The mechanisms of ‘neck-like’ deformation in high-speed melt spinning. 2. Effects of polymer crystallization

Abstract

As is shown in the first paper of the series, the main factor responsible for concentrated (‘neck-like’) deformation in high-speed melt spinning is the gradient of elongational viscosity along the spinline. In the present paper, stress-induced polymer crystallization is analyzed as a potential source of the rapid viscosity increase. A model of crystallization-controlled solidification is proposed, in which viscosity of the polymer increases with the degree of crystallinity, Θ, as ▪, reaching infinity (complete solidification) at Θ  Θ cr. The critical crystallinity level has been interpreted as one required for ‘crosslinking’ of polymer chains present in the melt. In addition to viscosity increase, crystallization modifies the local temperature in the spinline and reduces viscosity. The analysis of stress effects shows that critical crystallization temperature, T m, and crystallization rate, K, increase with the square of normal stress difference in the spinline, Δ p = p xx − p rr . The onset of crystallization can be shifted by 20–40 K towards higher temperatures, and crystallization rate can increase by orders of magnitude when high take-up speeds increase the stress level. A simple model illustrating velocity profiles in crystallizing Newtonian jets is discussed. The analysis strongly supports the hypothesis that the high viscosity gradient resulting from rapid stress-induced crystallization provides the major mechanism of ‘neck-like’ deformation.