Abstract
Dynamics of a novel pneumatic process of superthin fibres formation from polymer melts in supersonic air jets in the Laval nozzle is studied using computer simulation. The approach bases on the mathematical k- models of air flow in the nozzle and air drawing of polymer filaments in the coaxial air jet. The aerodynamic fields can be considered as undisturbed by presence of a single row of thin polymer filaments and predetermined air conditions are used in the modeling. The air fields are simulated for several values of the air compressions in the nozzle inlet and two nozzle geometries. Driving force of the Laval nozzle process results from the air drag forces acting onto the filament surface. Mathematical model of stationary melt spinning in single-, thin-filament approximation is applied with the effects of non-linear viscoelasticity of the polymer melt accounted for. The model allows also to discuss non-linear stress-optical relationship reflecting online molecular orientation, as well as online crystallization of the polymer filament if it occurs. Negative rheological extra-pressure in the air-drawn filament is predicted, as resulting from non-linear viscoelasticity of the polymer melt subjected to high elongation rates. The negative extra-pressure could lead to cavitation and longitudinal burst splitting of each filament into a high number of superthin sub-filaments. A hypothetical mean diameter of the sub-filaments is estimated from an energetic criterion. Example computations of the dynamic profiles of air drawing and discussion concern isotactic polypropylene (iPP) subjected to the Laval nozzle process.