Polyamide-imide-based film, preparation method thereof, and cover window and display device comprising the same
The embodiments aim to provide a polyamide-imide-based film, which has a content of nitrogen (N) elements in the film of 6 to 7.5% by weight based on the sum of the weights of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) elements in the film, whereby it is uniform in quality and excellent in mechanical properties and optical properties, a process for preparing the same, and a cover window and a display device comprising the same.
1 . A polyamide-imide-based film, which has a content of nitrogen (N) elements in the film of 6 to 7.5% by weight based on the sum of the weights of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) elements in the film,
wherein when the transmittance, haze, yellow index, and modulus are each measured at six points in total, in which two arbitrary points are selected in each of region N, region C, and region S partitioned in the TD direction of the film,
the average value of transmittance is 80% or more, and the deviation rate of transmittance is 0.3% or less,
the average value of haze is 1% or less, and the deviation rate of haze is 10% or less,
the average value of yellow index is 5% or less, and the deviation rate of yellow index is 10% or less, and
the average value of modulus is 5 GPa or more, and the deviation rate of modulus is 10% or less.
2 . The polyamide-imide-based film of claim 1 , wherein the content of nitrogen (N) elements in the film is 4 to 7% by weight based on the total weight of the film.
3 . The polyamide-imide-based film of claim 1 , wherein the content of nitrogen (N) elements in the film is 8 to 11 parts by weight relative to 100 parts by weight of carbon (C) elements in the film.
4 . The polyamide-imide-based film of claim 1 , wherein the sum of the contents of nitrogen (N) and hydrogen (H) elements in the film is 6 to 9% by weight based on the total weight of the film.
5 . The polyamide-imide-based film of claim 1 , wherein when the film is immersed in MIBK for 5 seconds and dried at 80° C. for 3 minutes, the amount of change in haze (ΔHZ M ) is 0.2% or less when its haze is measured, and
when the film is immersed in IPA for 5 seconds and dried at 80° C. for 3 minutes, the amount of change in haze (ΔHZ I ) is 0.1% or less when its haze is measured.
6 . The polyamide-imide-based film of claim 1 , which has an EC A value of 15.9 or more as defined by the following Equation 1:
EC
A
=
22
C
W
(
2
O
W
+
12
H
W
+
2
N
W
)
[
Equation
1
]
in Equation 1,
C w is the content (% by weight) of carbon (C) elements contained in the film based on the total weight of the film,
O w is the content (% by weight) of oxygen (O) elements contained in the film based on the total weight of the film,
H w is the content (% by weight) of hydrogen (H) elements contained in the film based on the total weight of the film, and
N w is the content (% by weight) of nitrogen (N) elements contained in the film based on the total weight of the film.
7 . The polyamide-imide-based film of claim 1 , which has an EC B value of 15.3 or more as defined by the following Equation 2, an EC C value of 14 or more as defined by the following Equation 3, and an EC D value of 10.5 or more as defined by the following Equation 4:
EC
B
=
33
C
W
(
4
O
W
+
12
H
W
+
2
N
W
)
[
Equation
2
]
EC
C
=
30
C
W
(
4
O
W
+
12
H
W
+
2
N
W
)
[
Equation
3
]
EC
D
=
(
C
W
+
O
W
)
N
W
[
Equation
4
]
in Equations 2 to 4,
C w is the content (% by weight) of carbon (C) elements contained in the film based on the total weight of the film,
O w is the content (% by weight) of oxygen (O) elements contained in the film based on the total weight of the film,
H w is the content (% by weight) of hydrogen (H) elements contained in the film based on the total weight of the film, and
N w is the content (% by weight) of nitrogen (N) elements contained in the film based on the total weight of the film.
8 . The polyamide-imide-based film of claim 1 , which has an R MO value of 95% to 105% as defined by the following Equation 5, and an R EL of 90% to 110% as defined by the following Equation 6:
R
MO
(
%
)
=
MO
T
MO
M
*
100
[
Equation
5
]
R
EL
(
%
)
=
EL
T
EL
M
*
100
[
Equation
6
]
in Equations 5 and 6,
MO T is the modulus measured with the TD direction of the film as the longitudinal direction,
MO M is the modulus measured with the MD direction of the film as the longitudinal direction,
EL T is the elongation measured with the TD direction of the film as the longitudinal direction, and
EL M is the elongation measured with the MD direction of the film as the longitudinal direction.
9 . The polyamide-imide-based film of claim 1 , wherein when the film is irradiated with a UV-A lamp (wavelength 340 nm) at 0.63 W/m 2 at 60° C. for 72 hours, the amount of change in yellow index (AYI) is 5 or less when the yellowness is measured.
10 . The polyamide-imide-based film of claim 1 , which has a transmittance of 80% or more, a haze of 1% or less, a yellow index of 5 or less, a modulus in the MD direction of 5 GPa or more, a modulus in the TD direction of 5 GPa or more, an elongation in the MD direction of 20% to 30%, and an elongation in the TD direction of 20% to 30%.
11 . A cover window for a display device, which comprises a polyamide-imide-based film and a functional layer, wherein the content of nitrogen (N) elements in the polyamide-imide-based film is 6 to 7.5% by weight based on the sum of the weights of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) elements in the film.
12 . A display device, which comprises a display unit; and a cover window disposed on the display unit,
wherein the cover window comprises a polyamide-imide-based film and a functional layer, and the content of nitrogen (N) elements in the polyamide-imide-based film is 6 to 7.5% by weight based on the sum of the weights of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) elements in the film.
13 . A process for preparing a polyamide-imide-based film of claim 1 , which comprises:
polymerizing a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent to prepare a polyamide-imide-based polymer solution;
casting the solution and then drying it to prepare a gel sheet; and
thermally treating the gel sheet,
wherein the step of thermally treating the gel sheet comprises thermally treating it through a first heater, a second heater, and a third heater spaced apart in the TD direction of the gel sheet, and
when the temperature of the first heater corresponding to the center of the gel sheet is T HC , and when the temperatures of the second heater and the third heater corresponding to both ends of the gel sheet are T HN and T HS , respectively, T HN and T HS are higher than T HC .
14 . The process for preparing a polyimide-based composite film according to claim 13 , wherein T HC is 300° C. to 360° C., and T HN and T HS are higher than T HC by 2% to 18%.