If necessary, optionally, each of the plurality of grooves G may continuously extend to the non-display area across the display area DA without being disconnected, along the X-axis direction which is the same as the folding axis.

Even though in the drawing, the cross-sectional shape of each of the plurality of grooves G is a rectangular, is not limited thereto. The cross-sectional shape of each of the plurality of grooves G may be formed with various shapes such as a polygonal shape other than a semi-circular shape or a quadrangular shape.

In the related art, in order to disperse the folding stress concentrated on the folding area, opening patterns, that is, a plurality of holes is formed on the bottom plate. In this case, the folding stress is dispersed to satisfy the folding reliability, but the visibility of the opening pattern is increased so that the appearance quality is deteriorated.

The flexible display device 100 according to the exemplary embodiment of the present disclosure forms grooves on the upper surface of the bottom plate 110 so that the visibility of the pattern is reduced to improve the appearance quality. That is, the plurality of grooves G is formed with a predetermined depth from the upper surface of the bottom plate 110 in a thickness direction (a Z-axis) of the bottom plate 110. Therefore, the recognition of the plurality of grooves G formed in the folding area FA by the user is reduced so that the appearance quality may be improved.

The plurality of grooves G may be formed by removing a part of a material which forms the bottom plate 110. For example, the plurality of grooves G may be formed by a known method such as photolithography, laser etching, or plasma etching, but is not limited thereto.

A filler 111 is filled in each of the plurality of grooves G.

When the plurality of grooves G is formed in the bottom plate 110 which overlaps the folding area FA, the stress during the folding is dispersed so that the foldability is improved. However, a step is formed so that the flatness is degraded. Accordingly, problems such as separation or cracks of the components are caused due to the stress irregularity during the folding so that the folding reliability is not satisfied and it is restricted to implement a flexible display device having a larger curvature. Further, when the flatness of the folding area FA is degraded, the distortion of the image which is displayed in the folding area FA is caused to deteriorate the display quality.

Therefore, the plurality of grooves G is filled with the filler 111 to reduce the step and reduce the degradation of the flatness. At least a part of each of the plurality of grooves G is filled with the filler 111. A thickness of the filler 111 which is filled in the plurality of grooves G may be adjusted as needed. For example, in order to reduce the visibility of the step and the groove pattern and maintain the folding reliability to be high, the filler 111 is filled in the plurality of grooves G to be equal to or higher than 0.5 times the depth of each of the plurality of grooves, but is not limited thereto.

For example, the filler 111 may be a soft polymer or a solder material.

For example, the filler 111 is a polymer which is curable by heat or UV light and is selected from an acrylic resin, a urethane resin, and a silicon resin, but is not limited thereto.

For example, the solder material may be formed from a liquid solder or a solder paste in which conductive particles are uniformly dispersed in the binder resin. For example, the binder resin may be selected from an epoxy-based resin, a urethane-based resin, an acrylic resin, a silicone-based resin, a phenol-based resin, a melamine-based resin, an alkyd-based resin, a urea resin, and an unsaturated polyester resin. For example, the conductive particles may be selected from an alloy of tin and one or more metals selected from silver, copper, lead, bismuth, zinc, and indium.

Referring to FIG. 3, a plurality of nano helix structures 112 is disposed on the filler 111. The nano helix structure 112 minimizes the degradation of the flatness by filling the step caused by the plurality of grooves G without degrading the foldability so that the plurality of groove (G) patterns is not visible from the outside of the flexible display device 100. Therefore, the appearance quality of the flexible display device will be improved.

The nano helix structure 112 has a spring or coil shape and has a complex shape as compared with another three-dimensional nano structure such as a nano rod, a nano wire, or a nano tube. The nano helix structure is disposed on the filler 111 so that the recognition of the plurality of grooves G formed on the bottom plate 110 from the outside of the flexible display device 100 may be more effectively reduced.

The plurality of nano helix structures 112 is disposed on the filler 111 so as to correspond to each of the plurality of grooves G. The plurality of nano helix structures 112 is disposed in each of the plurality of grooves G. Each of the plurality of nano helix structures 112 is disposed toward the thickness direction of the bottom plate 110, that is, the Z-axis direction from the upper surface of the filler 111. Accordingly, each of the plurality of nano helix structures 112 is disposed so as to be directed to the lower surface of the back plate 120 from the upper surface of the filler 111.

Referring to FIGS. 2 and 3, the nano helix structures 112 are disposed on the filler 111 which is filled in each of the plurality of grooves G with a zigzag pattern. That is, the plurality of nano helix structures 112 is disposed on the filler 111 filled in each of the plurality of grooves G with a zigzag pattern in the X-axis direction and the Y-axis direction. As described above, when the plurality of nano helix structures 112 is disposed with a zigzag pattern, the step filling effect is maximized while maintaining a high foldability to minimize the recognition of the groove pattern, but is not limited thereto. If a degree of recognizing the plurality of groove (G) patterns is not so high, the plurality of nano helix structures 112 may be randomly disposed or regularly disposed in a line.

Unlike the general nano structures, the nano helix structure has a characteristic that when a magnetic field or an electric field is applied, the nano helix structure is aligned in a regular direction. Accordingly, the plurality of nano helix structures 112 is regularly aligned in the thickness direction (Z-axis direction) of the bottom plate 110. Hereinafter, a physical property of the nano helix structure 112 will be described in detail with reference to FIGS. 4, 5A, and 5B. FIG. 4 is a view for explaining a characteristic of the nano helix structures which are aligned by a magnetic field or an electric field. FIG. 5A is an enlarged cross-sectional view illustrating a state in which an alignment of nano helix structures is deformed. FIG. 5B is an enlarged cross-sectional view illustrating a state in which an alignment of nano helix structures is restored.

Referring to FIG. 4, when a magnetic field or an electric field is applied, the nano helix structures 112 are regularly aligned in a single direction. When a magnetic field or an electric field is applied, the nano helix structures 112 are aligned in the applied electric field or magnetic field. For example, when the magnetic field is applied to the nano helix structures 112, the nano helix structures are aligned from the N pole to the S pole. As another example, an electric field is applied to the nano helix structures 112, the nano helix structures 112 are aligned in a direction from a positive terminal (+) to a negative terminal (−).

Even though the regular alignment of the nano helix structures 112 is disordered by the external environment, if an electric field or a magnetic field is applied, the nano helix structures are regularly disposed again. Referring to FIG. 5A, the plurality of nano helix structures 112 regularly disposed in each of the plurality of grooves G may be disordered when the flexible display device 100 is used. In this case, when a magnetic field is applied from the outside by wireless charging, as illustrated in FIG. 5B, the plurality of nano helix structures 112 may be regularly aligned. That is, as the flexible display device 100 is used, even though the nano helix structures 112 are disordered, the alignment is restored by the external magnetic field. Accordingly, the appearance quality is consistently maintained to be high.

For example, the nano helix structure 112 may include one or more materials selected from zinc oxide, magnesium oxide, and carbon nano tube, but is not limited thereto. Among them, the zinc oxide is easy to be manufactured as the nano helix structure 112 and has an excellent degree of dispersion. Further, the zinc oxide nano-helix structure 112 has advantages in that the reactivity to an external magnetic field is excellent and the restoring force of the alignment is also excellent. For reference, even though the nano helix structure 112 including magnesium oxide may reduce a visibility of the groove pattern, a ratio of defects is relatively high when the nano helix structure 112 is manufactured so that a magnetic susceptibility is relatively low. Even though the nano helix structure 112 including carbon nano tube may reduce a visibility of the groove pattern and a ratio of defects is relatively low when the nano helix structure 112 is manufactured, a magnetic susceptibility and a degree of dispersion are relatively low. Accordingly, in order to easily align the nano helix structure 112, reduce the visibility of the pattern, and maintain the appearance quality to be high for a long term, the nano helix structure 112 including zinc oxide may be most desirable.

For example, an average length of the nano helix structure 112 may be 3 μm to 13 μm, and desirably, 7 μm to 10 μm. In this case, it is susceptible to the external magnetic field so that it is easy to regularly align the nano helix structures 112 and when the alignment is deformed, the restoring force is also excellent. Accordingly, the visibility of the groove pattern is further reduced and the appearance quality is further improved.

For example, the nano helix structure 112 may be manufactured by various known methods such as a melt spinning method, a method of growing a nano helix structure 112 on a substrate, and a manufacturing method using a template. Desirably, the nano helix structure 112 may be manufactured by the melt spinning method, which has advantages in that it is easy to control a thickness or a length of the nano helix structure 112 and the mass production is possible. Specifically, the melting spinning method is performed by melting, spinning, and cooling materials such as zinc oxide, magnesium oxide, and carbon nano tube. The spun nano structure has a wire shape. If the spun nano structure is rapidly cooled, the spun nano structure is rolled into a spring form so that the nano helix structure is obtained. At this time, a rotation speed of a wheel from which the material is spun, a cooling speed, and a rotation radius of the wheel are controlled to obtain desired thickness and length.

For example, a weight ratio of the filler 111 to the nano helix structure 112 included in each of the plurality of grooves G may be 90:10 to 99:1. Desirably, a weight ratio of the filler 111 to the nano helix structure 112 included in each of the plurality of grooves G may be 97:3 to 93:7 or 96:4 to 93:7. Within this range, as the nano helix structure 112 is included, the effect of reducing the visibility of the groove pattern is more excellent and the restoring force is also excellent so that the appearance quality is excellent.

The adhesive layer Adh is disposed between the bottom plate 110 and the back plate 120. The adhesive layer Adh bonds the bottom plate 110 and the back plate 120. Further, the adhesive layer Adh is filled in an empty space of the plurality of grooves G to be filled in the step. Accordingly, the adhesive layer Adh is disposed to be in direct contact with the filler 111 and the plurality of nano helix structures 112. The adhesive layer Adh may be selected from an optical clear adhesive (OCA), an optical clear resin (OCR), and a pressure sensitive adhesive (PSA), but is not limited thereto.

In order to ensure the foldability, a low modulus adhesive is used to form the adhesive layer. In this case, there is a problem in that the plurality of groove patterns is more seriously visible from the outside of the flexible display device. According to the exemplary embodiment of the present disclosure, in each of the plurality of grooves G, the filler 111 and the plurality of nano helix structures 112 are disposed so that even though the adhesive layer Adh is formed with the low modulus adhesive, the recognition of the plurality of groove (G) patterns is minimized. Accordingly, according to the exemplary embodiment of the present disclosure, the flexible display device 100 with the excellent foldability and reliability and the excellent appearance quality may be provided.

Hereinafter, a method of manufacturing a bottom plate according to an exemplary embodiment of the present invention will be described with reference to FIGS. 6 to 9. FIGS. 6 to 9 are schematic cross-sectional process views for explaining a manufacturing method of a bottom plate in a flexible display device according to an exemplary embodiment of the present disclosure.

First, a bottom plate 110′ is prepared and a plurality of grooves G is formed in an area corresponding to a folding area FA. As described above, the plurality of grooves G may be formed by a method selected from photolithographic, laser etching, and plasma etching processes.

Next, each of the plurality of grooves G is filled with the filler 111. Specifically, a mask on which an opening pattern corresponding to the plurality of grooves G is formed is covered on the bottom plate 110′ and the filler 111 is filled in the plurality of grooves G and cured. For example, the filler 111 may be a polymer, a liquid solder, or a solder paste which is cured by heat or light. As another example, the filler 111 may be formed by a micro spot soldering process which locally applies a liquid solder or a solder paste in a selected area.

Next, a magnetic field or an electric field is applied to a magnetic reactive film 190 including a plurality of protrusions 191 to attach the nano helix structure 112 to each of the plurality of protrusions 191. The plurality of nano helix structures 112 which is manufactured by the melt-spinning method is randomly aligned and has an electromagnetic connectivity. Accordingly, in order to dispose the plurality of nano helix structures 112 in a specific area to have a regular alignment, the magnetic reactive film 190 is used. The magnetic reactive film 190 includes a plurality of protrusions 191. When the electric field or the magnetic field is applied to the magnetic reactive film 190, the nano helix structure 112 is attached to each protrusion 191 by the electromagnetic interaction.

The magnetic reactive film 190 is a film including a magnetic material. For example, the magnetic reactive film 190 may be a plastic film in which Fe-based magnetic particles are dispersed. Specifically, the magnetic reactive film 190 may be a polysiloxane based film in which carbonyl iron particles are dispersed. The carbonyl iron is a magnetic material so that the magnetic reactive film 190 including the carbonyl iron has a magnetic reactivity. When the magnetic field is not applied from the outside, the protrusion 191 does not protrude from the surface of the magnetic reactive film 190, but is folded to be in contact with the magnetic reactive film 190. When the magnetic field is applied to the magnetic reactive film 190, an electric dipole is formed at an end of the protrusion 191 so that as illustrated in FIG. 8, the protrusion 191 is deformed in an upright structure perpendicular to the surface of the magnetic reactive film 190, as the same direction as the applied magnetic field. The nano helix structure 112 is attached to each of the protrusion 191 of the magnetic reactive film 190 by an induced magnetic field between the electric dipole formed in the protrusion 191 and the nano helix structure 112.

Next, the nano helix structure 112 attached to each of the plurality of protrusions 191 is transferred onto the filler 111. After placing the magnetic reactive film 190 attached with the nano helix structure 112 on the bottom plate 110, the nano helix structure 112 attached to the plurality of protrusions 191 is transferred onto the filler 111. When the magnetic field applied to the magnetic reactive film 190 is removed after disposing the magnetic reactive film 190 attached with the nano helix structure 112 in a desired position, the nano helix structure 112 is transferred onto the filler 111.

At this time, in order to minimize the recognition of the plurality of grooves G from the outside of the flexible display device 100, the nano helix structure 112 may be transferred onto the filler 111 filled in each of the plurality of grooves G to be disposed with a zigzag pattern. In the meantime, the nano helix structure 112 is disposed on the filler 111 by selectively attaching and transferring the nano helix structure 112 in the protrusion 191 formed on the magnetic reactive film 190. Accordingly, the distribution and the shape of the protrusions 191 formed in the magnetic reactive film 190 may be formed to correspond to a desired alignment of the nano helix structure 112.

Next, the electric field or the magnetic field is applied to the bottom plate 110, the nano helix structures 112 are aligned in a single direction. As described above with reference to FIG. 4, the nano helix structures 112 have a characteristic of being aligned in a single direction when the electric field or the magnetic field is applied. Accordingly, after transferring the nano helix structures 112 onto the filler 111, the electric field or the magnetic field is applied in the thickness direction of the bottom plate 110 to align the plurality of nano helix structures in the thickness direction of the bottom plate 110. After transferring the nano helix structures 112 onto the filler 111 using the magnetic reactive film 190, if the nano helix structures 112 maintain a sufficiently regularly aligned state, this step may be omitted.

The bottom plate 110 manufactured as described above has an advantage of an excellent appearance quality because the filler 111 and the plurality of nano helix structures 112 are filled in the plurality of grooves G. Therefore, the shape of the plurality of grooves G formed in the bottom plate 110 is not visible to the user while maintaining the foldability and the reliability of the flexible display device 100 to be high.

Hereinafter, the effects of the present disclosure will be described in more detail with reference to Examples and Comparative Examples. However, the following Examples are set forth to illustrate the present disclosure, but the scope of the disclosure is not limited thereto.

Example 1

A plurality of grooves was formed in a metal bottom plate. A silicon based resin was filled in each of the plurality of grooves. A zinc oxide nano helix structure (an average length of 9 μm) was transferred onto the filler using a magnetic reactive film. At this time, a weight ratio of the filler and the nano helix structure was controlled to be 98:2. FIG. 10 is an SEM image illustrating a zinc oxide nano helix structure transferred onto a surface of the filler on a bottom plate according to Example 1. Referring to FIG. 10, it was confirmed that the zinc oxide nano helix structures were uniformly transferred onto a surface of the filler. Next, an electric field was applied in a thickness direction of the metal bottom plate to align the nano helix structures in the thickness direction of the metal bottom plate. The bottom plate with the structure as illustrated in FIGS. 2 and 3 was manufactured by means of this process.

Example 2

The bottom plate was manufactured by the same method as Example 1 except that a weight ratio of the filler and the nano helix structure was changed to 96:4.

Example 3

The bottom plate was manufactured by the same method as Example 1 except that a weight ratio of the filler and the nano helix structure was changed to 94:6.

Example 4

The bottom plate was manufactured by the same method as Example 1 except that a weight ratio of the filler and the nano helix structure was changed to 92:8.

Example 5

The bottom plate was manufactured by the same method as Example 1 except that a zinc oxide nano helix structure having an average length of 5 μm was used.

Example 6

The bottom plate was manufactured by the same method as Example 1 except that a zinc oxide nano helix structure having an average length of 11 μm was used.

Comparative Example 1

The bottom plate was manufactured by the same method as Example 1 except that the nano helix structure was not disposed on the filler. That is, the bottom plate according to Comparative Example 1 was formed such that only the filler was filled in each of the plurality of grooves.

Comparative Example 2

The bottom plate was manufactured by the same method as Example 1 except that a silicon based resin in which silver (Ag) nano particles (a diameter was 100 nm to 200 nm) were dispersed was filled in each of the plurality of grooves and the nano helix structure was not aligned on the filler. At this time, a weight ratio of the silicon based resin and the silver nano particles was controlled to be 98:2.

Comparative Example 3

The bottom plate was manufactured by the same method as Example 1 except that a silicon based resin in which silver (Ag) nano wires (a length was 300 nm to 500 nm) were dispersed was filled in each of the plurality of grooves and the nano helix structure was not aligned on the filler. At this time, a weight ratio of the silicon based resin and the silver nano wires was controlled to be 98:2.

Experimental Example 1

A visibility of the patterns of the bottom plates prepared according to Example 1 and Comparative Examples 1 to 3 to the user was evaluated. The visibility of the pattern was evaluated by preparing a specimen by laminating a display panel on the bottom plate, optically obtaining a contour graph for a surface of the display panel using Optimap surface analysis equipment, and then measuring a height deviation therefrom. The higher the height deviation, the larger the visibility of the pattern.

The result was represented in Table 1 and FIGS. 11 to 14.

FIG. 11 is a photograph illustrating a visibility of a groove pattern of a bottom plate according to Example 1 and FIG. 12 is a photograph illustrating a visibility of a groove pattern of a bottom plate according to Comparative Example 1. FIG. 13 is a photograph illustrating a visibility of a groove pattern of a bottom plate according to Comparative Example 2 and FIG. 14 is a photograph illustrating a visibility of a groove pattern of a bottom plate according to Comparative Example 3. In Table, a degree of recognizing a pattern of the bottom plate is represented by numerical values and it means that the larger the numerical value, the lower the appearance quality.

TABLE 1

Comp. Ex. 1
Comp. Ex. 2
Comp. Ex. 3
Example 1

Visibility of
1.77
1.45
1.41
0.94

pattern

Referring to Table 1 and FIGS. 11 to 14 together, it is confirmed that in Example 1, the groove pattern is hardly visible as compared with Comparative Examples 1 to 3. Specifically, in Example 1, as compared with Comparative Example 1 in which only the silicon based resin was filled in each of the plurality of grooves, the visibility is reduced by half. From this, it is confirmed that when the nano helix structure is disposed on the filler, the appearance quality of the display device is significantly improved. In Comparative Example 2 in which the silicon based resin in which silver nano particles are dispersed is filled, it is confirmed that the visibility is lower than that of Comparative Example 1, but the shape of the groove is still visible. Further, in Comparative Example 3 in which the silicon based resin in which silver nano wires are dispersed is filled, it is confirmed that the visibility of the shape of the groove is lower than that of Comparative Example 1, but the appearance quality is inferior to Example 1.

Experimental Example 2

The visibility of the pattern of the bottom plate prepared according to Examples 1 to 4 to the user, a dent characteristic, and a restoration degree were evaluated. The visibility of the pattern was measured in the same manner as Experimental Example 1. The result thereof was represented in Table 2 and FIG. 15. The dent characteristic was evaluated by scratching a surface of a specimen with a pencil under the condition of an angle of 45 degrees and a speed of 300 mm/min while applying a load of 500 g to the specimen and then observing whether the surface was scratched with the naked eye. The restoration degree was evaluated to find how much the alignment of the nano helix structure was restored by the external magnetic field when the alignment of the nano helix structures was deformed and was calculated by the following Equation 1.

Restoration degree=M1−M2 [Equation 1]

In Equation 1, M1 refers to a visibility of a pattern in a zero magnetic field state in which there is no influence of the magnetic field and M2 refers to a visibility of the pattern after exposing the specimen to a magnetic field with a specific magnitude for one hour.

FIG. 15 is a photograph illustrating a visibility of patterns of Comparative Example 1 and Example 1 according to the presence of a magnetic field. FIG. 15(a) is a photograph showing a visibility of a specimen according to Comparative Example 1 in which the nano helix structure is not transferred onto the filler. FIG. 15(b) is a photograph showing a visibility of a pattern in a zero magnetic field state of Example 1. FIG. 15(c) is a photograph showing a visibility of a pattern after applying a magnetic field to a specimen of Example 1.

First, by comparing FIGS. 15(a) and 15(b), it was confirmed that after transferring the nano helix structure onto the filler, the visibility of the groove pattern was reduced and the visibility of the pattern of Comparative Example 1 was 1.7 and the visibility of the pattern of Example 1 was 0.9 so that the visibility of the pattern was reduced by half.

By comparing FIGS. 15(b) and 15(c), it was confirmed that when a magnetic field of 100 emu was applied to the specimen of Example 1 for one hour, the visibility of the pattern was further reduced and specifically, the visibility of the pattern was reduced from 0.9 to 0.8. It means that this is related to the magnetic susceptibility of the nano helix structures and when the alignment of the nano helix structures is deformed, the nano helix structures are aligned in the single direction again in response to the external magnetic field. That is, according to the exemplary embodiment of the present disclosure, even though the alignment of the nano helix structures is deformed by the operation of repeatedly folding or unfolding the flexible display device, if the magnetic field is applied from the outside by wireless charging, the nano helix structures are realigned in the single direction. Therefore, the high appearance quality may be maintained for a long term.