Polyimide-Based Film and Window Cover Film Including the Same

Abstract

The present invention relates to a polyimide-based film which does not substantially cause a mura or rainbow phenomenon and has excellent visibility, a window cover film including the same, and a display panel including the same.

Claims

1. A polyimide-based film satisfying the following Expressions 1 and 2,
Max[R(θ, ψ.sub.0 to 360°)]<R.sub.0×[sin(θ)cos(θ)+5 sin(θ)+1].sup.3   [Expression 1] wherein R.sub.0 represents an in-plane phase difference (R.sub.in) when light is incident perpendicularly to the polyimide-based film (θ=0°), θ represents an inclination angle of the incident light, ψ represents an azimuth angle, R(θ, ψ) represents a phase difference according to an incident angle, and Max[R(θ, ψ.sub.0 to 360°)] represents a maximum phase difference value among phase difference values measured with the fixed inclination angle θ and the azimuth angle ψ of 0° to 360° at an interval of 5°,
1−Min[R(θ.sub.50°,ψ.sub.0 to 360°)]/Avg[R(θ.sub.50°, ψ.sub.0 to 360°)]≤50% or Max[R(θ.sub.50°, ψ.sub.0 to 360°)]/Avg[R(θ.sub.50 °, ψ.sub.0 to 360°)]−1≤50%   [Expression 2] wherein Min[R(θ.sub.50°, ψ.sub.0 to 360°)], Avg[R(θ.sub.50°, ψ.sub.0 to 360°)], and Max[R(θ.sub.50°, ψ.sub.0 to 360°)] represent a minimum phase difference, an average phase difference, and a maximum phase difference, respectively, among phase difference values measured with the fixed inclination angle of 50° and the azimuth angle ψ of 0° to 360° at an interval of 5°.

2. The polyimide-based film of claim 1, wherein the polyimide-based film has a modulus measured according to ASTM D882 of 3 GPa or more.

3. The polyimide-based film of claim 1, wherein the polyimide-based film has an elongation at break of 8% or more, a light transmittance measured at 388 nm according to ASTM D1746 of 80% or less, a total light transmittance measured at 400 to 700 nm according to ASTM D1746 of 87% or more, a haze of 2.0% or less, a yellow index of 5.0 or less, and a b* value of 2.0 or less.

4. The polyimide-based film of claim 1, wherein the polyimide-based film has a modulus measured according to ASTM D882 of 3 GPa or more and an elongation at break measured according to ASTM D882 of 8% or more.

5. The polyimide-based film of claim 1, wherein a thickness of the polyimide-based film is 10 to 500 μm.

6. The polyimide-based film of claim 1, wherein the polyimide-based film has a polyamide-imide structure.

7. The polyimide-based film of claim 6, wherein the polyimide-based film includes a unit derived from a fluorine-based aromatic diamine.

8. The polyimide-based film of claim 6, wherein the polyimide-based film includes a unit derived from an alicyclic dianhydride.

9. The polyimide-based film of claim 6, wherein the polyimide-based film includes a unit derived from a fluorine-based aromatic diamine, a unit derived from an aromatic dianhydride, and a unit derived from an aromatic diacid dichloride.

10. The polyimide-based film of claim 9, wherein the polyimide-based film further includes a unit derived from an alicyclic dianhydride.

11. A window cover film comprising: the polyimide-based film of claim 1; and a coating layer formed on one surface of the polyimide-based film.

12. The window cover film of claim 11, wherein the coating layer is one or more selected from an antistatic layer, an anti-fingerprint layer, an anti-fouling layer, an anti-scratch layer, a low-refractive layer, an anti-reflective layer, and an impact absorption layer.

13. A flexible display panel comprising the polyimide-based film of claim 1.

Description

EXAMPLE 1

[0142] Isophthaloyl chloride (IPC), terephthaloyl chloride (TPC), and 2,2′-bis(trifluoromethyl)-benzidine (TFMB) were mixed with a solution in which dichloromethane and pyridine were mixed with each other in a molar ratio (IPC:TPC:TFMB) of 120:160:380, and stirring was performed in a reactor under a nitrogen atmosphere at 25° C. for 2 hours. A solid content was 10 wt %. After the reaction, the reactant was precipitated in water and then filtered, and the solid content was vacuum-dried at 50° C. for 6 hours or longer, thereby obtaining an oligomer. A formula weight (FW) of the oligomer was 11,580 g/mol.

[0143] N,N-dimethylacetamide (DMAc) as a solvent, 100 moles of the oligomer, and 20 moles of 2,2′-bis(trifluoromethyl)-benzidine (TFMB) were injected into the reactor and stirring was sufficiently performed. After confirming that the solid raw material was completely dissolved, fumed silica (surface area: 95 m.sup.2/g, <1 μm) was added to DMAc in an amount of 1,000 ppm with respect to the solid content, and the fumed silica was dispersed using ultrasonic waves and then injected into the reactor. 120 moles of cyclobutanetetracarboxylic dianhydride (CBDA) were injected, stirring was sufficiently performed, and then polymerization was performed at 40° C. for 10 hours. A solid content was 20 wt %.

[0144] Subsequently, pyridine and acetic anhydride were sequentially added to a solution, respectively, at 2.5-fold moles relative to a total content of the dianhydride, stirring was performed at 60° C. for 12 hours, the mixture was precipitated in an excessive amount of methanol and then filtered, and vacuum-drying was performed at 50° C. for 6 hours or longer, thereby obtaining a polyamide-imide powder. The powder was diluted and dissolved with DMAc at 20 wt % to prepare a polyimide-based resin solution. A weight average molecular weight and a polydispersity index (PDI) of the prepared resin solution were 280,000 g/mol and 2.28, respectively.

[0145] The resin solution was coated onto a PET substrate film at room temperature using a slot die at a shear rate of 120 s.sup.−1 and an air gap of 1.0 mm at a lip portion of the slot die, drying was performed in a hot air drying section under conditions of a temperature of 140° C. and a wind speed of 15 m/s for 6 minutes, and then cooling was performed at room temperature.

[0146] A final film was produced by stretching the film obtained by coating the resin solution to 102% in a machine direction at 170° C., additionally stretching the film to 103% at a speed at which the film entered into a stretching region at 210° C., and shrinking and stretching the film to 100.5% at the speed at which the film entered into the stretching region at 230° C. A thickness of the film was 49 μm.

EXAMPLE 2

[0147] A film was produced in the same manner as that of

[0148] Example 1 except that the wind speed in the drying section was 10 m/s.

EXAMPLE 3

[0149] A film was produced in the same manner as that of Example 1 except that the wind speed in the drying section was 5 m/s.

EXAMPLE 4

[0150] A film was produced in the same manner as that of Example 1 except that the wind speed in the drying section was 2.5 m/s.

COMPARATIVE EXAMPLE 1

[0151] A film was produced in the same manner as that of Example 1 except that the wind speed in the drying section was 1.0 m/s.

COMPARATIVE EXAMPLE 2

[0152] A film was produced in the same manner as that of Example 1 except that the shear rate was 30 s.sup.−1, the wind speed in the drying section was 30.0 m/s, and the thickness of the final film was 99.8 um.

[0153] In the case of each of the films produced in Examples 1 to 4 and Comparative Examples 1 and 2, a light transmittance measured at 388 nm was 65 to 75%, a total light transmittance was 89.5 to 90.5%, a haze was 0.4 to 0.8%, a yellow index (YI) was 1.4 to 1.6, a b* value was 0.8 to 1.0, a modulus was 6.0 to 6.5 GPa, an elongation at break was 15% or more, and a pencil hardness was HB/750 g, which showed that properties of each item were measured at a similar level.

[0154] In addition, a phase difference value according to an in-plane incident angle and an azimuth angle and a transmission mura of each of the films produced in Examples 1 to 4 and Comparative Examples 1 and 2 were evaluated. The results are shown in Table 1.

TABLE-US-00001 TABLE 1 Expression 1 (based on θ = 5°) Expression 2 R.sub.0 Left Right Determination of Determination of Transmission (nm) term term satisfaction Value satisfaction mura Example 1 50 63.2 176.5 Satisfied 4.2% Satisfied ∘ Example 2 100 113.8 352.8 Satisfied 8.3% Satisfied ∘ Example 3 200 213.7 705.2 Satisfied 16.7% Satisfied ∘ Example 4 400 413.3 1,411.3 Satisfied 33.2% Satisfied ∘ Comparative 800 813.6 2,824.0 Satisfied 68.0% Not satisfied x Example 1 Comparative 10 72.4 35.3 Not satisfied 0.2% Satisfied Δ Example 2

[0155] The present invention provides a polyimide-based (polyimide is a term including polyimide or polyamide-imide) film having the above physical properties. The polyimide-based film hardly causes a mura phenomenon. In addition, the window cover film including the polyimide-based film also does not substantially cause a mura phenomenon.

[0156] Further, an object of the present invention is to provide a display panel produced using the polyimide-based film having the above physical properties.

[0157] Further, an object of the present invention is to provide a polyimide-based film as an optical film which does not substantially cause a mura phenomenon, a rainbow phenomenon, and a blackout phenomenon, a window cover film including the same, and a display panel including the same.

[0158] Further, an object of the present invention is to provide a polyimide-based film used for an optical application such as a cover window, which does not cause optical stains such as a rainbow phenomenon and a mura phenomenon that occur according to a viewing angle, and has excellent optical properties such as visibility, a window cover film including the same, and a display device including the same.

[0159] In addition, it is more preferable that the polyimide-based film of the present invention has a modulus measured according to ASTM D882 of 3 GPa or more, an elongation at break measured according to ASTM D882 of 8% or more, a light transmittance measured at 388 nm according to ASTM D1746 of 80% or less, a total light transmittance measured at 400 to 700 nm according to ASTM D1746 of 87% or more, a haze of 2.0% or less, a yellow index of 5.0 or less, and a b* value of 2.0 or less.

[0160] Hereinabove, although the present invention has been described by specific matters, exemplary embodiments, and drawings, they have been provided only for assisting in the entire understanding of the present invention. Therefore, the present invention is not limited to the exemplary embodiments. Various modifications and changes may be made by those skilled in the art to which the present invention pertains from this description.

[0161] Therefore, the spirit of the present invention should not be limited to these exemplary embodiments, but the claims and all modifications equal or equivalent to the claims are intended to fall within the scope and spirit of the present invention.