Multilayer film and use of same

1. A hard coating film for a foldable display, comprising:

a polyester film having a thickness of from 10 μm to 100 μm; and

an adhesive layer formed on at least one surface of the polyester film,

wherein the polyester film satisfies 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 adhesive layer has a thickness of from 1 μm to 50 μm,

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 arranged on the polyester film at least on an opposite side to a side of the hard coating film having the adhesive layer.

2. The hard coating film for 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 6% or less.

3. The hard coating film for a foldable display according to claim 1 , wherein the application amount of the easy-to-adhere layer is from 0.005 g/m 2 to 0.20 g/m 2 .

4. The hard coating film for a foldable display according to 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.

5. The hard coating film for a foldable display according to 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.

6. The hard coating film for a foldable display according to claim 1 , wherein the easy-to-adhere layer comprises fine particles with an average particle diameter of 2 μm or less.

7. The hard coating film for a foldable display according to claim 6 , 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.

8. The hard coating film for a foldable display according to 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.

9. A foldable display comprising the hard coating film for a foldable display of claim 1 arranged as a surface protective film so that the hard coating layer is positioned on a surface of the foldable display, wherein the hard coating film is a single hard coating film continuous across a folding portion of the foldable display.

10. The foldable display according to claim 9 , 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.

11. A mobile device comprising the foldable display of claim 9 .