Nonisothermal orientation-induced crystallization in melt spinning of polypropylene

Two kinds of polypropylene with different molecular weight (MI = 15 and 30) were melt-spun at the spinning temperatures of 210–290°C and take-up velocities of 0.15–3 km/min. In the cases of the spinning temperatures of 270 and 290°C for MI15 and 250 and 290°C for MI30, the density showed a minimum with increasing take-up velocity at around 0.5–1 km/min. This result suggests that crystallization behavior is influenced by two competitive effects, i.e., cooling rate and crystallization rate both of which are enhanced by the increase in take-up velocity. Crystal structures of slightly oriented monoclinic, slightly oriented pseudohexagonal, highly oriented pseudohexagonal, and highly oriented monoclinic were successively observed with increasing take-up velocity. The change of crystallization temperature may result in the different kinds of crystal modifications. Numerical calculations on nonisothermal orientation-induced crystallization in the melt spinning process and experimental results showed qualitative agreement in the change of crystallinity with take-up velocity, spinning temperature, and molecular weight.