Crystallization effects in the spinning of fibers from polymer melts

Molecular orientation in, and crystallinity of, polymers decide about fiber properties. Formation of an ordered molecular structure is essential for spinning processes. Mathematical modelling of fiber spinning involves the dynamic equations of (i) energy conservation that governs the heat transfer between the filament and cooling medium (air), (ii) momentum conservative accounting for inertia, air drag, gravity and surface tension, (iii) constitution of a viscous crystallizing fluid, (iv) kinetics of crystallization (coupled with energy and momentum conservation). Crystallization accompanying melt spinning provides for a heat source embedded in the equation of heat conservation. A one-dimensional model involving temperature-and stress-dependent crystallization allowed to describe fiber formation dynamics in a slow-crystallizing polymer melt. A newtonian liquid model was adopted with temperature-only, or both temperature-and crystallization-dependent viscosity of (PET) melt. Crystallization elevates the viscosity of the melt. The calculations performed for fiber spinning in molten PET showed higher take-up velocities to be conducive to shifting the stress-induced oriented crystallization, and thus to the fiber forming process, toward the spinneret outlet. Crystallization increased the filament velocity gradient and the tensile stress rose accordingly.