Process for preparing a poly(amide-imide) film and a poly(amide-imide) film prepared by the same

1. A process for preparing a poly(amide-imide) film, which comprises:

preparing a polymer solution;

transferring the polymer solution to a tank;

carrying out vacuum degassing after the polymer solution is transferred to the tank for 30 minutes to 3 hours, and the internal pressure of the tank reaches to 0.1 to 0.7 bar;

purging the tank with an inert gas;

storing the polymer solution in the tank for 12 hours to 360 hours after the purging of the tank;

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

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

cooling the cured film while it is moved; and

winding the cooled cured film using a winder,

wherein the tank has an internal temperature of from −20° C. to 20° C.,

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 a target viscosity is reached;

(c) determining that the viscosity of the first polymer solution is less than the target viscosity, and adding more of the dicarbonyl compound to prepare a second polymer solution having the target viscosity, and

wherein the target viscosity is 100,000 cps to 300,000 cps at room temperature.

2. The process for preparing a poly(amide-imide) film of claim 1 ,

which further comprises the step of purging the tank with an inert gas in the step of preparing the polymer solution.

3. The process for preparing a poly(amide-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 poly(amide-imide) film of claim 1 ,

wherein the purging is carried out by purging the tank with the inert gas at an internal pressure of 1 atm to 2 atm.

5. The process for preparing a poly(amide-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.

6. The process for preparing a poly(amide-imide) film of claim 1 ,

wherein the thermal treatment is carried out for 5 to 40 minutes at a temperature in a temperature range of 80° C. to 500° C. and while the temperature is increasing from an initial temperature to a maximum temperature at a temperature elevation rate of 2° C./min to 80° C./min.

7. The process for preparing a poly(amide-imide) film of claim 6 ,

wherein the initial temperature of the thermal treatment of the gel-sheet is 80° C. or higher, and the maximum temperature in the thermal treatment is 300° C. to 500° C.

8. The process for preparing a poly(amide-imide) film of claim 1 ,

wherein the step of cooling the cured film while it is moved comprises

a first temperature lowering step of reducing the temperature at a rate of 100° C./min to 1,000° C./min, and

a second temperature lowering step of reducing the temperature at a rate of 40° C./min to 400° C./min;

the second temperature lowering step is performed after the first temperature lowering step; and

the temperature lowering rate of the first temperature lowering step is faster than the temperature lowering rate of the second temperature lowering step.

9. The process for preparing a poly(amide-imide) film of claim 1 ,

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

10. The process for preparing a poly(amide-imide) film of claim 1 , wherein the thickness variation (%) according to the following Equation 1 is 3% to 30%:

Thickness variation (%)=( M 1− M 2)/ M 2×100  [Equation 1]

in the above Equation 1, M1 is the thickness (μm) of the gel-sheet, and M2 is the thickness (μm) of the cooled cured film at the time of winding.

11. A poly(amide-imide) film prepared by the preparation process of claim 1 .