Rheological and Theoretical Estimation of the Spinnability of Polyolefines

Abstract

Melt spinning is a polymer processing technique that makes great demands on the extensibility of the polymer melt in the distance between die exit and solidi®cation point [1]. The polymer material is exposed to a rapidly growing deformation rate over a large range of deformation within a short time of about 100 milliseconds. Simultaneously an extreme cooling occurs with cooling rates of about 1000 K=s. For this reason only a few polymer materials are usable for this kind of polymer processing with suf®cient take-up speeds. Most polymers show a ®ber break in the molten state either by brittle cohesive rupture or ductile failure when approaching critical conditions of deformation. The rheological behaviour of a polymer melt at the critical conditions of deformation in the ®ber forming process can not be predicted by means of usual rheological material functions. This paper reports the attempt to ®nd out material functions, which describe the critical deformation states of the melt spinning process. The established material functions are compared with the results of spinning experiments to estimate their practicality. Discussion of Spinnability in Relation to RheologyIn the melt spinning process a short time load occurs for the polymer with rapid increase of the extensional strain and simultaneous extreme cooling between spinneret and winding point [1]. From the point of view of rheology this uniaxial extension is a`strong¯ow' because it tends to generate a high degree of molecular stretching and orientation [2].At the approach to the critical¯ow condition a ®ber break happens by ductile failure or brittle cohesive fracture in the molten state [3]. At Rauschenberger [3] the ductile failure is probably the essential cause for the ®ber break in the melt spinning process. This failure of the material can not be predicted using standard rheological material functions.Small oscillatory frequency sweeps in the linear viscoelastic range are not able to describe the¯ow properties at large strains. The steady-state viscosity functions from the shear experiment admittedly represent the behaviour at large strains and strain rates, but only at the equlibrium state for t 3 I which is not reached in melt spinning. Extensional viscosimeters make possible experiments on polymer melts only with higher viscosities and at small extensional rates. Typical melt spinning polymers can not be tested because of their low melt viscosities.Nonisothermal extensional experiments by means of a Rheotens apparatus as the take-up device result in an increase of melt strength with increasing weight average molecular weight [4]. This statement can not be transferred to the melt spinning process where the inverse effect is observed.Theoretical models of polymer networks predict the stress-strain behaviour at large strains [5, 6], but the interpretation and prediction of the failure at critical¯ow conditions is not possible.The aim of our study was to obtain material functions which come close to the critical¯ow conditions at melt spinning. The transie...

Design of a Series of Preceramic B-Tri(methylamino)borazine-Based Polymers as Fiber Precursors: Shear Rheology Investigations

“…Beyreuther et al proposed requirements for melt-spinning polyolefin-type polymers based on shear rheology investigations. Results were checked by Hoffmann et al based on the correlation between the shear rheology in small-angle oscillation, tan δ, and spinnability of polymer.…”

“…Criteria defined by Beyreuther et al are well distinct from our predictions established in the present paper. Results reported in Table even predict that poly[ B -(methylamino)borazines] represent nonspinnable compounds, whereas spinning experiences showed that some of them display reliable spinnability (polymers 3 − 5 ) .…”

Improved Spinnability of Metallocene Polyethylenes by Using Processing Aids

“…Because of the too high viscoelasticity of this sample ductile failure occurs in the spinline [11]. However samples with 30 % A and 70 mass-% C were spinnable.…”

Evaluation of the processability of boron‐containing organosilazane polymers based on shear rheology

“…Influence of temperature variation along the fiber (including solidification and crystallization phenomena) on the melt‐spinning simulations are investigated in several studies29, 32–35; however, direct measurements of temperature variation along a thin fiber are not yet published.…”