COVER WINDOW FOR FLEXIBLE DISPLAY DEVICE AND FLEXIBLE DISPLAY DEVICE

Abstract

The present disclosure provides a cover window for a flexible display device comprising a laminate including a light-transmitting substrate or polymer substrate; and a first hard coating layer and a second hard coating layer each formed on both surfaces of the light-transmitting substrate or the polymer substrate, wherein a ratio of a modulus of the first hard coating layer to the total modulus of the laminate is 1.5 to 2.0, and wherein a surface pencil hardness measured from the first hard coating layer side is 5H or more based on 750 g, and a flexible display device comprising the cover window.

Claims

1. A cover window for a flexible display device comprising a laminate including a substrate, a first hard coating layer and a second hard coating layer, wherein the substrate is a light-transmitting substrate or polymer substrate, wherein the first hard coating layer is formed on one surface of the substrate and the second hard coating layer is formed on a surface of the substrate opposite to the one surface, wherein a ratio of a modulus of the first hard coating layer to a total modulus of the laminate is 1.5 to 2.0, and wherein a surface pencil hardness measured from the first hard coating layer side is at least 5H based on 750 g.

2. The cover window for a flexible display device according to claim 1, wherein the total modulus of the laminate is measured by applying a strain rate of 12.5 mm/min according to ISO 527-3, and the modulus of the first hard coating layer is measured at a strain rate of 0.05/s by applying a target load of 45 mN according to ISO 14577-1 and applying a target depth of 400 μm from one surface of the first hard coating layer.

3. The cover window for a flexible display device according to claim 1, wherein the ratio of the modulus of the first hard coating layer to the total modulus of the laminate is 1.750 to 1.950.

4. The cover window for a flexible display device according to claim 1, wherein the total modulus of the laminate is 5.5 to 6.5 MPa, and the modulus of the first hard coating layer is 10.0 to 12.0 MPa.

5. The cover window for a flexible display device according to claim 1, wherein the light-transmitting substrate has a thickness of 5 to 100 μm, and each of the first and second hard coating layers has a thickness of 1 to 20 μm.

6. The cover window for a flexible display device according to claim 1, wherein the light-transmitting substrate has an elastic modulus of at least 5 GPa as measured by applying a strain rate of 12.5 mm/min.

7. The cover window for a flexible display device according to claim 6, wherein the light-transmitting substrate includes at least one polymer selected from polyimide, polyamide, and polyamideimide.

8. The cover window for a flexible display device according to claim 1, wherein cracks do not occur in a 100,000-time bending durability test conducted around a rod having a diameter of 2 mm at a temperature of 25° C.

9. The cover window for a flexible display device according to claim 1, wherein the cover window has a transmittance of at least 90.0% with respect to light in a wavelength range of 550 nm, and a haze value of 1.00% or less.

10. The cover window for a flexible display device according to claim 1, wherein the first hard coating layer includes a binder resin containing a cured product of a monomer mixture including a 6- or less functional (meth)acrylate compound and a 8- or more functional (meth)acrylate compound, and fine inorganic particles dispersed in the binder resin, and a weight ratio of the 6- or less functional (meth)acrylate compound and the 8- or more functional (meth)acrylate compound is 1:2 to 1:10.

11. The cover window for a flexible display device according to claim 1, wherein the second hard coating layer includes a binder resin derived from a (meth)acrylate compound and fine inorganic particles dispersed in the binder resin, and a ratio of a modulus of the second hard coating layer to the total modulus of the laminate is 1.400 to 1.800.

12. The cover window for a flexible display device according to claim 11, wherein theft ratio of the modulus of the first hard coating layer to the total modulus of the laminate is at least 0.1000 higher than the ratio of the modulus of the second hard coating layer to the total modulus of the laminate.

13. The cover window for a flexible display device according to claim 10, wherein the binder resin contained in the first hard coating layer further includes a copolymer formed from (meth)acrylic polymer having a weight average molecular weight of 10,000 to 200,000, and a monomer mixture.

14. The cover window for a flexible display device according to claim 10, wherein the first hard coating layer contains 50 to 80 parts by weight of the fine inorganic particles relative to 100 parts by weight of the binder resin, and the second hard coating layer contains 50 to 80 parts by weight of the fine inorganic particles relative to 100 parts by weight of the binder resin.

15. A flexible display device comprising the cover window for a flexible display device according to claim 1.

16. The cover window for a flexible display device according to claim 4, wherein the light-transmitting substrate has a thickness of 5 to 100 μm, and each of the first and second hard coating layers has a thickness of 1 to 20 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0088] FIG. 1 is a view schematically showing a method for performing a bending durability test of Experimental Example 6.

[0089] Hereinafter, the operation and effect of the invention will be described in more detail by way of concrete examples. However, these examples are merely presented for illustrative purposes only, and the scope of the invention is not determined thereby.

Preparation Example: Preparation of Coating Liquid for Forming Hard Coating Layer

Preparation Example 1-1

[0090] 20 g of trimethylolpropane triacrylate (TMPTA, manufactured by Cytec, Mw=296 g/mol, acrylate group equivalent=99 g/mol) as a trifunctional acrylate compound, 30 g of MU9800 (manufactured by Miwon, Mw=3500 g/mol, acrylate group equivalent=389 g/mol) as a 9-functional urethane acrylate compound, 50 g of MU9020 (manufactured by Miwon, Mw=4500 g/mol, acrylate group equivalent=450 g/mol) as a 10-functional urethane acrylate compound, 1 g of Irgacure 184 (manufactured by Ciba) as a photoinitiator, and 10 g of methyl ethyl ketone (MEK) were mixed to prepare an acrylate solution.

[0091] In the acrylate solution, 60 g of a solution in which 50% by weight of silica particles S1 (average particle diameter of 20 nm, surface-modified with methacrylate silane coupling agent) was dispersed in n-butyl acetate, and 100 g of a solution in which 30 wt % of silica particles S2 (average particle diameter of 40 nm, surface-modified with acrylate silane coupling agent) was dispersed in methyl ethyl ketone (MEK) were mixed to prepare a coating solution for forming a hard coating layer.

Preparation Example 1-2

[0092] 20 g of trimethylolpropane triacrylate (TMPTA, manufacturer: Cytec, Mw=296 g/mol, acrylate group equivalent=99 g/mol) as a trifunctional acrylate compound, 30 g of MU9800 (manufactured by Miwon, Mw=3500 g/mol, acrylate group equivalent=389 g/mol) as a 9-functional urethane acrylate compound, 50 g of MU9020 (manufactured by Miwon, Mw=4500 g/mol, acrylate group equivalent=450 g/mol) as a 10-functional urethane acrylate compound, 40 g of a binder solution in which 50 wt % of SMP-250AP (Acrylic polymer, manufactured by Kyoeisha Chemical, acrylate group equivalent=240-260 g/mol, weight average molecular weight: 37,000) as an acrylate-based polymer compound was dissolved in propylene glycol monomethyl ether, 1 g of Irgacure 184 (manufactured by Ciba) as a photoinitiator, and 20 g of methyl ethyl ketone (MEK) were mixed to prepare an acrylate solution.

[0093] In the acrylate solution, 60 g of a solution in which 50 wt % of silica particles S1 (average particle diameter of 20 nm, surface-modified with methacrylate silane coupling agent) was dispersed in n-butyl acetate, and 100 g of a solution in which 30 wt % of silica particles S2 (average particle diameter of 40 nm, surface-modified with acrylate silane coupling agent) was dispersed in methyl ethyl ketone (MEK) were mixed to prepare a coating solution for forming a hard coating layer.

Preparation Examples 1-3 to 1-5

[0094] A coating solution for forming a hard coating layer was prepared in the same manner as in Preparation Example 1-1, except that the content of the ingredients used was adjusted as shown in Table 1 below.

Preparation Example 2-1

[0095] 40 g of MU9800 (manufactured by Miwon, Mw=3500 g/mol, acrylate group equivalent=389 g/mol) as a 9-functional urethane acrylate compound, 40 g of MU9020 (manufactured by Miwon, Mw=4500 g/mol, acrylate group equivalent=450 g/mol) as a 10-functional urethane acrylate compound,), 20 g of PU340 (manufactured by Miwon, Mw=2400 g/mol, acrylate group equivalent=800 g/mol) as a trifunctional urethane acrylate compound, 1 g of Irgacure 184 (manufactured by Ciba) as a photoinitiator, and 10 g of methyl ethyl ketone (MEK) were mixed to prepare an acrylate solution.

[0096] In the acrylate solution, 60 g of a solution in which 50 wt % of silica particles S1 (average particle diameter of 20 nm, surface-modified with methacrylate silane coupling agent) was dispersed in n-butyl acetate, and 100 g of a solution in which 30 wt % of silica particles S2 (average particle diameter of 40 nm, surface-modified with acrylate silane coupling agent) was dispersed in methyl ethyl ketone (MEK) were mixed to prepare a coating solution for forming a hard coating layer.

TABLE-US-00001 TABLE 1 Manufacture, Preparation Preparation Preparation Preparation Preparation Preparation Product Example Example Example Example Example Example name 1-1 1-2 1-3 1-4 1-5 2-1 Acrylate TMPTA 20 20 10 70 compound/p MU9800 30 30 30 30 40 olymer MU9020 50 30 40 30 30 40 (unit: g) SMP- 20 20 40 250AP PU340 20 Inorganic S1 30 30 30 30 30 30 fine S2 30 30 30 30 30 30 particles* (unit:g) Photoinitiator Irgacure 1 1 1 1 1 1 184 Organic solvent MEK 10 25.5 25.5 10 41.5 10 *In Table 1, the content of inorganic fine particles was shown by the net weight of only inorganic fine particles excluding the solvent according to the wt % of the inorganic fine particles dispersed in the solvent.

Examples and Comparative Examples: Cover Window for Flexible Display Device

[0097] A coating solution for forming a hard coating layer shown in Table 2 below was coated onto both surfaces of a 50 μm-thick polyimide substrate (elastic modulus based on a strain rate of 12.5 mm/min-listed in Table 2) by a bar coating method, and dried at 90° C. for 2 minutes under an air atmosphere. An optical laminate was manufactured by photocuring with a metal halide lamp (light quantity: 200 mJ/cm.sup.2) having a wavelength of 290 to 320 nm. After the curing was completed, the thickness of the coating layers formed on both surfaces was 10 μm, respectively.

Experimental Example: Measurement of Physical Properties of Cover Window for Flexible Display Device

Experimental Example 1: Pencil Hardness

[0098] For the hard coating layer formed in the front face of the optical laminate of each of Examples and Comparative Examples, the maximum hardness without scratches was confirmed after moving the pencil back and forth three times at an angle of 45 degrees under a load of 750 g using a pencil hardness tester in accordance with standard JIS K5400-5-4.

Experimental Example 2: Transmittance and Haze

[0099] The transmittance and haze were measured using a spectrophotometer (device name: COH-400) for each of the cover windows of Examples and Comparative Examples.

Experimental Example 3: Bending Test

[0100] According to the test method of JIS K5600-5-1, the cover window of each of Examples and Comparative Examples were wound around a cylindrical mandrel having various diameters, and then the minimum diameter at which no cracks occurred was measured.

Experimental Example 4: Adhesive Strength of Coating Layer

[0101] The front surface of the hard coating layer formed in the front surface of the cover window of each of Examples and Comparative Examples was scratched using a cutter knife so that 100 grids were formed within the size of 1 cm*1 cm˜2 cm*2 cm, and attached with Nichiban Tape (CT-24), and then proceeded with the peeling test. Peeling tests were performed twice on the same surface, and the adhesive strength was evaluated from 5B (not peeled) to 0B (front surface peeled) according to the peeled level (excellent up to 5B). [0102] 5B (not peeled) [0103] 4B (1 to 5 grids containing peeled parts) [0104] 3B (6 to 15 grids containing peeled parts) [0105] 2B (16 to 35 grids containing peeled parts) [0106] 1B (36 to 50 grids containing peeled parts) [0107] 0B (51 or more grids containing peeled parts)

Experimental Example 5: Evaluation of Scratch Resistance

[0108] For the hard coating layer formed on the front surface of each cover window of Examples and Comparative Examples, a load of 500 gf was applied to the steel wool (#0000), and the surface of the hard coating film was rubbed back and forth 500 times at a speed of 30 rpm, and it was observed whether scratches occurred on the surface. When one or less scratches of 1 cm or less observed with the naked eye were observed, it was judged to be excellent.

Experimental Example 6: Bending Durability Test

[0109] FIG. 1 is a view schematically showing a method for performing a bending durability test of a film according to one embodiment of the present disclosure.

[0110] Each of the films of Examples and Comparative Examples was cut, but laser cutting was performed into a size of 80×140 mm so as to minimize fine cracks at the edge portions. The laser cut film was placed on the measuring equipment and set so that the interval between the folded portions was 4 mm. Then, processes of folding and spreading both sides of the films at 90 degrees toward the bottom surface at room temperature were repeated 100,000 times by continuous operations (the speed at which the film was folded was once every 1.5 seconds)

[0111] After repeating 100,000 times, the film was peeled off, and it was observed whether or not cracks of 3 mm or more in length occurred (OK, NG). When cracks did not occur, the film was again bended 100,000 times and whether or not cracks occurred was repeatedly observed, thereby measuring the maximum number of repetitions that cracks do not occur.

Experimental Example 7: Modulus Measurement

[0112] (1) Measure the Modulus of the Cover Window for the Flexible Display Device

[0113] The modulus of the film was measured according to ISO 527-3 using Universal Testing Systems (Instron® 3360). Each film of Examples and Comparative Examples was cut, but laser cutting was performed into a size of 10×150 mm so as to minimize fine cracks at the edge portions. The edge of the laser-cut film was fixed to the measuring equipment (Instron® 3360), and measurement was performed at a strain rate of 12.5 mm/min until the specimen was broken in the tensile direction.

[0114] (2) Measure the Modulus of the First Hard Coating Layer of the Cover Window for the Flexible Display Device

[0115] The modulus of the first hard coating layer was measured according to ISO 14577-1 using a Nano Indentation System (MTS Nanoindenter XP). More specifically, measurement was performed at a strain rate of 0.05/s to a depth of 400 μm by applying a load of 45 mN from the surface of the first hard coating layer using a 50 nm Berkovich Indenter Tip.

[0116] The measurement results of the physical properties for Examples and Comparative Examples are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Example Example Example Example Comparative Comparative Comparative Comparative 1 2 3 4 Example 1 Example 2 Example 3 Example 4 First hard Preparation Preparation Preparation Preparation Preparation Preparation Preparation Preparation coating layer Example1-1 Example1-2 Example1-3 Example1-4 Example1-5 Example1-6 Example1-6 Example1-7 (upper layer) Second hard Preparation Preparation Preparation Preparation Preparation Preparation Preparation Preparation coating layer Example2-1 Example2-1 Example2-1 Example2-1 Example2-1 Example2-1 Example2-1 Example2-1 (lower layer) Transmittance 91.9 91.8 91.9 92.2 92.1 92.0 92.2 92.0 (%) Haze (%) 0.82 0.48 0.42 0.90 0.48 0.31 0.45 0.31 Attaching the OK OK OK OK OK OK OK OK coating layer Scratch Excellent Excellent Excellent Excellent Excellent Excellent Excellent Excellent resistance First hard 7H 7H 6H 8H 8H 4H 6H 6H coating layer pencil hardness Bending test 4 mm 4 mm 3 mm 5 mm 8 mm 4 mm 6 mm 8 mm Bending 100,000 100,000 100,000 100,000 NG 100,000 NG NG durability times times times times times (25° C., 5 mm) Ok Ok Ok Ok Ok Elastic 6.1 6.1 6.1 6.2 6.1 6.1 3.9 3.5 modulus of polyimide substrate (GPa) Elastic 6.0 6.0 5.8 6.0 6.1 5.7 4.0 3.7 modulus of cover window (GPa) (A) Modulus of 11.63 11.35 10.99 11.12 12.5 7.83 10.54 10.48 the first hard coating layer (GPa) (B) Modulus of 9.62 9.73 9.42 9.83 10.25 9.47 9.48 9.36 the second hard coating layer (GPa) (C) B/A 1.938 1.892 1.895 1.853 2.049 1.374 2.635 2.832

[0117] As shown in Table 2, the cover window for a flexible display device of Examples exhibits high hardness while being implemented so as to satisfy sufficient flexibility at the same time while exhibiting a glass level of a high hardness, in particular, hardly have a risk of damaging the film even by repetitive bending or folding operations, and thus, can be easily applied to bendable, flexible, rollable or foldable mobile devices, display devices, and the like.

[0118] On the contrary, the cover windows of Comparative Examples did not have a relatively low surface hardness, unlike Examples, or did not exhibit bending durability enough to be used as a cover window for a flexible display device.