Method for producing aromatic polyimide film wherein linear expansion coefficient in transverse direction is lower than linear expansion coefficient in machine direction

A process for producing aromatic polyimide film having a linear expansion coefficient in transverse direction lower than a linear expansion coefficient in machine direction, comprising the steps in order of:

• spreading an aromatic polyimide precursor solution in which an aromatic polyimide precursor is dissolved in a solvent on a surface of a running continuous support to form an aromatic polyimide precursor solution layer;
• heating the aromatic polyimide precursor solution layer to removing a portion of the solvent by evaporation, thereby converting it to a self-supporting aromatic polyimide precursor layer;
• separating the self-supporting aromatic polyimide precursor layer from the continuous support, thereby obtaining a self-supporting aromatic polyimide precursor film;
• stretching the self-supporting aromatic polyimide precursor film under heating;
• and heating the stretched self-supporting aromatic polyimide precursor film at an elevated temperature, thereby converting it to a self-supporting aromatic polyimide film,
• wherein the aromatic polyimide precursor solution is a solution prepared by reaction of a carboxylic acid component comprising 3,3′,4,4′-biphenyltetracarboxylic acid as a predominant ingredient and a diamine component comprising p-phenylene diamine as a predominant ingredient;
• and
• wherein the self-supporting aromatic polyimide precursor film has a solvent content in the range of 25 to 45 wt. % and an imidation ratio in the range of 5 to 40%, the step of stretching the self-supporting aromatic polyimide precursor film under heating is performed by stretching the self-supporting aromatic polyimide precursor film in transverse direction initially at a temperature in the range of 80 to 200° C. and terminated at a temperature in the range of 80 to 300° C., and the step of converting the stretched self-supporting aromatic polyimide precursor film to the self-supporting aromatic polyimide film is performed at temperatures in the range of 350 to 580° C.