Abstract:

Objective The forming mechanism of profiled fibers of dry spinning involves heat transfer, mass transfer, phase change, deformation, and so on. The cross-sectional shape changing process, in particular, is affected by surface tension, solvent diffusion and evaporation, and it is difficult to develop quantitative research on this process. The previous studies on dry spinning process dynamics were mainly focused on circular cross-section fibers, and research on dry spinning of profiled diacetate fibers is yet to be carried out. The purpose of this research is to establish a dry spinning dynamics model of profiled diacetate fibers, and compare and validate the simulation results of fibers cross-section against the experimental data.

Method A profile degree calculation equation was derivated from the triangle spinneret hole to describe the variation of fibers cross-section, and combined with a typical one-dimensional dry spinning model to establish a hybrid dynamic model which has advantage in efficient computing. Based on this model, the variations of filament solvent mass fraction, temperature, velocity, tension and cross-sectional area during the spinning process could be obtained, so could the variation in cross-section profile degree. Profiled fibers were prepared by industrial spinning device. Stereomicroscopy was adopted to observe the cross-section of fibers, and the experimental results were compared with the simulation calculation values.

Results Spinning solution quickly evaporated after being extruded from the spinneret since its temperature is higher than the atmospheric boiling point of acetone. Besides, the small diameter of spinneret hole contributed to solvent diffusion and evaporation, and the curing point of filament was only about 50 cm away from spinneret. The glass transition temperature showed an opposite trend to the solvent content, reaching the maximum value after the filament was cured. Filament temperature was rapidly decreased by flash evaporation, with the minimum temperature dropping to -12 ℃. Because of the high specific surface area and fast heat transfer rate of profiled diacetate fibers, the constant temperature evaporation zone of spinning process was only about 20 cm. As the rapid reduction of solvent content during flash evaporation, the filament tension quickly increased under the and accelerated stretching. After that, the tension increased along with the spinline, and the friction force between filament and air was the main influence factor. The filament instantaneously accelerated to take-up velocity after being extruded from spinneret and keeps a plateau value throughout the spinning process. The variation trend of the cross-sectional area was similar to that of the solvent content. The profile degree of diacetate fibers was controlled by solvent evaporation and surface tension with solvent evaporation was found the main control factor. Length of the major and minor axes of the filament cross-section were affected by flash evaporation and significantly shortened in a few centimeters from the spinneret hole, then slowly decreased along the spinline to a plateau value at z=50 cm. However, the decrease ratio of minor axes was larger than major axes so that the profile degree could be increased. The experimental profile degree was 74.4%, and the simulation value was 72.8%. While the experimental fibers cross-sectional area is 642.0 μm², and the simulation value is 614.8 μm².

Conclusion A dry spinning dynamic model was established to simulate the spinning process of profiled diacetate fiber. The results show that solvent evaporation and temperature changes dramatically, and the constant temperature evaporation section is not obvious. Filament approaches the take-up velocity at z=1 cm and cures rapidly near the spinneret while the cross-sectional area reaches a constant value. The simulation results of the dry spinning model of fibers cross-section are consistent with the experimental values. As shown in the experiment, the cross-section of diacetate fibers exhibits as a regular "Y" shape with a high consistency of profile degree, and while there are a little differences in the cross-sectional area among single fibers. It is found that the coefficient of variation of profile degree(CV=1.12%) is smaller than the cross-sectional area(CV=4.17%).

Key words: profiled diacetate fiber, dry spinning, dynamic model, spinning forming process simulation, heteromorphism