COVER WINDOW FOR FLEXIBLE DISPLAY DEVICE AND FLEXIBLE DISPLAY DEVICE

Abstract

The present disclosure relates to a cover window for a flexible display device comprising: a light-transmitting substrate; and a first hard coating layer and a second hard coating layer which are formed on both sides of the light-transmitting substrate, respectively, wherein each of the first hard coating layer and the second hard coating layer has IR spectra in which a ratio of the amide C═O peak to the ester C═O peak is in a predetermined range, and a flexible display device including the cover window for a flexible display device.

Claims

1. A cover window for a flexible display device comprising: a light-transmitting substrate, a first hard coating layer and a second hard coating layer, wherein the first hard coating layer is formed one side of the light-transmitting substrate and the second hard coating layer is formed on the other side of the light-transmitting substrate, wherein the first hard coating layer has IR spectra in which a ratio of an amide C═O peak to an ester C═O peak is 0.8 or less, and wherein the second hard coating layer has IR spectra in which a ratio of an amide C═O peak to an ester C═O peak is greater than 0.8.

2. The cover window for a flexible display device according to claim 1, wherein the ratio of the amide C═O peak to the ester C═O peak in the IR spectra of the first hard coating layer is 0.500 to 0.800.

3. The cover window for a flexible display device according to claim 1, wherein the ratio of the amide C═O peak to the ester C═O peak in the IR spectra of the second hard coating layer is 0.820 to 1.500.

4. The cover window for a flexible display device according to claim 1, wherein cracks do not occur in 50,000 times bending durability tests that are performed around a mandrel with a diameter of 5 mm at a temperature of −20° C.

5. The cover window for a flexible display device according to claim 1, wherein: cracks do not occur in 100,000 times bending durability tests that are performed around a mandrel with a diameter of 5 mm at a temperature of 60° C. and 90 RH %.

6. The cover window for a flexible display device according to claim 1, wherein a pencil hardness of a surface measured on the first hard coating layer side is 5 H or more under a load of 750 g, and cracks do not occur in 200,000 times bending durability tests that are performed around a mandrel with a diameter of 2 mm at a temperature of 25° C.

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

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

9. The cover window for a flexible display device according to claim 1, which has a light transmittance of 90.0% or more at a wavelength region 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 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.

11. The cover window for a flexible display device according to claim 1, wherein the first hard coating layer comprises a binder resin containing a cured product between a (meth)acrylic-based polymer having a weight average molecular weight of 10,000 to 200,000 and at least one (meth)acrylate compound, and fine inorganic particles dispersed in the binder resin, and the second hard coating layer comprises a binder resin derived from at least one (meth)acrylate compound and fine inorganic particles dispersed in the binder resin.

12. The cover window for a flexible display device according to claim 11, wherein: the binder resin contained in the first hard coating layer comprises a copolymer formed from a mixture containing 5 to 40% by weight of a (meth)acrylic-based polymer having a weight average molecular weight of 10,000 to 200,000; 50 to 85% by weight of an 8- to 12-functional (meth)acrylate compound; and 5 to 40% by weight of a 1- to 6-functional (meth)acrylate compound.

13. The cover window for a flexible display device according to claim 11, wherein: the binder resin contained in the second hard coating layer comprises a copolymer formed from a mixture containing 40 to 85% by weight of an 8- to 12-functional (meth)acrylate compound; and 15 to 60% by weight of a 1- to 6-functional (meth)acrylate compound.

14. The cover window for a flexible display device according to claim 11, wherein: the first hard coating layer comprises 30 to 100 parts by weight of the fine inorganic particles relative to 100 parts by weight of the binder resin, and the second hard coating layer comprises 30 to 100 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 the flexible display device according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0095] FIG. 1 shows the IR spectrum of the hard coating layer obtained in Preparation Example 1-1, Preparation Example 1-2, and Preparation Example 1-5, respectively.

[0096] FIG. 2 schematically shows the method of performing the bending durability and bending stability tests of Experimental Examples 7 and 8.

[0097] 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

[0098] 20 g of trimethylolpropane triacrylate (TMPTA) (manufactured by Cytec, Mw=296 g/mol, acrylate group equivalent weight=99 g/mol) as a trifunctional acrylate-based compound, 30 g of MU9800 (manufactured by Miwon, Mw=3500 g/mol, acrylate group equivalent weight=389 g/mol) as a 9-functional urethane acrylate-based compound, 30 g of MU9020 (manufactured by Miwon, Mw=4500 g/mol, acrylate group equivalent weight=450 g/mol) as a 10-functional urethane acrylate-based compound, 40 g of a binder solution in which SMP-250AP (acrylic polymer, manufactured by Kyoeisha Chemical, acrylate group equivalent weight=240 to 260 g/mol, weight average molecular weight (Mw): 37,000) as an acrylate-based polymer compound was dissolved in propylene glycol monomethyl ether in an amount of 50 wt %, 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.

[0099] 60 g of a solution in which a silica particle S1 (average particle diameter of 20 nm, surface-modified with methacrylate silane coupling agent) was dispersed in n-butyl acetate in an amount of 50 wt.%, and 100 g of a solution in which a silica particle S2 (average particle diameter of 40 nm, surface-modified with an acrylate silane coupling agent) was dispersed in methyl ethyl ketone (MEK) in an amount of 30 wt.% were mixed with the resulting acrylate solution to prepare a coating liquid for forming hard coating layer.

Preparation Examples 1-2 to 1-7

[0100] A coating solution for forming a hard coating layer was prepared in the same manner as in Preparation Example 1-1, except that the contents of the used components were adjusted as shown in Table 1 below.

Preparation Example 2-1

[0101] 30 g of MU9800 (manufactured by Miwon, Mw=3500 g/mol, acrylate group equivalent weight=389 g/mol) as a 9-functional urethane acrylate-based compound, 40 g of MU9020 (manufactured by Miwon, Mw=4500 g/mol, acrylate group equivalent weight=450 g/mol) as a 10-functional urethane acrylate-based compound, PU340 (manufactured by Miwon, Mw=2400 g/mol, acrylate group equivalent weight=800 g/mol) as a 3-functional urethane acrylate-based compound, 1 g of Irgacure 184 (manufactured by Ciba) as a photoinitiator, and 40 g of methyl ethyl ketone (MEK) were mixed to prepare an acrylate solution.

[0102] 50 g of a solution in which a silica particle Si (average particle diameter of 20 nm, surface-modified with methacrylate silane coupling agent) was dispersed in n-butyl acetate in an amount of 50 wt.%, and 100 g of a solution in which a silica particle S2 (average particle diameter of 40 nm, surface-modified with an acrylate silane coupling agent) was dispersed in methyl ethyl ketone (MEK) in an amount of 30 wt % were mixed with the resulting acrylate solution to prepare a coating liquid for forming hard coating layer.

TABLE-US-00001 TABLE 1 Prepar- Prepar- Prepar- Prepar- Prepar- Prepar- Prepar- Prepar- Manu- tion tion tion tion tion tion tion tion facturer, Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Product ple ple ple ple ple ple ple ple name 1-1 1-2 1-3 1-4 1-5 1-6 1-7 2-1 Acrylate- TMPTA 20 10 10 20 30 50 based MU9800 30 30 30 30 30 40 30 30 compound/ MU9020 30 40 30 30 40 10 20 40 polymer SMP- 20 20 30 20 50 (unit: g) 250AP PU340 30 Inorganic S1 30 30 30 30 30 30 30 30 fine S2 30 30 30 30 30 30 30 30 particles* (unit: g) Photoinitiator 1 1 1 1 1 1 1 1 Irgacure 184 Organic solvent 20 20 10 20 40 40 40 40 MEK

[0103] *In Table 1, the content of the inorganic fine particles is represented by the net weight of only the inorganic fine particles excluding the solvent according to the weight percentage of the inorganic fine particles dispersed in the solvent.

Examples and Comparative Examples: Cover Window for Flexible Display Device

[0104] A coating solution for forming a hard coating layer described in Table 2 below was coated onto both sides of a 50 μm thick polyimide substrate (product name: A-50-D, manufactured by Kolon Industries, modulus (measured by UTM: 6.1 GPa)) by a bar coating method, and dried at 90° C. for 2 minutes under an air atmosphere. It was photo-cured with a metal halide lamp (light quantity: 200 mJ/cm.sup.2) having a wavelength of 290 to 320 nm to form an optical laminate. After curing was completed, the thickness of the coating layers formed on both sides was 10 μm, respectively.

[0105] At this time, the hard coating layer formed on one side of the polyimide substrate using the coating solution for forming the hard coating layer of Preparation Example 1-1 to Preparation Example 2-1 was analyzed using FT-IR (Fourier-transform infrared spectroscopy; FTS 3000), and the resolution was measured using a resolution of 4 cm.sup.−1 and a scan number of 64 in the wavenumber range of 650 to 4000 cm.sup.−1 under the ATR (attenuated total reflectance)-IR mode conditions. From the measurement result, the ratio of the intensity of the amide C═O peak (about 1690 cm.sup.−1) to the intensity of the ester C═O peak (about 1724 cm.sup.−1) was calculated.

TABLE-US-00002 TABLE 2 Prepar- Prepar- Prepar- Prepar- Prepar- Prepar- Prepar- Prepar- tion tion tion tion tion tion tion tion Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple ple ple ple ple ple ple ple Category 1-1 1-2 1-3 1-4 1-5 1-6 1-7 2-1 Ratio of amide C═O 0.654 0.736 0.748 0.675 0.399 0.352 0.874 0.834 peak to ester C═O peak in IR spectrum

<Experimental Example: Measurement of Physical Properties of Optical Laminates>

Experimental Example 1: Pencil Hardness

[0106] For the hard coating layer formed in the front face of the cover window 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

[0107] For the cover window of each of Examples and Comparative Examples, the transmittance and haze were measured using a spectrophotometer (apparatus name: COH-400).

Experimental Example 3: Bending Test

[0108] The cover window of each of Examples and Comparative Examples was interposed and wound between cylindrical mandrels of various diameters, and then the minimum diameter at which no cracks occurred was measured.

Experimental Example 4: Adhesion of Coating Layer

[0109] The front face of the hard coating layer formed in the front face of the cover window of each of Examples and Comparative Examples was cut and scratched with a knife to form 100 grids within the size of 1 cm*1 cm˜2 cm*2 cm. Then, Nichiban tape (CT-24) was attached to the cut surface to proceed a peel test. The peel test was performed twice on the same side, and the adhesive strength was evaluated from 5B (no peeling) level to 0B (front peeling) according to the peeled level (5B excellent). [0110] 5B: (no peeling) [0111] 4B (1 to 5 grids containing peeled parts) [0112] 3B (6 to 15 grids containing peeled parts) [0113] 2B (16 to 35 grids including peeled parts) [0114] 1B (36 to 50 grids containing peeled parts) [0115] 0B (51 or more grids containing peeled parts)

Experimental Example 5: Evaluation of Scratch Resistance

[0116] The hard coating layer formed in the front face of the cover window of each of Examples and Comparative Examples was rubbed back and forth 500 times at a speed of 30 rpm with a steel wool (#0000) under a load of 500 gf to confirm the surface of the hard coating film. It was determined as excellent if the number of scratches of 1 cm or less observed with the naked eye was 1 or less.

Experimental Example 6: Bending Durability Test

[0117] FIG. 2 is a diagram schematically illustrating a method for performing a bending durability and bending stability test for a film according to an exemplary embodiment of the present disclosure.

[0118] 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 10,000 times (the speed at which the film was folded was once every 1.5 seconds).

[0119] After repeating 10,000 times, the film was peeled off, and it was observed whether cracking of 3 mm or more in length occurred (OK, NG). When cracks did not occur, the film was again bent 10,000 times and whether or not cracks occurred were repeatedly observed, thereby measuring the maximum number of repetitions that cracks do not occur.

(Temperature and Humidity Conditions)

[0120] 1) room temperature: 25° C. [0121] 2) low temperature: −20° C. [0122] 3) high temperature/high humidity: 60° C./90 RH %

[0123] The measurement results of the physical properties of Examples and Comparative Examples are shown in Table 3 below.

TABLE-US-00003 TABLE 3 Compar- Compar- Compar- ative ative ative Example Example Example Example Example Example Example 1 2 3 4 1 2 3 First hard Prepar- Prepar- Prepar- Prepar- Prepar- Prepar- Prepar- coating layer ation ation ation ation ation ation ation Example Example Example Example Example Example Example 1-1 1-2 1-3 1-4 1-5 1-6 1-7 Second hard Prepar- Prepar- Prepar- Prepar- Prepar- Prepar- Prepar- coating layer ation ation ation ation ation ation ation Example Example Example Example Example Example Example 2-1 2-1 2-1 2-1 2-1 2-1 2-1 Transmittance 91.9 91.7 92.1 91.9 91.9 92.0 91.8 (%) Haze(%) 0.50 0.40 0.36 0.65 0.50 0.40 0.40 Adhesion of OK OK OK OK OK OK OK coating layer Scratch Excellent Excellent Excellent Excellent Excellent Excellent Excellent resistance Pencil 6 H 7 H 6 H 7 H 6 H 8 H 3 H hardness Bending test 2 mm 2 mm 2 mm 2 mm 4 mm 6 mm 3 mm Bending 100,000 100,000 100,000 100,000 100,000 NG 100,000 durability times times times times times times (25° C., 5 mm) Ok Ok Ok Ok Ok Ok Bending 100,000 100,000 100,000 100,000 100,000 NG 100,000 durability times times times times times times (100,000 Ok Ok Ok Ok Ok Ok times, 25° C., 2 mm) Bending 200,000 200,000 200,000 200,000 NG NG 200,000 durability times times times times times (200,000 Ok Ok Ok Ok Ok times, 25° C., 2 mm) Low- 50,000 50,000 50,000 50,000 NG NG NG temperature times times times times bending OK OK OK OK durability (−20° C., 5 mm) High- 100,000 100,000 100,000 100,000 NG NG 100,000 temperature/ times times times times times high humidity Ok Ok Ok Ok Ok durability (60° C., 90 RH%) 5 mm)

[0124] As shown in Table 3, it was confirmed that the cover windows for flexible display devices of Examples have sufficient flexibility while exhibiting a glass level of high hardness, in particular, hardly has a risk of damaging the film even in repetitive bending or folding operations, and thus, can be easily applied to bendable, flexible, rollable or foldable mobile devices, display devices, and the like.

[0125] On the other hand, the films of Comparative Examples had lowered pencil hardness or did not exhibit sufficient bending durability to be suitable for cover windows for flexible display devices.