Polyamide-imide film and preparation method thereof

1. A process for preparing a polyamide-imide film, which comprises:

simultaneously or sequentially mixing and reacting a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent in a polymerization apparatus to prepare a polymer solution;

transferring the polymer solution to a tank;

casting the polymer solution in the tank and then drying it to prepare a gel-sheet;

thermally treating the gel-sheet, while it is moved on a belt, to prepare a cured film; and

winding the cured film using a winder,

wherein the ratio of the moving speed of the gel-sheet on the belt at the time of thermal treatment to the moving speed of the cured film at the time of winding is 1:0.95 to 1:1.40,

wherein the step of preparing the polymer solution comprises:

(a) simultaneously or sequentially mixing and reacting a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent in a polymerization apparatus to prepare a first polymer solution;

(b) measuring the viscosity of the first polymer solution and evaluating whether the target viscosity is reached; and

(c) if the viscosity of the first polymer solution does not reach the target viscosity, further adding the dicarbonyl compound to prepare a second polymer solution having the target viscosity.

2. The process for preparing a polyamide-imide film of claim 1 , wherein the target viscosity at room temperature is 100,000 cps to 300,000 cps.

3. The process for preparing a polyamide-imide film of claim 1 , wherein, once the polymer solution is prepared, the step of transferring the polymer solution to the tank is carried out without any additional steps.

4. The process for preparing a polyamide-imide film of claim 1 , wherein the temperature inside the tank is −20° C. to 0° C.

5. The process for preparing a polyamide-imide film of claim 1 , which, after the polymer solution is transferred to the tank, further comprises carrying out vacuum degassing; and

purging the tank with an inert gas.

6. The process for preparing a polyamide-imide film of claim 1 , which, after the polymer solution is transferred to the tank, further comprise storing the polymer solution in the tank for 12 hours to 60 hours.

7. The process for preparing a polyamide-imide film of claim 1 , wherein the polymer solution is cast and then dried at a temperature of 60° C. to 150° C. for 5 minutes to 60 minutes to prepare a gel-sheet.

8. The process for preparing a polyamide-imide film of claim 1 , wherein the thermal treatment is carried out in a temperature range of 80° C. to 500° C. at a temperature elevation rate of 2° C./min to 80° C./min for 5 to 40 minutes.

9. The process for preparing a polyamide-imide film of claim 1 , which, after the cured film is prepared by thermal treatment, further comprises cooling the cured film while it is moved on a belt.

10. The process for preparing a polyamide-imide film of claim 1 , wherein the ratio of the moving speed of the gel-sheet on the belt at the time of thermal treatment to the moving speed of the cured film at the time of winding is 1:0.99 to 1:1.10.

11. The process for preparing a polyamide-imide film of claim 1 , wherein the area value up to the yield point derived by the 0.2% off-set method on a stress-strain curve of the polyamide-imide film as measured using a universal testing machine (UTM) is 80 to 150 J/m2.

12. The process for preparing a polyamide-imide film of claim 1 , wherein the polyamide-imide film has a modulus of 5.0 GPa or more based on a thickness of 50 μm.

13. The process for preparing a polyamide-imide film of claim 1 , wherein the polyamide-imide film has a restoration angle of 60° or more at the time of evaluating the folding restoring force, and

the restoration angle at the time of evaluating the folding restoring force refers to the angle at which the film bent and inserted between zigs of a 5 mm interval is restored after 24 hours under the conditions of 85° C. and 85% RH.