Melt spinning of continuous filaments by cold air attenuation

Melt spinning is the most convenient and economic method for polymer fiber manufacturing at industrial scales. In its standard setup, however, the thermomechanical history is often hard to control along the long spinline, resulting in poor controllability of the processing and fiber structure, and limited capability of producing very fine fibers. To address these process drawbacks, we developed and investigated an alternative melt spinning process where attenuation of continuous filaments is conducted solely by an annular high-speed cold air jet. This differs from the standard melt spinning process where filament stretching is driven by mechanical pulling force applied along the spinline. With the new process, the fiber is quenched by a symmetric cold air jet and simultaneously attenuated where an inverse parabolic velocity profile in molten fiber is expected. Since the formation of fiber structure is highly dependent on the process conditions, the new process will provide a unique and controllable operation window to study fiber attenuation and structural formation under high-speed cold air drawing.;Assisted by computational analysis on air/polymer fluid dynamics, we designed a high speed jet attenuation spinneret pack consisted of an extrusion die, an isolation plate, an air chamber, and an air nozzle. We built a piston-driven melt extruder, mounted to a hydraulic press and retrofitted with a single orifice extrusion die and the above described spinneret pack. The diameter of the die orifice was 0.5 mm. Parametric experimental studies were carried out to investigate effects of process variables on resulting fiber properties, including fiber diameter, molecular orientation, crystallinity and mechanical properties. Theoretical modeling was conducted to analyze the non-isothermal fiber attenuation mechanism under cold air drawing conditions.;Polypropylene was chosen as the polymer for the major part of the process study. The fibers produced by the new process showed a uniform diameter and a smooth surface appearance. The fiber diameter was found highly dependent on the polymer viscosity and processing conditions, including the processing temperature and the air/polymer flow ratio. The fiber diameter decreases with increasing of the air/polymer flow ratio and the processing temperature, and with decreasing of the polymer viscosity and the initial fiber velocity. Fibers with diameter of 7 mum were produced from the 0.5 mm diameter spinneret, yielding an equivalent drawing ratio exceeding 5,000.;The molecular orientation of cold air attenuated fibers was found to increase with the increase of the air/polymer flow ratio. The maximum molecular orientation was observed at mild processing temperature (200-220oC). The measured fiber mechanical properties, in general, correlated well with the molecular orientation. The tensile strength and modulus increased with increasing of the air/polymer flow rate. Although no post drawing and heat setting steps were performed, the single filament with diameter of 10mum (without post drawing) showed moderately good mechanical properties, with a tensile strength of 100 MPa and a modulus of 2.5 GPa.;Orientation induced crystallization was found to be dominant in this new process. The fiber crystallinity increases with increase of the air/polymer flow ratio accompanied with formation of high molecular orientation. Low crystallinity fiber was associated with high processing temperature at relatively low air/polymer flow ratio. Only alpha-monoclinic crystalline was formed in the produced polypropylene fibers.;For further demonstration, this process was also successfully applied to other polymer materials, including Nylon-6.;An isothermal Newtonian model was developed to analyze the single factor effect of processing conditions on fiber diameter. A non-isothermal model was implemented to predict the fiber diameter under different processing conditions. The predicted values compared favorably with the experimental data.;The new knowledge obtained in this study would likely yield a new process for producing innovative fiber products.