METHODS OF STRETCHING FILMS

摘要

A method of biaxially stretching a polymeric film along an overbias stretch profile. The method comprises the steps of: ) imparting a sufficiently high temperature to the film to allow a significant amount of biaxial stretch; and b) biaxial tenter stretching the film to a final first direction stretch parameter and a final second direction stretch parameter, wherein at least 75% of the final first direction stretch parameter is attained before no more than 50% of the final second direction stretch parameter is attained, and wherein the final first direction stretch parameter is no greater than the final second direction stretch parameter. An alternative method comprises a method of biaxially stretching a polymeric film along an overbias stretch profile. The method comprising the steps of: a) imparting a sufficiently high temperature to the film to allow a significant amount of biaxial stretch; and b) biaxial tenter stretching the film according to a stretch profile to a final first direction stretch parameter and a final second direction stretch parameter, wherein the final first direction stretch parameter is no greater than the final second direction stretch parameter. In such a method: i) a straight line between the point defining zero stretch parameter and the point defining the final first and second direction stretch parameters represents a proportional stretch profile and defines a proportional stretch area; and ii) the curve representing the stretch profile between the point defining zero stretch parameter and the point defining the final first and second direction stretch parameters defines an area at least 1.4 times the proportional stretch area.