Polyimide film, process for preparing the same and controlling isotropy of the same

A process for preparing a polyimide film which comprises the steps of:

• a) forming a green sheet which has opposite ends and contains volatile components;
• and
• b) heating the green sheet while moving the sheet through a furnace with the opposite ends of the green sheet being fixed relative to each other;
• c) wherein during the heating step the original film has an orientation axis angle θ of a positive value at both ends in the original film width direction, and the furnace temperature is lower than the boiling point of a main volatile component in a zone where the original film is transferred for a distance equal to the film width between both fixed ends from a film fixed end of the film.

A process for preparing a polyimide film which comprises the steps of:

• a) forming a green sheet which has opposite ends and contains a volatile component;
• and
• b) heating the green sheet while moving the sheet through a furnace with the opposite ends of the green sheet being fixed;
• c) wherein during the heating step, in the case where the original film has an orientation axis angle θ of a positive value at both ends in the original film width direction, the temperature is lower than the boiling point of a main volatile component in a zone where the original film is transferred for a distance equal to the film width between both fixed ends, and in the case where the original film has an orientation axis angle θ of a negative value at both ends in the original film width direction, the temperature is at least 100° C. higher than the boiling point of a main volatile component in a zone where the original film is transferred for a distance equal to the film width between both fixed ends from a film fixed end of the film.