Based on the social demand for the reduction of waste and resource, there is an increasing demand for decreasing the film thickness of materials, particularly the materials for packaging uses. Presently, for example, biaxially stretched polypropylene films having a thickness of 20 μm are used as the packaging materials. Most of the biaxially stretched polypropylene films are manufactured by conventional longitudinal-transverse sequential biaxial stretching method. In conventional longitudinal-transverse sequential biaxial stretching method, polymer is melted by an extruder, filtered, extruded from a slit die, and wound around a metal drum to prepare a cooled and solidified unstretched film. The unstretched film is passed between rolls of different rotating speeds and is stretched in the longitudinal direction. The film is then fed into a tenter, is stretched in the transverse direction, is heat-set, cooled, and winded. This process is the typical process for manufacturing biaxially stretched polypropylene films.
Compared with the above-described biaxially stretched polypropylene films having a thickness of 20 μm, a 25% reduction of wastes and resources can be achieved if the same performance and the same converting ability can be achieved with biaxially stretched polypropylene films having a thickness of 15 μm.
To achieve this, biaxially stretched polypropylene films must be tensilized to decrease the elongation against tension applied during the converting process. During the converting process, the tension works in the longitudinal direction of the film. Thus, biaxially stretched polypropylene films must be tensilized mainly in the longitudinal direction.
In general, the heat shrinkage of polypropylene films tends to increase as the polypropylene films are tensilized. When the dimensional stability of the film decreases at high temperatures, the film shrinks during the converting process such as printing, coating, and laminating, thereby drastically decreasing the commercial value of the film. Accordingly, the heat shrinkage must be comparable to or even lower than that of common biaxially stretched polypropylene films.
Japanese Patent Publication of Examined Application Nos. 41-21790, 45-37879, and 49-18628 disclose methods for making films tensilized in the longitudinal directions whereby the film is re-stretched in the longitudinal direction after it is stretched in the longitudinal and transverse direction to increase the longitudinal strength of the film. A drawback of these films tensilized in the longitudinal direction is their low strength in the transverse direction. To overcome this drawback, Japanese Unexamined Patent Application Publication No. 56-51329 discloses a method whereby a polypropylene sheet having predetermined melting/recrystallization temperatures re-stretched in the longitudinal direction after it has been biaxially stretched.
However, in conventional longitudinal-transverse sequential biaxial stretching method, it has been difficult to obtain films tensilized in the longitudinal direction. In other words, in conventional longitudinal-transverse sequential biaxial stretching method, the film must be kept at a certain temperature to maintain a half-melted state because the oriented crystals produced by longitudinal stretching is stretched by transverse stretching. Since most of the crystals become oriented in the transverse direction after transverse stretching, the resulting biaxially stretched polypropylene film has a markedly high strength in the transverse direction when compared to that in the longitudinal direction.
The microstructure, hereinafter referred to as the “fibril structure”, of a common biaxially stretched polypropylene film manufactured by a conventional longitudinal-transverse sequential biaxial stretching method is observed with an atomic force microscope (AFM). A network structure consisting of fibrils having a diameter of approximately 20 nm and being mainly oriented in the transverse direction is observed. The fibrils have a high strength in the length direction, but readily deform in the width direction. This fact is considered as the cause of bias of the film strength in the transverse direction.
Moreover, the methods described in Japanese Patent Publication of Examined Application No. 41-21790 and Japanese Unexamined Patent Application Publication No. 56-51329 in which re-stretching in the longitudinal direction is performed are complex, and require high equipment costs. Moreover, the heat shrinkage is higher than that of common biaxially stretched polypropylene films, which is a problem.