Process for preparing biaxially drawn polyamide films

Claim 1

A process for the preparation of a biaxially drawn polyamide film, which comprises: drawing a substantially amorphous unoriented polyamide film in the longitudinal direction at a temperature of 45° C. to 65° C., a rate of deformation of at least 10,000% /min and a draw ratio of 2.7 to 3.5 by a roll type longitudinal drawing method to form a longitudinally drawn film; transferring the longitudinally drawn film to a position of starting the drawing in the lateral direction at a temperature adjusted to 45° C. to 60° C., within a time represented by formula (I): t=e.sup.(3.9-0.053T 1) (I) wherein e represents the base of the natural logarithm, t represents the time in between the point of termination of the drawing in the longitudinal direction and the point of starting of the drawing in the lateral direction, and T 1 represents the temperature of the film during this period of time, which is within the range of from 45° C. to 60° C., the thickness profile of the longitudinally drawn film at said position of starting the drawing in the lateral direction, is gradually reduced from the side end portions (tenter clip holding portions) with respect to the lateral direction, toward the central portion with respect to the lateral direction, so that the thickness of the film at the central portion with respect to the lateral direction is 75 to 90% of the thickness of the film at the side end portions with respect to the lateral direction; and drawing the longitudinally drawn film in the lateral direction at an average rate of deformation of 2,000 to 10,000%.min and a draw ratio of 3 to 5 by a tenter type lateral drawing method wherein, until the mechanically set draw ratio between respective tenter clips exceeds 1.4 times the orginal distance, the film is expanded at an angle not greater than 6° with respect to the center line, in the lateral direction of the film, at a tenter clip temperature T 2 satisfying the condition T 2 <T 1 , the film temperature being elevated stepwise from the position of starting the drawing in the lateral direction, to a film temperature at the position of termination of the drawing in the lateral direction, of from 70° C. to 100° C.