COVER WINDOW FOR FLEXIBLE DISPLAY DEVICE AND FLEXIBLE DISPLAY DEVICE

Abstract

The present disclosure relates to a cover window for a flexible display device including a polymer substrate including a polyamide resin; and a hard coating layer formed on at least one surface of the polymer substrate, wherein a yellow index of the polymer substrate measured in accordance with STM E 313 is 4.00 or less, and an elastic modulus of the polymer substrate measured at a strain rate of 12.5 mm/min in accordance with ISO 527-3 is 4 to 9 GPa.

Claims

1. A cover window for a flexible display device, comprising a polymer substrate comprising a polyamide resin; and a hard coating layer formed on at least one surface of the polymer substrate, wherein the polyamide resin comprises a first polyamide segment and a second polyamide segment, each polyamide segment containing a benzene-1,3-dicarbonyl group and a benzene-1,4-dicarbonyl group in a different molar ratio, the polymer substrate has a yellow index of 4.00 or less as measured in accordance with STM E 313, and an elastic modulus of 4 to 9 GPa as measured at a strain rate of 12.5 mm/min in accordance with ISO 527-3.

2. The cover window for a flexible display device of claim 1, wherein no crack occurs on the cover window when the cover window is wound on a mandrel having a diameter of 3 mm.

3. The cover window for a flexible display device of claim 1, wherein the hard coating layer has a pencil hardness of at least 5H under a load of 750 g.

4. The cover window for a flexible display device of claim 1, wherein the cover window has a transmittance of at least 90.0% with respect to light having a wavelength of 550 nm and a haze of 0.6% or less.

5. The cover window for a flexible display device of claim 1, wherein the cover window has a yellow index of 3.0 or less.

6. The cover window for a flexible display device of claim 1, wherein the polyamide resin is a polyamide block copolymer comprising A) an amide bond between an aromatic diamino group and a benzene-dicarbonyl group; and B) b1) the first polyamide segment comprising 20 mol % or less of the benzene-1,3-dicarbonyl group based on a total of the benzene-1,3-dicarbonyl group and the benzene-1,4-dicarbonyl group, and b2) the second polyamide segment comprising more thab 20 mol % of the benzene-1,3-dicarbonyl group based on a total of the benzene-1,3-dicarbonyl group and the benzene-1,4-dicarbonyl group.

7. The cover window for a flexible display device of claim 6, wherein a molar ratio of the first polyamide segment to the second polyamide segment is 1:0.5 to 1:10.

8. The cover window for a flexible display device of claim 6, wherein the polyamide block copolymer comprises the benzene-1,3-dicarbonyl group of 5 to 25 mol % based on a total of the benzene-1,3-dicarbonyl group and the benzene-1,4-dicarbonyl group.

9. The cover window for a flexible display device of claim 6, wherein the first polyamide segment has 2 mol % or more and 20 mol % or less of the benzene-1,3-dicarbonyl group based on a total of the benzene-1,3-dicarbonyl group and the benzene-1,4-dicarbonyl group contained in the first polyamide segment.

10. The cover window for a flexible display device of claim 6, wherein the second polyamide segment has a ratio of isophthaloyl chloride to a total of the benzene-1,3-dicarbonyl group and the benzene-1,4-dicarbonyl group of more than 20 mol % and 40 mol % or less.

11. The cover window for a flexible display device of claim 6, wherein the aromatic diamino group has a diamino group derived from at least one selected from the group consisting of 2,2′-bis(trifluoromethyl)-4,4′-biphenyldiamine, 2,2′-dimethyl-4,4′- diaminobenzidine, 4,4′-diaminodiphenyl sulfone, 4,4′-(9-fluorenylidene)dianiline, bis(4-(4-aminophenoxy)phenyl)sulfone, 2,2′,5,5′-tetrachlorobenzidine, 2,7-diaminofluorene, 4,4-diaminooctafluorobiphenyl, m-phenylenediamine, p-phenylenediamine, 4,4′-oxydianiline, 2,2′-dimethyl-4,4′-diaminobiphenyl, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 1,3-bis(4-aminophenoxy)benzene, and 4,4′-diaminobenzanilide.

12. The cover window for a flexible display device of claim 6, wherein the polyamide block copolymer has a weight average molecular weight of 10,000 to 700,000 g/mol.

13. The cover window for a flexible display device of claim 1, wherein the polymer substrate has an elongation of at least 10% as measured in accordance with ISO 527-3.

14. The cover window for a flexible display device of claim 1, wherein the polymer substrate has a thickness of 5 to 300 μm.

15. The cover window for a flexible display device of claim 1, wherein the hard coating layer comprises a binder resin and inorganic nanoparticles, and has a thickness of 5 μm to 50 μm.

16. A flexible display device comprising the cover window for a flexible display device of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0106] FIG. 1 schematically shows a method of performing a bending durability test and a bending stability test of Experimental Examples 7 and 8.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0107] Hereinafter, the function and effect of the present invention will be described in more detail through specific examples. However, these examples are for illustrative purposes only, and the invention is not intended to be limited by these examples.

PREPARATION EXAMPLE A

Preparation of Coating Solution for Forming Hard Coating Layer

PREPARATION EXAMPLE A-1

[0108] As shown in Table 1 below, 30 g of trimethylolpropane triacrylate (TMPTA, manufactured by Cytec, Mw=296 g/mol, acrylate group equivalent weight=99 g/mol) as a trifunctional acrylate-based binder, 40 g of MU9800 (manufactured by Miwon, Mw=3500 g/mol, acrylate group equivalent weight=389 g/mol) as a 9-functional urethane acrylate-based binder, 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 binder, 1 g of Irgacure 184 (manufactured by Ciba) as a photoinitiator, and 17.5 g of methyl ethyl ketone (MEK) were mixed to prepare an acrylate solution.

[0109] 60 g of a solution in which silica particles 51 (average particle diameter: 20 nm, surface-modified with a methacrylate silane coupling agent) were dispersed in n-butyl acetate (normal butyl acetate) in an amount of 50 wt %, and 100 g of a solution in which silica particles S2 (average particle diameter: 40 nm, surface-modified with an acrylate silane coupling agent) were dispersed in methyl ethyl ketone (MEK) in an amount of 30 wt % were mixed with the resulting acrylate solution to prepare a coating solution for forming a hard coating layer.

PREPARATION EXAMPLE A-2

[0110] A coating solution for forming a hard coating layer was prepared in the same manner as in Preparation Example A-1, except that 1) an acrylate solution was prepared using 26.1 g of methyl ethyl ketone, and 2) 50 g of the solution in which silica particles S2 were dispersed in methyl ethyl ketone (MEK) in an amount of 30 wt % was used.

PREPARATION EXAMPLE A-3

[0111] A coating solution for forming a hard coating layer was prepared in the same manner as in Preparation Example A-1, except that 1) an acrylate solution was prepared using 21.8 g of methyl ethyl ketone, and 2) 75 g of a solution in which silica particles S3 (average particle diameter: 100 nm, surface-modified with an acrylate silane coupling agent) were dispersed in an amount of 40 wt % was used instead of 100 g of the solution in which silica particles S2 were dispersed in methyl ethyl ketone (MEK) in an amount of 30 wt %.

PREPARATION EXAMPLE A-4

[0112] As shown in Table 1 below, 30 g of TMPTA (manufactured by Cytec, Mw=296 g/mol, acrylate group equivalent weight=99 g/mol), 40 g of MU9800 (manufactured by Miwon, Mw=3500 g/mol, acrylate group equivalent weight=389 g/mol), 30 g of MU9020 (manufactured by Miwon, Mw=4500 g/mol, acrylate group equivalent weight=450 g/mol), 1 g of Irgacure 184 (manufactured by Ciba), and 43.3 g of methyl ethyl ketone (MEK) were mixed to prepare a coating solution for forming a hard coating layer.

PREPARATION EXAMPLE A-5

[0113] 30 g of TMPTA (manufactured by Cytec, Mw=296 g/mol, acrylate group equivalent weight=99 g/mol), 40 g of MU9800 (manufactured by Miwon, Mw=3500 g/mol, acrylate group equivalent weight=389 g/mol), 30 g of MU9020 (manufactured by Miwon, Mw=4500 g/mol, acrylate group equivalent weight=450 g/mol), 1 g of Irgacure 184 (manufactured by Ciba), and 30.5 g of methyl ethyl ketone (MEK) were mixed to prepare an acrylate solution.

[0114] 90 g of a solution in which silica particles S1 (average particle diameter: 20 nm, surface-modified with a methacrylate silane coupling agent) were dispersed in n-butyl acetate (normal butyl acetate) in an amount of 50 wt % was mixed with the resulting acrylate solution to prepare a coating solution for forming a hard coating layer.

PREPARATION EXAMPLE A-6

[0115] 30 g of TMPTA (manufactured by Cytec, Mw=296 g/mol, acrylate group equivalent weight=99 g/mol), 40 g of MU9800 (manufactured by Miwon, Mw=3500 g/mol, acrylate group equivalent weight=389 g/mol), 30 g of MU9020 (manufactured by Miwon, Mw=4500 g/mol, acrylate group equivalent weight=450 g/mol), 1 g of Irgacure 184 (manufactured by Ciba), and 24 g of methyl ethyl ketone (MEK) were mixed to prepare an acrylate solution.

[0116] 112.5 g of a solution in which silica particles S3 (average particle diameter: 100 nm, surface-modified with an acrylate silane coupling agent) were dispersed in an amount of 40 wt % was mixed with the resulting acrylate solution to prepare a coating solution for forming a hard coating layer.

TABLE-US-00001 TABLE 1 Acrylate- Acrylate- Acrylate- Acrylate- Acrylate- Acrylate- Acrylate- based based based based based based based binder binder binder binder binder binder binder (unit: g) (unit: g) (unit: g) (unit: g) (unit: g) (unit: g) (unit: g) Acrylate- Cytec, 30 30 30 30 30 30 based binder TMPTA (unit: g) Miwon, 40 40 40 40 40 40 MU9800 Miwon, 30 30 30 30 30 30 MU9020 Inorganic fine S1 30 30 30 45 particles* S2 30 15 — (unit: g) S3 30 45 *In Table 1, the content of inorganic fine particles is represented by a 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.

[0117] [Preparation Example B: Preparation of Polyamide Block Copolymer]

PREPARATION EXAMPLE B-1

[0118] In a 500 mL 4-neck round-bottom flask (reactor) equipped with a stirrer, a nitrogen injector, a dropping funnel, and a temperature controller, 184 g of N,N-dimethylacetamide (DMAc) was placed with slowly blowing nitrogen. The temperature of the reactor was adjusted to −10° C., and then 0.030343 mol of 2,2′-bis(trifluoromethyl)-4,4′-biphenyldiamine (TFDB) was dissolved.

[0119] 0.000759 mol of isophthaloyl chloride (IPC) and 0.014716 mol of terephthaloyl chloride (TPC) were sequentially added thereto at about 5 minute-intervals, and stirred. Thereafter, an amide formation reaction was carried out at about −10° C. for about 60 minutes (first segment).

[0120] After 0.030343 mol of TFDB was added thereto and dissolved, 0.005007 mol of IPC and 0.010468 mol of TPC were sequentially added thereto at about 5 minute-intervals, and stirred. Thereafter, an amide formation reaction was carried out at about −10° C. for about 60 minutes (second segment).

[0121] After completion of the reaction, DMAc was added to dilute to a solid content of 5% or less, and then precipitated using 1 L of methanol. The precipitated solid was filtered and then dried at 100° C. under vacuum for about 6 hours or more to obtain a polyamide block copolymer in the form of solid (weight average molecular weight: about 417,201 g/mol).

[0122] Then, the polyamide block copolymer obtained above was dissolved in N,N-dimethylacetamide to prepare a polymer solution of about 12% (w/V).

[0123] The polymer solution was applied on a polyimide-based substrate (UPILEX-75s, manufactured by UBE), and the thickness of the polymer solution was uniformly controlled using a film applicator.

[0124] This was dried in a mathis oven at about 80° C. for about 15 minutes, cured at about 250° C. for about 30 minutes while flowing nitrogen, and then peeled from the substrate to obtain a polymer substrate including a polyamide block copolymer having a thickness of about 50.0 μm.

PREPARATION EXAMPLEs B-2 AND B-3

[0125] Polyamide block copolymers in the form of solid were obtained in the same manner as in Preparation Example B-1, except that the amounts of reactants added were changed as shown in Table 2 below (weight average molecular weight: about 401,117 g/mol).

[0126] Polymer substrates including a polyamide block copolymer having a thickness of about 50.0 μm were prepared in the same manner as in Preparation Example B-1.

PREPARATION EXAMPLE B-4

[0127] In a 500 mL 4-neck round-bottom flask (reactor) equipped with a stirrer, a nitrogen injector, a dropping funnel, and a temperature controller, 184 g of N,N-dimethylacetamide (DMAc) was placed with slowly blowing nitrogen. The temperature of the reactor was adjusted to −10° C., and then 0.030343 mol of 2,2′-bis(trifluoromethyl)-4,4′-biphenyldiamine (TFDB) was dissolved.

[0128] 0.001517 mol of isophthaloyl chloride (IPC) and 0.029432 mol of terephthaloyl chloride (TPC) were sequentially added thereto at about 5 minute-intervals, and stirred. Thereafter, an amide formation reaction was carried out at about −10° C. for about 60 minutes.

[0129] After completion of the reaction, DMAc was added to dilute to a solid content of 5% or less, and then precipitated using 1 L of methanol. The precipitated solid was filtered and then dried at 100° C. under vacuum for about 6 hours or more to obtain a polyamide block copolymer in the form of solid (weight average molecular weight: about 431,122 g/mol).

[0130] A polymer substrate including a polyamide block copolymer having a thickness of about 50.0 μm was prepared in the same manner as in Preparation Example B-1.

TABLE-US-00002 TABLE 2 IPC TPC TFDB IPC TPC Segment (mol) (mol) (mol) (mol %) (mol %) Prep. Ex. First 7.5900E−04 1.4716E−02 0.03034 4.905 95.095 B-1 segment Second 5.0070E−03 1.0468E−02 0.03034 32.355 67.645 segment Prep. Ex. First 1.9000E−04 3.6790E−03 0.00379 4.911 95.089 B-2 segment Second 5.5750E−03 2.1505E−02 0.02650 20.587 79.413 segment Prep. Ex. First 7.7000E−04 1.4330E−02 0.01541 5.099 94.901 B-3 segment Second 4.4685E−03 1.0632E−02 0.01541 29.592 70.408 segment Prep. Ex. — 1.5170E−03 2.9432E−02 0.03034 4.902 95.098 B-4

EXPERIMENTAL EXAMPLE A

[0131] Measurement of Physical Properties of Polymer Substrate

[0132] The following properties were measured or evaluated for the polymer substrates including the polyamide block copolymer according to Preparation Example B, and the results are summarized in Table 3 below.

[0133] (1) Yellow Index (Y.I.)

[0134] The yellow index (Y.I.) of the polymer substrate was measured according to the method of ASTM E 313 using a COH-400 Spectrophotometer (manufactured by NIPPON DENSHOKU INDUSTRIES).

[0135] (2) Haze

[0136] The haze of the polymer substrate was measured according to the method of ASTM D1003 using a COH-400 Spectrophotometer (manufactured by NIPPON DENSHOKU INDUSTRIES).

[0137] (3) Pencil Hardness

[0138] The maximum hardness without scratches was confirmed for the hard coating layer formed on the front surface in each cover window of Examples and Comparative Example after moving a 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 JIS K5400-5-4.

[0139] (4) Modulus and Elongation

[0140] The modulus (GPB) and elongation (%) of the film were measured according to the method of ISO 527-3 using Universal Testing Systems (Instron® 3360).

[0141] (5) Folding Endurance

[0142] The folding endurance of the film was measured according to the method of ISO 5626 using a folding endurance tester.

[0143] Specifically, a specimen (1 cm*7 cm) of the film was loaded into the folding endurance tester at 25° C., and folded to an angle of 135° at a rate of 175 rpm on the left and right sides of the specimen, with a radius of curvature of 0.8 mm and a load of 250 g, until the specimen was fractured. The number of reciprocating bending cycles was measured as the folding endurance.

TABLE-US-00003 TABLE 3 Pencil hard- Modulus Elongation YI Haze ness (Gpa) (%) MIT (0.8R) Prep. Ex. 2.41 0.63 3H 7.15 about 15 about 9,000 B-1 cycles Prep. Ex. 2.81 0.82 3H 7.21 about 15 about 9,000 B-2 cycles Prep. Ex. 3.79 0.81 3H 7.11 about 15 about 9,000 B-3 cycles Prep. Ex. 6.12 2.23 3H 7.77 about 10 about 7,000 B-4 cycles

EXAMPLES AND COMPARATIVE EXAMPLE 1

Cover Window for Flexible Display Device

[0144] The coating composition prepared in Preparation Example A was applied to both surfaces of the polymer substrate prepared in Preparation Example B by a bar coating method, and dried at 90° C. under an air atmosphere for 2 minutes. An optical laminate was prepared by photocuring with a metal halide lamp having a wavelength of 290 to 320 nm (amount of light: 200 mJ/cm.sup.2). After the curing was completed, the thickness of the coating layer formed on both surfaces was 10 μm, respectively.

[0145] Examples and Comparative Example are shown in Tables 4 and 5 below.

TABLE-US-00004 TABLE 4 Example 1 Example 2 Example 3 Example 4 Coating Prep. Prep. Prep. Prep. composition Ex. A-1 Ex. A-2 Ex. A-3 Ex. A-1 Polymer Prep. Prep. Prep. Prep. substrate Ex. B-1 Ex. B-1 Ex. B-1 Ex. B-2 Total thickness 70 μm 70 μm 70 μm 70 μm Thickness of 10 μm 10 μm 10 μm 10 μm coating layer

TABLE-US-00005 TABLE 5 Comparative Example 1 Coating Prep. Ex. A-1 composition Polymer Prep. Ex. B-4 substrate Total thickness 70 μm Thickness of 10 μm coating layer

EXPERIMENTAL EXAMPLE

Measurement of Physical Properties of Optical Laminate

EXPERIMENTAL EXAMPLE 1

Pencil Hardness

[0146] The maximum hardness without scratches was confirmed for the hard coating layer formed on the front surface in each cover window of Examples and Comparative Example after moving a 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 JIS K5400-5-4.

EXPERIMENTAL EXAMPLE 2

Transmittance and Haze

[0147] The transmittance and haze of each cover window of Examples and Comparative Example were measured using a spectrophotometer (apparatus name: COH-400).

EXPERIMENTAL EXAMPLE 3

Yellow Index (Y.I.)

[0148] The yellow index (Y.I.) of each cover window of Examples and Comparative Example was measured using a spectrophotometer (apparatus name: COH-400 manufactured by NIPPON DENSHOKU INDUSTRIES) in accordance with ASTM E 313.

EXPERIMENTAL EXAMPLE 4

Bending Test

[0149] Each cover window of Examples and Comparative Example was interposed and wound on cylindrical mandrels with various diameters, and then the minimum diameter at which no crack occurred was measured in accordance with JIS K5600-5-1.

EXPERIMENTAL EXAMPLE 5

Coating Layer Adhesion

[0150] Scratches were made to make 100 lattices using a cutter knife on the entire surface of the hard coating layer formed on the front surface of each cover window of Examples and Comparative Example within a size of 1 cm*1 cm to 2 cm*2 cm, and Nichiban Tape (CT-24) was attached thereon to perform a peeling test. The peeling test was performed twice on the same surface to evaluate the adhesion from 5B (no peeling) to OB (full peeling) depending on the peeled level. [0151] 5B (no peeled parts) [0152] 4B (1 to 5 lattices containing peeled parts) [0153] 3B (6 to 15 lattices containing peeled parts) [0154] 2B (16 to 35 lattices containing peeled parts) [0155] 1B (36 to 50 lattices containing peeled parts) [0156] 0B (51 or more lattices containing peeled parts)

EXPERIMENTAL EXAMPLE 6

Scratch Resistance Test

[0157] For the hard coating layer formed on the front surface of each cover window of Examples and Comparative Example, a load of 500 gf was applied to steel wool (#0000) and reciprocated 500 times at 30 rpm to evaluate the surface of the hard coating film. When 1 or less scratch with 1 cm or less was observed with the naked eye, it was judged as excellent.

EXPERIMENTAL EXAMPLE 7

Bending Durability Test

[0158] FIG. 1 schematically shows a method of performing a bending durability test and a bending stability test on a film according to an embodiment of the present disclosure.

[0159] Each film 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 a measuring equipment and set so that an interval between folded portions was 4 mm. Then, a process of folding and unfolding both sides of the film at 90 degrees with respect to the bottom at room temperature was repeated 10,000 times by continuous operations (the speed at which the film was folded was once every 1.5 seconds).

[0160] After repeating 10,000 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 folded 10,000 times and whether or not cracks occurred was repeatedly observed, thereby measuring the maximum number of repetitions that cracks do not occur. When cracks did not occur up to 100,000 times of repetitions, the bending durability was judged to be excellent.

EXPERIMENTAL EXAMPLE 8

Bending Stability Test

[0161] Similarly to the bending durability test, each film 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.

[0162] The laser cut film was placed on a fixing device and set so that an interval between folded portions was 4 mm. After leaving both sides of the film folded at 90 degrees with respect to the bottom for 24 hours, the film was then peeled off and turned over so that the folded portion went downward. Then, a □(square)-shaped SUS structure was placed thereon and the film was fixed. The 3D image of the film shape was measured with a noncontact-type surface roughness measuring instrument (PLUTO 681, Dukin Co., Ltd., use of 605 nm laser, resolution 0.1 μm), and the maximum value of the height Z lifted from the bottom was measured as the bending stability.

[0163] In order to measure the recovery of the film, the film in which the bending stability was measured was allowed to stand at room temperature for 1 hour, and then the maximum value of the lifted height Z was again measured. A change in appearance of the folded portion was visually observed.

[0164] When Z is 0.1 mm or less and the change in appearance such as traces on the folded portions is small, it is judged to be OK, and when Z exceeds 0.1 mm or a large number of traces remain on the folded portions, it is judged to be NG.

[0165] The results of measuring the physical properties of Examples 1 to 5 and Comparative Example are shown in Tables 6 and 7 below.

TABLE-US-00006 TABLE 6 Example 1 Example 2 Example 3 Example 4 Transmittance 91.89% 91.79% 92.05% 91.90% Haze 0.42% 0.40% 0.46% 0.56% YI 2.25 2.00 2.35 2.40 Pencil 7H 8H 7H 8H Hardness Coating layer OK OK OK OK adhesion Scratch Excellent Excellent Excellent Excellent resistance Flexibility test 4 mm 4 mm 4 mm 4 mm Bending 100,000 times 100,000 times 100,000 times 100,000 times durability Ok Ok Ok Ok Bending 0.1 mm 0.1 mm 0.1 mm 0.2 mm stability

TABLE-US-00007 TABLE 7 Flexibility test   6 mm Bending durability NG (40,000 times) Bending stability 0.3 mm

[0166] As shown in Table 3, the cover windows for a flexible display device of Examples satisfied sufficient flexibility while simultaneously exhibiting a low yellow index of 2.5 or less and glass-level high hardness. Particularly, they were confirmed that the film was hardly damaged by repetitive bending or folding operations, thereby easily applied to a bendable, flexible, rollable, or foldable mobile device, or display device.

[0167] On the other hand, the cover window of Comparative Example had a relatively low surface hardness or did not exhibit bending durability enough to be used as a cover window for a flexible display device. In addition, it was confirmed that the cover window of Comparative Example had a lower transmittance, a higher haze, or a higher yellow index than that of Examples.