Optical film, method for manufacturing same, and liquid crystal display including same
Disclosed are an optical film, a method for preparing the same, and a liquid crystal display including the same. The optical film is a single film of a non-liquid crystalline thermoplastic resin, and includes: first and second surface part placed on both surfaces of the film in a thickness direction; and an inner part of the film placed between the first and second surface part, wherein the first and second surface part are not aligned with a tilt in the thickness direction, and the inner part of the film is aligned with a tilt in the thickness direction.
1. An optical film being a single film of a non-liquid crystalline thermoplastic resin, the single film comprising:
first and second surface parts on both surfaces of the single film in a thickness direction; and
an inner part of the single film between the first surface part and the second surface part,
wherein the first and second surface parts are not aligned with a tilt in the thickness direction, the inner part of the single film is aligned with a tilt in the thickness direction, and the first and second surface parts and the inner part of the single film have the same thermoplastic resin composition.
2. The optical film according to claim 1 , wherein each of the first and second surface parts has a thickness of 1 μm to 20 μm from the surface thereof.
3. The optical film according to claim 1 , wherein the first and second surface parts occupy 1% to 30% of a total thickness of the optical film.
4. The optical film according to claim 1 , wherein the inner part of the single film has a film β angle of 5° to 35°, as defined as an angle at which transmittance under crossed nicols is minimized.
5. The optical film according to claim 4 , wherein the inner part of the single film has a ratio of a maximum β angle (β1) to a minimum β angle (β2)(β1/β2) of 1:1 to 7:1.
6. The optical film according to claim 4 , wherein the thermoplastic resin comprises at least one of cycloolefin resins, polycarbonate resins, polyolefin resins, aromatic vinyl resins, polyamide resins, polyimide resins, polyester resins, and acrylic resins.
7. The optical film according to claim 1 , wherein the inner part of the single film has a thickness of 5 μm to 100 μm.
8. The optical film according to claim 1 , wherein the optical film has an in-plane retardation value (Ro′) of 20 nm to 110 nm at a wavelength of 550 nm, as defined by Equation 1:
Ro ′=( nx−ny ′)× d, [Equation 1]
wherein nx and ny′ are refractive indices in x-axis and y′-axis directions, respectively, and d is film thickness.
9. The optical film according to claim 1 , wherein the optical film has an out-of-plane retardation value (Rth′) of 80 nm to 190 nm at a wavelength of 550 nm, as defined by Equation 2:
Rth ′=[( nx+ny ′)/2 −nz′]×d, [Equation 2]
wherein nx, ny′ and nz′ are refractive indices in x-axis, y′-axis and z′-axis directions, respectively, and d is film thickness.
10. The optical film according to claim 1 , wherein the optical film has a thickness of 30 μm to 110 μm.
11. A liquid crystal display comprising:
a polarizing film; and
the optical film according to claim 1 stacked on the polarizing film.
12. The liquid crystal display according to claim 11 , further comprising: a liquid crystal layer on another surface of the optical film.
13. A method for preparing the optical film of claim 1 , the method comprising:
melt extruding the non-liquid crystalline thermoplastic resin; and
passing the melt extruded thermoplastic resin between a first roll and a second roll to form the optical film,
wherein the first and second rolls have a surface temperature lower than a glass transition temperature of the thermoplastic resin, and impart the tilt angle to the inner part of the film by generating a different shear force on the first and second surface parts of the optical film than in the inner part of the optical film depending on operation of the first and second rolls.
14. The method according to claim 13 , wherein the first and second rolls have a surface temperature (Tr) satisfying Equation 3:
Te× 0.4< Tr<Te× 0.5, [Equation 3]
wherein Te is a temperature of the thermoplastic resin immediately after melt extrusion, and Tr is a surface temperature of the rolls.
15. The method according to claim 13 , wherein the first and second rolls have different elasticities.
16. The method according to claim 13 , further comprising: stretching the thermoplastic resin having passed between the first and second rolls.
17. The method according to claim 16 , wherein the thermoplastic resin is stretched in a transverse direction (TD) by 5% to 20% relative to a machine direction.
18. The method according to claim 16 , wherein the thermoplastic resin is stretched at a temperature lower than a glass transition temperature (Tg) thereof.