Structure and Mechanical Properties of Polypropylene Fiber in the Direct Spin Draw Process Equipped with the Cooling Take-up Roll

In the field of nylon and polyester fibers, there has been a tendency in recent years for the Direct Spin Draw (DSD) process to become the mainstream technology. However, in the case of Polypropylene (PP), fibers with high tensile strength could not be obtained using the DSD process because as-spun fibers could not be drawn to high draw ratio.To solve this problem, a new DSD (I-DSD) process was developed. In a previous paper, studies on the effect of installation of a cooling take-up roll on the structure and drawability of as-spun fibers were carried out. In this paper, the structure of fibers obtained in the following three processes was examined: (1) the I-DSD process, (2) the conventional-type DSD (C-DSD) process and (3) the offline drawing (OFD) process in which drawing is carried out on an offline basis. There was a large difference in the structure of fibers drawn to the same draw ratio in the C-DSD process and the other two processes. The fiber obtained by the C-DSD process was found to have higher crystallinity and higher birefringence values compared with the other two. On the other hand, the highest melting point of fiber was obtained in the OFD process, followed by the I-DSD process, and then by the C-DSD process. It was found that a fiber with a lower melting point had a shorter “long period”.In addition, the tensile strength of fiber obtained by the C-DSD process was the highest among the fibers obtained by the three processes when compared at the same draw ratio. However, because the draw ratio could only be increased to about 3-4, its tensile strength could only be raised to around 0.30GPa. On the contrary, the draw ratio of fiber obtained in the I-DSD process, as in the OFD process, could be increased to about 9, so that it was possible to obtain fibers whose tensile strength was high or about 0.51GPa.