130. Thus, a thickness of the multi panel140may be 40 μm or less. This may be explained in detail later.

The cover window300may be located on the multi panel140including the touch layer120and the polarization layer130to protect the display panel110. The cover window300may protect the display panel110and the multi panel140against an external light, and may transmit a light emitted from the display panel110so that an image produced by the display panel110is seen from the outside.

The cover window300may be divided into a first cover portion310having a first hardness and a second cover portion320having a second hardness less than the first hardness. A surface hardness (i.e., pencil hardness) of the first hardness may be 9H or greater, and a surface hardness of the second hardness may be 7H or less.

The first cover portion310of the first hardness may be formed of a glass material, and particularly, may include a non-alkali glass, and more particularly, may include aluminosilicate glass. The aluminosilicate glass has excellent scratch resistance and damage resistance.

The first cover portion310may have a thickness of 30 μm or less. With a surface hardness of 9H or greater, the first cover portion310may have a prescribed elastic modulus and thus have a flexible property.

In other words, while the first cover portion310may have a high hardness, the first cover portion310may has a flexibility not to form cracks even when it is fully folded.

R means a curvature and may be defined as a radius of curvature that indicates a curvature of an arc of a circle having a radius of R. 1R mean an arc of a circle having a radius of 1 mm.

In case that the first cover portion310may have a thickness of 31 μm or greater, it may be difficult for the first cover portion310to have a flexibility, and thus a folding of the flexible display device may be difficult.

The first cover portion310may have an elastic modulus of 20 GPa to 100 GPa.

The second cover portion320may be formed of one of polymethylmethacrylate (PMMA), polycarbonate (PC), cycloolefin polymer (COP), polyethylene terephthalate (PET), polyimide (PI), or polyaramid (PA).

The second cover portion320may be formed to have a thickness of 30 μm or less in view of a step with the first cover portion310. In order for the thin second cover portion320to have a surface hardness of 7H or greater, in one embodiment the second cover portion320further includes a hard coating layer.

Even though not shown in the drawings, the second cover portion320may be formed with a plurality of layers that include a first layer made of PI and a second layer as a hard coating layer. Alternatively, the second cover portion320may be formed with a single layer that is made of PI and hard coating ingredients contained in the PI base.

The second cover portion320may have an elastic modulus of 20 GPa to 100 GPa. As an elastic modulus may be in inverse proportion to a hardness, the second cover portion320may have a hardness less than that of the first cover portion310so that the second cover portion320may have an elastic modulus greater than that of the first cover portion310.

In order to prevent that a surface step is produced by the first and second cover portions310and320that are made of different materials, a surface treatment layer330for a surface planarization is further formed on the first and second cover portions310and320.

With prevention of the surface step of the first and second cover portions310and320, the surface treatment layer330may have at least one of an anti-reflection function, an anti-glare function and an anti-finger function.

The surface treatment layer330may be formed to have a thickness of about 5 μm. In case that the surface treatment layer330may have a thickness over 5 μm, a total thickness of a cover window300including the surface treatment layer330may be great. Thus, the surface treatment layer330has a thickness of about 5 μm that is a minimum to achieve an object and effect as desired.

The flexible display device100of this embodiment may be configured to include an outer folding portion where a display surface is exposed to the outside when folded and an inner folding portion where a display surface is located at an inner side when folded. Accordingly, the first cover portion310of a first hardness may be located corresponding to the outer folding portion, and the second cover portion320of a second hardness less than the first hardness may be located corresponding to the inner folding portion.

In detail, when the flexible display device100is folded, stresses in opposite directions are respectively applied to the outer folding portion and the inner folding portion.

In other words, with respect to a front surface (i.e., a display surface) of the flexible display device100, a tensile stress is applied to the outer folding portion while a compressive stress is applied to the inner folding portion.

When the opposite stresses are applied, a TFT of the display panel110may be damaged or cracks may be generated at a signal line of the display panel110connected to each pixel region, and delamination between layers of the display panel110may be produced. Further, cracks may be generated at the touch electrodes121and123or a signal line of the touch layer120.

The tensile stress may more affect generation of such the crack. Thus, in order to offset the tensile stress, the cover window300having a high elastic modulus may be located at the front surface of the flexible display device100.

In this regard, in the flexible display device100of this embodiment, since the outer folding portion is entirely exposed to the outside, an external impact is relatively more applied to the outer folding portion. Thus, the outer folding portion may need a high elastic modulus and a high hardness as well.

Accordingly, the first cover portion310having a first hardness may be located at the outer folding portion, and the second cover portion320having a relatively greater elastic modulus may be located at the inner folding portion.

Since the second cover portion320is cheaper than the first cover portion310, the first cover portion310may be formed corresponding to the outer folding portion of the flexible display device100. Thus, production cost may be reduced, and as a result, production efficiency may be improved.

Particularly, regarding the outer folding portion, instead of the first cover portion310entirely covering the first folding region FA1, the second cover portion320extending over a region, out of the folding region FA, where a stress is produced relatively less when folded, may be preferable.

A stress means a resistance force generated in a material corresponding to a magnitude of a load (i.e., an external force), such as compression, tension, bending, twisting or the like, when the load is applied to the material.

A stress increases as an external force increases, but there is a limit to a stress. When a stress reaches an inherent limit of a material, a material is resistless to an external force and is destroyed finally. A unit stress means a stress per a unit area, and hereinafter, a stress means a unit stress.

In the flexible display device100, when folded and unfolded, stresses are produced at the first and second folding regions FA1and FA2according to an external force. Ideally, in case that the first and second folding regions FA1and FA2are folded to each have a uniform folding radius with respect to the first and second folding axes FX1and FX2, stresses are most at center portions O of the first and second folding regions FA1and FA2, and intensities of stresses decrease in a direction from the first and second folding regions FA1and FA2to the first to third non-folding regions NFA1to NFA3.

This refers toFIG. 3D, in which a decrease of stress occurs from the bottom of the figure towards the top of the figure.

In the flexible display device100of this embodiment, the first cover portion310may be formed corresponding to the first non-folding region NFA1and the first folding region FA1as an outer folding portion of the flexible display device100which an external impact is relatively more applied to when folded. Accordingly, the first cover portion310may be located at a region from the first non-folding region NFA1to the center portion O of the first folding region FA1, and an optimum position of the first cover portion310in a region A from the center portion O of the first folding region FA1to the second non-folding region NFA2may preferably meet following formulas 3 and 4.

In other words, when a region A may be defined to be divided into 8 equal parts having the same width, since a stress applied to the region A may relatively decrease from a fourth part, a region, in the region A, where the first cover portion310may located, may be defined according to a radius of curvature of the first folding region FA1by the following formulas 3 and 4.
2πR*¼=1.57R(mm).  Formula 3:

In the formula 3, R is a radius of curvature and is defined in millimeters, and ¼ is a distance from the center portion O of the first folding region FA1to the second non-folding region NFA2and is a ½ of the first folding region FA1.
1.57R(mm)*⅝=0.98R(mm).  Formula 4:

In the formula 4, when dividing the region A into 8 equal parts, ⅝ indicates a distance to a ⅝ point from the center portion O of the first folding region FA1.

By the formulas 3 and 4, the first cover portion310may be preferably located at the first non-folding region NFA1, from the first non-folding region NFA1to the center portion O of the first folding region FA1, and to 0.98R (mm) from the center portion O of the first folding region FA1, and other region may be occupied by the second cover portion320.

Accordingly, in the flexible display device100of this embodiment, since the first cover portion310having a prescribed elastic modulus and a surface hardness of 9H or greater as well may be located at the first non-folding region NFA1and the first folding region FA1as the outer folding portion which an external impact is relatively more applied to, the extreme outer folding may be possible, and durability may be improved.

The cover window300having a high hardness of at least 7H or at least 9H has an excellent scratch resistance. A scratch resistance of the cover window300is evaluated by measuring a number of scratches which occurs at a surface of the cover window300when a steel wool travels back and forth under a load of 500 gf/cm2on the cover window300. The evaluation result is that even when travelling back and forth is conducted 3000 or more times, scratches does not occur and it is confirmed that the cover window300has an excellent mechanical property of a scratch resistance and the like.

Accordingly, the flexible display device100of this embodiment may have an improved durability by the cover window300.

Particularly, the cover window300may be located at the first and second folding regions FA1and FA2and the first to third non-folding regions NFA1to NFA3as well, and thus durability of the flexible display device100may be further improved. Accordingly, reduction of continuity of image and no touch recognition by the first and second folding regions FA1and FA2may be solved.

Further, since the cover window300may be formed with the separated first and second cover portions310and320, production cost may be reduced along the above technical advantages, and thus production efficiency may be improved.

The display panel110and the multi panel140may be attached to each other using the optical adhesive layer150, and the cover window300having the first and second cover portions310and320may be configured integrally with the multi panel140by a roll to roll method or roll to sheet method, and thus the flexible display device100may be modulized integrally.

The optical adhesive layer150may be formed of an optically cleared adhesive (OCA) and at a thickness of 10 μm to 25 μm. In this regard, in case that a thickness of the optical adhesive layer150may be 10 μm or less, an adhesive strength is weak and thus it is difficult to integrally modulize the display panel110and the multi panel140. In case that a thickness of the optical adhesive layer150may be 25 μm or greater, a folding of the flexible display device100is difficult.

Thus, the flexible display device100may have a thin profile.

In other words, in the flexible display device100, the multi-panel140may be formed at a thickness of about 40 μm, the cover window300may be formed at a thickness of about 30 μm, the surface treatment layer330may be formed at a thickness of about 5 μm, and the optical adhesive layer150may be formed at a thickness of at most 25 μm. Thus, the window member200of the flexible display device100of this embodiment may be formed at a total thickness of about 100 μm or less.

Particularly, the window member200may have an excellent scratch resistance because the cover window300has a high hardness of at least 7H or at least 9H, and may have a thin profile at a total thickness of about 100 μm or less.

Accordingly, the flexible display device100may realize a thin profile, and further, may be extremely folded inwardly and outwardly.

The flexible display device100including the window member200having a thickness of about 100 μm or less may have a radius of curvature up to 1R when inwardly folded, and thus an extreme inward folding may be possible. Such the flexible display device100may have a radius of curvature up to 3R when outwardly folded, and thus an extreme outward folding may be possible.

FIG. 4is a view schematically illustrating processes of manufacturing a window member according to a second embodiment of the present invention, andFIG. 5is a cross-sectional view illustrating a general window member.

Referring toFIG. 4, processes of manufacturing a window member of the flexible display device of this embodiment may roughly include a step ST1of forming a multi panel and a step ST2of forming a cover window.

In the step ST1of forming the multi panel, a touch layer120may be provided over a polarization layer130, which is wound around a roller, in a roll to roll method, a UV adhesive401amay be coated between the polarization layer130and the touch layer120, and a lamination step may be conducted using a first laminating roller403a.

Through the lamination step, the polarization layer130and the touch layer120are attached to each other. In the touch layer120, a plurality of first touch electrodes (121ofFIG. 3C) may be located on a base film405and be spaced apart from each other, a dielectric layer (125ofFIG. 3C) may be located on the plurality of first touch electrodes, and a plurality of second touch electrode (123ofFIG. 3C) may be located on the dielectric layer and be arranged alternately with the plurality of first touch electrodes.

The polarization layer130and the touch layer120attached to each other may be placed in a first oven OVEN1and a drying step may be conducted. Then, the base film405of the touch layer120may be wound and removed from the touch layer120, and thus the multi panel140may be formed finally.

The step ST2of forming a cover window may be conducted. A first cover portion310may be provided over a region of the multi panel140, an adhesive401bmay be coated between the multi panel140and the first cover portion310, and a lamination step may be conducted using a second laminating roller403b.

Then, the multi panel140attached to the first cover portion310may be placed in a second oven OVEN2and a drying step may be conducted. Then, a second cover portion320may be provided over other region of the multi panel140, where the first cover portion310is not formed, and the second cover portion320and the multi panel140may be attached to each other.

In this regard, a lamination step may be conducted using a third laminating roller403cfor the second cover portion320and the multi panel140so that the second cover portion320and the multi panel140may be attached to each other. Thus, both of the first and second cover portions310and320may be finally attached onto the multi panel140.

A surface treatment step may be conducted for a top of the first and second cover portions310and320attached to the multi panel140so that a surface treatment layer330may be formed on the first and second cover portions310and320.

Lastly, even though not shown in the drawings, a cutting step may be conducted suitably to a size of the flexible display device (100ofFIG. 3C). Accordingly, the window member (200ofFIG. 2C) of this embodiment may be manufactured.

Since the multi panel140of this embodiment may be configured with the polarization layer130and the touch layer120which are formed integrally through the lamination step using the roll to roll method, the multi panel140may be formed at a thickness of 40 μm or less.

Referring toFIG. 5, a general touch member500includes a general touch panel510and a polarization layer520. The general touch panel510includes a first touch film511where a first touch electrode511ais formed, and a second touch film513where a second touch electrode513ais formed, and the touch panel510has a thickness of at least 40 μm.

The polarization layer520has a thickness of at least 50 μm, and an optical adhesive layer530having a thickness of 10 μm to 50 μm is interposed between the polarization layer520and the touch panel510. The general window member500only including the polarization layer520and the touch panel510has a thickness of at least 100 μm.

In addition to the above components, the window member200further includes a cover window, an optical adhesive layer to attach the cover window and the polarization layer520, and an optical adhesive layer to attach the touch panel510and a display panel. Thus, the general window member usually has a thickness of 300 μm or greater.

On the other hand, the flexible display device of this embodiment may provide the window member (200ofFIG. 3C) having a high hardness and a total thickness of about 100 μm or less as well.

Accordingly, the flexible display device of this embodiment may realize a thin profile, and further, may be extremely folded inwardly and outwardly.

The flexible display device100including the window member200having a thickness of about 100 μm or less may have a radius of curvature up to 1R when inwardly folded, and thus an extreme inward folding may be possible. Such the flexible display device100may have a radius of curvature up to 3R when outwardly folded, and thus an extreme outward folding may be possible.

As described above, in the flexible display device of this embodiment, the first cover portion having an elastic modulus and a surface hardness of 9H or greater may be located at the first non-folding region and the first folding region as the outer folding portion which an external impact is relatively more applied to, and the second cover portion having a relatively greater elastic modulus may be located at the inner folding portion. Thus, the flexible display device capable of being extremely folded inwardly and outwardly and having an improved durability may be provided.

Particularly, the cover window may be located at the first and second folding regions and the first to third non-folding regions as well, and thus durability of the flexible display device may be further improved. Accordingly, reduction of continuity of image and no touch recognition by the first and second folding regions may be solved.

Further, since the cover window may be formed with the separated first and second cover portions, production cost may be reduced along the above technical advantages, and thus production efficiency may be improved.

Further, the window member including the cover window may be configured to have an integral structure by a roll to roll method or roll to sheet method. Thus, the flexible display device having a thin profile may be provided.