Polyamide-imide-based film, preparation method thereof, and cover window and display device comprising the same

1 . A polyamide-imide-based film, which comprises a polyamide-imide-based polymer,

wherein when the coefficients of thermal expansion in the MD direction and TD direction of the film are measured at an interval of 5° C. in a temperature range of 300° C. to 365° C., respectively, the temperature at which the coefficient of thermal expansion in the MD direction or the coefficient of thermal expansion in the TD direction changes from a positive value to a negative value is 330° C. to 345° C., and

wherein when the haze is 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 haze is 1% or less, and the deviation rate of haze is 10% or less.

2 . The polyamide-imide-based film of claim 1 , wherein the temperature at which the coefficient of thermal expansion in the MD direction changes from a positive value to a negative value is 335° C. to 345° C., and the temperature at which the coefficient of thermal expansion in the TD direction changes from a positive value to a negative value is 330° C. to 345° C.

3 . The polyamide-imide-based film of claim 1 , wherein when the yellow index is 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 yellow index is 5 or less, and the deviation rate of yellow index is 10% or less.

4 . The polyamide-imide-based film of claim 1 , wherein when the modulus is 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 modulus is 5 GPa or more, and the deviation rate of modulus is 10% or less.

5 . The polyamide-imide-based film of claim 1 , wherein when an average value (ppm/° C.) (CTEMD) of the coefficients of thermal expansion in the MD direction in a temperature range of 50° C. to 300° C. and an average value (ppm/° C.) (CTETD) of the coefficients of thermal expansion in the TD direction in a temperature range of 50° C. to 300° C. are 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 CTEMD/CTETD value is 0.96 to 1.03.

6 . The polyamide-imide-based film of claim 1 , wherein the polyamide-imide-based polymer comprises an imide-based repeat unit and an amide-based repeat unit at a molar ratio of 2:98 to 70:30.

7 . The polyamide-imide-based film of claim 1 , which further comprises at least one selected from the group consisting of a filler, a blue pigment, and a UVA absorber.

8 . A cover window for a display device, which comprises a polyamide-imide-based film and a functional layer,

wherein the polyamide-imide-based film comprises a polyamide-imide-based polymer, and when the coefficients of thermal expansion in the MD direction and TD direction of the polyamide-imide-based film are measured at an interval of 5° C. in a temperature range of 300° C. to 365° C., respectively, the temperature at which the coefficient of thermal expansion in the MD direction or the coefficient of thermal expansion in the TD direction changes from a positive value to a negative value is 330° C. to 345° C., and

wherein when the haze is 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 haze is 1% or less, and the deviation rate of haze is 10% or less.

9 . 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, the polyamide-imide-based film comprises a polyamide-imide-based polymer,

wherein when the coefficients of thermal expansion in the MD direction and TD direction of the polyamide-imide-based film are measured at an interval of 5° C. in a temperature range of 300° C. to 365° C., respectively, the temperature at which the coefficient of thermal expansion in the MD direction or the coefficient of thermal expansion in the TD direction changes from a positive value to a negative value is 330° C. to 345° C., and

wherein when the haze is 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 haze is 1% or less, and the deviation rate of haze is 10% or less.

10 . A process for preparing the 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 THC, and when the temperatures of the second heater and the third heater corresponding to both ends of the gel sheet are THN and THS, respectively, THN and THS are higher than THC.