This report shows the effects of crystalline orientation on the mechanical properties of the biaxially stretched polypropylene films prepared by dry-process.
In this study, polypropylene fiims T-die extruded with 150μ thickness and random orientation were used. In a recirculating hot-air bath at 145°C, and at constant rate 1600%/min, the films were stretched to various extensions using the filmstretcher, (
i.e., uniaxially stretched under the conditions of free width and/or constant width, two-way successively, and simultaneously biaxially stretched), and then heat set at 150°C for 10min.
Mechanical properties of the variously stretched films were evaluated by stress-strain curves measured at 20°C, 65% R. H. On the other hand, the behaviors of crystalline orientation and crystallization in these stretched films were investigated by the x-ray diffractogram and density measruement.
The following results were obtained:
(1) The crystalline orientation of uniaxially stretched films under the conditions of free width has uniaxial orientation of which crystal
c-axis is oriented parallel to the tensile axis, cylindrical symmetry around the stretching direction, as a fiber. For the uniaxially stretched films with the constant width, however, the crystallites have uniplanar-axialy orient in the way that the c- and a′-axies orient parallel to a filmsurface plane and the c-axis parallel to the tensile axis. The difference of both mechanical properties was connected with that of the orientation.
(2) The crystalline orientation of two-way successively biaxially stretched film varied depending on streching ratio, that is by successive stretching to transverse directon, the initiial uniplanaraxial orientation changes into the intermitted state, and finally approaches to the uniplanar-axial orientation to a transverse dirction. During the stretching, the crystallite begins to turn to transverse direction, and this process is presumably related to the unfolding of crystals. After the “blance point”, rectystallization seems to take place.
(3) The crystalline orientation of simultaneously biaxially stretched films is uniplanar orientation at all stretch ratio. Therefore, the equal mechanical properties are obtained within the stretched plane at all stretched ratio.
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