COVER WINDOW FOR FLEXIBLE DISPLAY DEVICE AND FLEXIBLE DISPLAY DEVICE

Abstract

The present disclosure relates to a cover window for a flexible display device and a flexible display device including the cover window, wherein the cover window includes a polymer substrate including a polyamideimide block copolymer; and a hard coating layer formed on at least one surface of the polymer substrate and having a pencil hardness of 5H or more under a load of 750 g, and no crack occurs when wound on a mandrel having a diameter of 3 mm.

Claims

1. A cover window for a flexible display device, comprising a polymer substrate comprising a polyamideimide block copolymer having a repeating unit containing an aromatic group; and a hard coating layer formed on at least one surface of the polymer substrate and having a pencil hardness of at least 5H under a load of 750 g, wherein the polyamideimide block copolymer comprises an imide block having an imide repeating unit containing an aromatic tetravalent functional group; and an imide block having an amide repeating unit containing an aromatic divalent functional group, at least one fluorine-containing functional group is substituted in at least one of the imide repeating unit containing the aromatic tetravalent functional group and the amide repeating unit containing the aromatic divalent functional group, and no crack occurs on the cover window when the cover window is wound on a mandrel having a diameter of 3 mm.

2. The cover window for a flexible display device of claim 1, wherein a yellow index of the polymer substrate measured in accordance with ASTM D1925 is 4.5 or less, and a haze of the polymer substrate measured in accordance with ASTM D1003 is 1.1% or less.

3. The cover window for a flexible display device of claim 1, wherein an elastic modulus of the polymer substrate measured at a strain rate of 12.5 mm/min is at least 5 GPa.

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

5. The cover window for a flexible display device of claim 1, wherein the polyamideimide block copolymer comprises an imide block having a first repeating unit represented by Chemical Formula 1; and an amide block having at least one selected from the group consisting of a second repeating unit represented by Chemical Formula 2 and a third repeating unit represented by Chemical Formula 3: ##STR00012## in the Chemical Formula 1, each R.sup.1 is the same as or different from each other in each repeating unit, and each is independently a single bond, —O—, —S—, —C(═O)—, —CH(OH)—, —S(═O).sub.2—, —Si(CH.sub.3).sub.2—, —(CH.sub.2).sub.p— (wherein, 1=p≤=10), —(CF.sub.2).sub.q— (wherein, 1=q≤=10), —C(CH.sub.3).sub.2—, —C(CF.sub.3).sub.2—, —C(═O)NH—, or a C6 to C30 divalent aromatic organic group; each R.sup.2 is the same as or different from each other in each repeating unit, and each is independently —H, —F, —Cl, —Br, —I, —CF.sub.3, —CCl.sub.3, —CBr.sub.3, —CI.sub.3, —NO.sub.2, —CN, —COCH.sub.3, —CO.sub.2C.sub.2H.sub.5, a silyl group containing three C1 to C10 aliphatic organic groups, a C1 to C10 aliphatic organic group, or a C6 to C20 aromatic organic group; n1 and m1 are each independently an integer of 0 to 3; each Y.sup.1 is the same as or different from each other in each repeating unit, and each independently comprises a C6 to C30 divalent aromatic organic group containing at least one trifluoromethyl group (—CF.sub.3); and the aromatic organic group exists alone, or two or more aromatic organic groups are bonded to each other to form a divalent condensed ring, or two or more aromatic organic groups are linked by a single bond, a fluorenylene group, —O—, —S—, —C(═O)—, —CH(OH)—, —S(═O).sub.2—, —Si(CH.sub.3).sub.2—, —(CH.sub.2).sub.p— (wherein, 1=p≤=10), —(CF.sub.2).sub.q— (wherein, 1=q≤=10), —C(CH.sub.3).sub.2—, —C(CF.sub.3).sub.2—, or —C(═O)NH—; and each E.sup.1 is independently a single bond, or —NH—; ##STR00013## in the Chemical Formulae 2 and 3, Y.sup.2 and Y.sup.2′ are the same as or different from each other in each repeating unit, and each is independently a C6 to C30 divalent aromatic organic group containing at least one trifluoromethyl group (—CF.sub.3); and the aromatic organic group exists alone, or two or more aromatic organic groups are bonded to each other to form a divalent condensed ring, or two or more aromatic organic groups are linked by a single bond, a fluorenylene group, —O—, —S—, —C(═O)—, —CH(OH)—, —S(═O).sub.2—, —Si(CH.sub.3).sub.2—, —(CH.sub.2).sub.p— (wherein, 1=p≤=10), —(CF.sub.2).sub.q— (wherein, 1=q≤=10), —C(CH.sub.3).sub.2—, —C(CF.sub.3).sub.2—, or —C(═O)NH—; and E.sup.2, E.sup.2′, E.sup.3, E.sup.3′, E.sup.4 and E.sup.4′ are independently a single bond, or —NH—.

6. The cover window for a flexible display device of claim 5, wherein a molar ratio of the second repeating unit:the third repeating unit is 10:90 to 50:50.

7. The cover window for a flexible display device of claim 5, wherein a molar ratio of the imide block:the amide block is 3:7 to 6:4.

8. The cover window for a flexible display device of claim 5, wherein a molar ratio of the imide block:the amide block is 3:7 to 4:6, and a molar ratio of the second repeating unit:the third repeating unit is 30:70 to 45:55.

9. The cover window for a flexible display device of claim 5, wherein a molar ratio of the imide block:the amide block is 4.5:5.5 to 6:4, and a molar ratio of the second repeating unit:the third repeating unit is 20:80 to 40:60.

10. The cover window for a flexible display device of claim 5, wherein the first repeating unit comprises a repeating unit represented by Chemical Formula 1-1: ##STR00014## in the Chemical Formula 1-1, R.sup.1, R.sup.2, n1 and m1 are as defined in the Chemical Formula 1.

11. The cover window for a flexible display device of claim 5, wherein the second repeating unit comprises a repeating unit represented by Chemical Formula 2-1 and the third repeating unit comprises a repeating unit represented by Chemical Formula 3-1: ##STR00015##

12. The cover window for a flexible display device of claim 1, wherein the polyamideimide block copolymer has a weight average molecular weight of 10,000 to 1,000,000 g/mol.

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

14. 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.

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0117] FIG. 1 schematically shows a method of performing a bending durability test and a bending stability test of Experimental Examples 7 and 8.

[0118] 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

[0119] 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.

[0120] 60 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 %, 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

[0121] 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

[0122] 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

[0123] 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

[0124] 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.

[0125] 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

[0126] 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.

[0127] 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 Manufacturer, Prep. Ex. Prep. Ex. Prep. Ex. Prep. Ex. Prep. Ex. Prep. Ex. product name A-1 A-2 A-3 A-4 A-5 A-6 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.

Preparation Example B: Synthesis of Polyamide-Imide Copolymer and Preparation of Polymer Substrate

Preparation Example B-1

[0128] 2,2′-bis(trifluoromethyl)benzidine, 4,4′-(hexafluoroisopropylidene)diphthalic anhydride and dimethylacetamide were placed in a round flask equipped with a dean-stark apparatus and a condenser, and the reaction was initiated at room temperature. The reaction mixture was stirred using ice water at 0° C. for 4 hours under a nitrogen atmosphere.

[0129] After 4 hours, the reaction product was taken out to room temperature, and 2,2′-bis(trifluoromethyl)benzidine, isophthaloyl dichloride (IPC), 4,4′-biphenyldicarbonyl chloride (BPC) and dimethylacetamide were added thereto. Thereafter, the reaction was initiated at room temperature under a nitrogen atmosphere.

[0130] After the formation of a polyamic acid polymer by the reaction for 4 hours, acetic anhydride and pyridine were added to the reaction mixture, and the mixture was stirred in an oil bath at about 40° C. for 15 hours to carry out a chemical imidization reaction.

[0131] After the completion of the reaction, the reaction mixture was precipitated in water and ethanol (1:1 (v/v)) to obtain a polyamide-imide block copolymer A-1 having the following first repeating unit, second repeating unit and third repeating unit (weight average molecular weight: about 200,000 g/mol). The obtained copolymer had a molar ratio of the first repeating unit:the second repeating unit and the third repeating unit ({circle around (1)}) of 50:50 and a molar ratio of the second repeating unit:and the third repeating unit ({circle around (2)}) of 20:80.

[0132] [The first repeating unit]—The imide repeating unit

##STR00009##

[0133] [The second repeating unit]—The amide repeating unit (derived from IPC)

##STR00010##

[0134] [The third repeating unit]—The amide repeating unit (derived from BPC)

##STR00011##

[0135] Thereafter, the polyamide-imide block copolymer obtained above was dissolved in dimethylacetamide to prepare a solution of about 10 wt %. The solution was cast on a glass plate using a bar coater and a drying temperature was controlled to 120° C. and 200° C. in sequence. A polymer substrate including the polyamide-imide block copoplymer having a thickness of 50 μm was obtained.

Preparation Examples B-2 to B-4

[0136] Polyamide-imide copolymers were prepared by controlling the amount of the respective monomers so as to satisfy the molar ratios shown in Table 2 below.

[0137] Then, polymer substrates including one of the polyamide-imide block copolymer were prepared in the same manner as in Preparation Example B-1.

TABLE-US-00002 TABLE 2 The ratio of amide Imide:Amide - repeating units - {circle around (2)} Mw of Elastic Index {circle around (1)} Kinds Ratio copolymer Y.I. Haze modulus Prep. Ex. B-1 50:50 IPC:BPC 20:80 520,000 4.00 0.90 5.69 Prep. Ex. B-2 40:60 IPC:BPC 20:80 420,000 4.30 0.85 5.88 Prep. Ex. B-3 40:60 IPC:BPC 30:70 360,000 3.50 0.55 5.53 Prep. Ex. B-4 40:60 IPC:BPC 40:60 270,000 3.30 0.51 5.36 * Imide: Amide {circle around (1)} represents a molar ratio of the first repeating unit:the second repeating unit + the third repeating unit. * The ratio of amide repeating units {circle around (2)} represents a molar ratio of the second repeating unit (or the 2-2 repeating unit):the third repeating unit (or the 2-2 repeating unit). * Y.I. (Yellow Index): The yellow index of the polyamide-imide film sample (thickness: 50 ± 2 μm) was measured according to the method of ASTM D1925 using a COH-400 Spectrophotometer (manufactured by NIPPON DENSHOKU INDUSTRIES). * Haze: The haze of the polyamide-imide film sample (thickness: 50 ± 2 μm) was measured according to the method of ASTM D1003 using a COH-400 Spectrophotometer (manufactured by NIPPON DENSHOKU INDUSTRIES). * Elastic Modulus (GPa): The elastic modulus of the polyamide-imide film sample (thickness: 50 ± 2 μm) was measured using Zwich/Roell z005 (5 kN) under a strain rate of 12.5 mm/min.

Examples 1 to 6 and Comparative Example 1: Cover Window for Flexible Display Device

[0138] 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.

[0139] Examples and Comparative Example are shown in Tables 3 and 4 below.

TABLE-US-00003 TABLE 3 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Coating Prep. Ex. Prep. Ex. Prep. Ex. Prep. Ex. Prep. Ex. Prep. Ex. composition A-1 A-2 A-3 A-1 A-1 A-1 Polymer Prep. Ex. Prep. Ex. Prep. Ex. Prep. Ex. Prep. Ex. Prep. Ex. substrate B-1 B-1 B-1 B-2 B-3 B-4 Total 70 μm 70 μm 70 μm 70 μm 70 μm 70 μm thickness Thickness 10 μm 10 μm 10 μm 10 μm 10 μm 10 μm of coating layer

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

Experimental Example: Measurement of Physical Properties of Optical Laminate

Experimental Example 1: Pencil Hardness

[0140] 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

[0141] 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.)

[0142] 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

[0143] 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

[0144] 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 0B (full peeling) depending on the peeled level. [0145] 5B (no peeled parts) [0146] 4B (1 to 5 lattices containing peeled parts) [0147] 3B (6 to 15 lattices containing peeled parts) [0148] 2B (16 to 35 lattices containing peeled parts) [0149] 1B (36 to 50 lattices containing peeled parts) [0150] 0B (51 or more lattices containing peeled parts)

Experimental Example 6: Scratch Resistance Test

[0151] 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

[0152] 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.

[0153] 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).

[0154] 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

[0155] 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.

[0156] 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.

[0157] 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.

[0158] 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.

[0159] The results of measuring the physical properties of Examples 1 to 5 and Comparative Examples 1 to 5 are shown in Tables 5 and 6 below.

TABLE-US-00005 TABLE 5 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Transmittance 91.60% 92.03% 92.09% 92.03% 92.15% 91.19% Haze 0.75% 0.71% 0.80% 0.72% 0.40% 0.48% YI 3.34% 3.30% 3.47% 3.54% 2.71% 2.67% Pencil Hardness 7H 8H 7H 6H 7H 6H Coating layer adhesion OK OK OK OK OK OK Scratch resistance Excellent Excellent Excellent Excellent Excellent Excellent Flexibility test 4 mm 4 mm 4 mm 4 mm 5 mm 5 mm Bending durability 100,000 100,000 100,000 100,000 100,000 100,000 times Ok times Ok times Ok times Ok times Ok times Ok Bending stability 0.1 mm 0.1 mm 0.1 mm 0.2 mm 0.1 mm 0.2 mm

TABLE-US-00006 TABLE 6 Comparative Example 1 Transmittance 89.12% Haze 0.90% YI 4 Pencil Hardness 2H Coating layer adhesion — Scratch resistance NG Flexibility test 2 mm Bending durability 100,000 times OK Bending stability 0.4 mm

[0160] As shown in Tables 5 and 6, the cover windows for a flexible display device of Examples satisfied sufficient flexibility while simultaneously exhibiting 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.

[0161] 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.