Polyester film and application therefor

1. A polyester film for a touchscreen module substrate of a foldable display, the polyester film satisfying the following conditions:

(1) a refractive index in a bending direction is from 1.591 to 1.606;

(2) a refractive index in a folding portion direction is from 1.670 to 1.700;

(3) a refractive index in a thickness direction is 1.520 or less; and

(4) a density is 1.380 g/cm 3 or more,

wherein the bending direction refers to a direction orthogonal to a folding portion of the polyester film at a time when the polyester film is folded,

wherein the polyester film is a biaxially stretched polyethylene terephthalate film, and

wherein the polyester film comprises an easy-to-adhere layer on at least one surface thereof and a hard coating layer having a thickness of from 1 μm to 50 μm.

2. The polyester film for a touchscreen module substrate of a foldable display according to claim 1 , wherein the polyester film has an elastic modulus in the bending direction of 2.7 GPa or less, and an elastic modulus in the folding portion direction of 4.5 GPa or more.

3. The polyester film for a touchscreen module substrate of a foldable display according to claim 1 , wherein the polyester film has a total light transmittance of 85% or more, a haze of 3% or less, and a maximum heat shrinkage ratio of 2% or less.

4. The polyester film for a touchscreen module substrate of a foldable display according to claim 1 , wherein the polyester film is free of a crack and deformation that are recognizable in bending resistance evaluation at a bend radius of 1.5 mm after heat treatment at 150° C. for 30 minutes.

5. The polyester film for a touchscreen module substrate of a foldable display of claim 1 , wherein the easy-to-adhere layer is applied in an amount from 0.005 g/m 2 to 0.20 g/m 2.

6. The polyester film for a touchscreen module substrate of a foldable display of claim 1 , wherein the easy-to-adhere layer comprises at least one water-soluble or water-dispersible resin selected from the group consisting of a polyester-based resin, a polyether polyurethane-based resin, a polyester polyurethane resin, a polycarbonate polyurethane resin, and an acrylic resin.

7. The polyester film for a touchscreen module substrate of a foldable display of claim 1 , wherein the easy-to-adhere layer comprises at least one water-soluble or water-dispersible compound selected from the group consisting of a melamine compound, an isocyanate compound, an oxazoline compound, an epoxy compound, and a carbodiimide compound.

8. The polyester film for a touchscreen module substrate of a foldable display of claim 1 , wherein the easy-to-adhere layer comprises fine particles with an average particle diameter of 2 μm or less.

9. The polyester film for a touchscreen module substrate of a foldable display of claim 8 , wherein the fine particles are at least one member selected from the group consisting of titanium dioxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, calcium fluoride, styrene-based particles, acrylic particles, melamine-based particles, benzoguanamine-based particles, and silicone-based particles.

10. The polyester film for a touchscreen module substrate of a foldable display of claim 1 , wherein the hard coating layer comprises at least one member selected from the group consisting of an acrylic resin, a siloxane-based resin, an inorganic hybrid-based resin, a urethane acrylate-based resin, a polyester acrylate-based resin, and an epoxy-based resin.

11. A foldable display comprising the polyester film for a touchscreen module substrate of a foldable display of claim 1 as a substrate film of a touchscreen module, wherein the polyester film is a single substrate film of a touchscreen module continuous across a folding portion of the foldable display.

12. The foldable display according to claim 11 , further comprising a polarizer, wherein the polyester film is provided on at least the visible side surface of the polarizer, and the slow axial direction of the polyester film is in the range of 0° ±10° or 90° ±10° with the absorption axis direction of the polarizer.

13. A mobile device comprising the foldable display of claim 11 .