Polyester film and flexible display device comprising the same
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.
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.