Abstract
Load-extension behavior of a biaxially balanced polypropylene film which has been prepared by the two-step biaxial stretching method is not always isotropic, i.e., the Young's modulus and the yield stress along the pp-axis (the first stretching direction) are smaller than those along the ps-axis (the restretching direction), while the tenacity and the ultimate elongation which relate to large deformation, do not differ according to the direction of the test.
Such an anisotropy appears more remarkably on the thermal shrinkage. When a piece of biaxially balanced film is shrunk freely by raising progressively its temperature, the lpp and lps, the dimensions along the pp- and ps-directions respectively, recover along the restretching curve, which indicates the lpp vs lps, relation during the restretching, until the film is heated to the stretching temperature. On further heating above this temperature, the lpp and lps change reversely along the first stretching curve. Both the optical index and x-ray diffraction in reference to the pp-axis orientation change correspondingly with this dimensional change.
