The mechanisms of ‘neck-like’ deformation in high-speed melt spinning. 1. Rheological and dynamic factors

Abstract

In the search for mechanisms responsible for abrupt (‘neck-like’) deformation of polymer jets in high-speed fibre spinning, two groups of factors are analyzed. Kinematics of inertialess jets from Newtonian, Reiner-Rivlin, ‘power-law’ and co-rotational Maxwell fluids are studied. It is shown that the main factor responsible for S-shaped velocity profiles and concentrated deformation (‘necking’), is a positive viscosity gradient along the fluid jet. Elasticity, and non-linear (elongation-rate dependent) viscosity do not admit ‘necking’ under isothermal conditions, and reduce the necking effects produced by temperature-controlled viscosity increase. A similar study of Newtonian jets with inertial, surface tension, gravity and air drag effects, confirmed the controlling role of the viscosity gradient. Inertia, surface tension and air drag slightly reduce necking in non-isothermal jets; gravity exerts a small positive effect. Necking effects modelled by exponentially varying viscosity cannot quantitatively reproduce the experimental observations on high-speed fibre spinning. An additional source of viscosity gradient — stress-induced crystallization — will be discussed in a separate paper.