Polyester film and flexible display device comprising the same

Patent metadata

Field	Value
Publication number	US-11548207-B2
Application number	US-202016863488-A
Country	US
Kind code	B2
Filing date	Apr 30, 2020
Priority date	May 8, 2019
Publication date	Jan 10, 2023
Grant date	Jan 10, 2023

Abstract

The polyester film according to the embodiment achieves the elasticity required for a cover of a flexible display by increasing the rate of increase in stress in an initial stage of strain to a certain level or higher. As a result, it is possible to maintain the original characteristics when it is applied to a cover of a flexible display device and subjected to a plurality of repeated folding. Accordingly, the polyester film can be applied to a cover of a flexible display device, in particular, a foldable display device to exhibit excellent characteristics.

First claim

The invention claimed is:

1. A polyester film having tensile stress values satisfying the following Relationships (1) and (2) in an in-plane first direction and in an in-plane second direction perpendicular to the first direction:
1.1≤ TS 1[20%]/ TS 1[6%]  (1)

1.1≤ TS 2[8%]/ TS 2[4%]  (2)

in the above relationships, TS1[6%] and TS1[20%] are the stresses (kgf/mm 2 ) at a strain of 6% and 20% in the first direction, respectively, and TS2[4%] and TS2[8%] are the stresses (kgf/mm 2 ) at a strain of 4% and 8% in the second direction, respectively,
wherein the polyester film is biaxially stretched in the in-plane first direction and in the in-plane second direction, and
wherein a ratio (d 2 /d 1 ) of a stretching ratio in the in-plane second direction (d 2 ) to a stretching ratio in the in-plane first direction (d 1 ) is 1.2 or less.

2. The polyester film of claim 1 , wherein TS1[20%] is greater than TS1[6%] by 1 kgf/mm 2 or more, and TS2[8%] is greater than TS2[4%] by 1.5 kgf/mm 2 or more.

3. The polyester film of claim 1 , wherein TS1[20%] and TS2[8%] are 10 kgf/mm 2 to 15 kgf/mm 2 .

4. The polyester film of claim 1 , wherein the ratio of TS1[20%]/TS1[6%] is 1.1 to 1.25, and the ratio of TS2[8%]/TS2[4%] is 1.1 to 1.2.

5. The polyester film of claim 1 , which further satisfies the following Relationship (3):
0.9≤( TS 2[8%]/ TS 2[4%])/( TS 1[20%]/ TS 1[6%])≤1.1  (3)

in the above relationship, TS1[6%] and TS1[20%] are the stresses (kgf/mm 2 ) at a strain of 6% and 20% in the first direction, respectively, and TS2[4%] and TS2[8%] are the stresses (kgf/mm 2 ) at a strain of 4% and 8% in the second direction, respectively.

6. The polyester film of claim 1 , wherein the first direction is the longitudinal direction (MD) of the film, and the second direction is the transverse direction (TD) of the film.

7. The polyester film of claim 1 , which withstands 100,000 times or more of repeated folding at an angle of 135° until it is broken.

8. The polyester film of claim 1 , which is applied to a cover of a flexible display device.

9. A laminate, which comprises a transparent cover for a flexible display device; and the polyester film of claim 1 disposed on the transparent cover.

10. The laminate of claim 9 , wherein the transparent cover is a polyimide-based film or ultra-thin glass (UTG), and the transparent cover withstands 100,000 times or more of repeated folding at an angle of 135° until it is broken.

11. A flexible display device, which comprises the polyester film of claim 1 on a cover.

12. A process for preparing a polyester film, which comprises molding a polyester resin to a sheet form and biaxially stretching it to obtain a polyester film, wherein the polyester film has tensile stress values satisfying the following Relationships (1) and (2) in an in-plane first direction and in an in-plane second direction perpendicular to the first direction:
1.1≤ TS 1[20%]/ TS 1[6%]  (1)

1.1≤ TS 2[8%]/ TS 2[4%]  (2)

In the above relationships, TS1[6%] and TS1[20%] are the stresses (kgf/mm 2 ) at a strain of 6% and 20% in the first direction, respectively, and TS2[4%] and TS2[8%] are the stresses (kgf/mm 2 ) at a strain of 4% and 8% in the second direction, respectively,
wherein a ratio (d 2 /d 1 ) of a stretching ratio in the in-plane second direction (d 2 ) to a stretching ratio in the in-plane first direction (d 1 ) is 1.2 or less.

Assignees

• Skc Co Ltd

Inventors

• Kim Gun Uk
• Heo Young Min
• Lee Sechul
• Park Jin-Seok
• Choi Sang Min

Classifications

• B32B2307/704

  Crystalline · CPC title

• B32B27/08

  of synthetic resin · CPC title

• H05K5/03

  Covers · CPC title

• B29C55/143 Primary

  firstly parallel to the direction of feed and then transversely thereto · CPC title

• B32B2307/412

  Transparent · CPC title