Extrusion instability in an aramid fibre spinning process

Abstract

The efficiency of polymer extrusion processes can be severely limited by the occurrence of viscoelastic extrusion instabilities. In a para-aramid fibre spinning process, for example, a ?m-scale extrusion instability is responsible for the waste of tons of polymer per year. At present, a considerable amount of research literature is available on such viscoelastic extrusion instabilities. However, this literature largely applies to isotropic polymers, whereas the polymer solution that is used for the production of para-aramid fibres is liquid crystalline. Liquid crystalline polymers (LCPs) are anisotropic at rest, and their flow behaviour is known to deviate from that of other viscoelastic fluids. Therefore, more research was needed to characterise the extrusion instability in the para-aramid fibre spinning process. The work presented in this thesis deals, first, with the question to which extent the contraction flow of a nematic aramid solution is similar to contrac- tion flow of isotropic polymeric fluids. More specifically, we looked into the flow stability of nematic contraction flow, and its influence on the extruded aramid jet. Second, as a fibre spinning process typically involves the extrusion of around 1 000 closely spaced jets, the influence of the presence of neighbouring outlets on the behaviour of viscoelastic contraction flow is addressed. For the first part of the research, the aramid fibre spinning process was modelled by a 100 ?m deep 100:1 planar contraction flow with free outflow, which was designed to capture the essential features of the extrusion process. At a depth of 100 ?m the optically anisotropic aramid solution is sufficiently transparent to allow for flow visualisation, while at the same time the pressure drop over the geometry permits flow velocities that are realistic for fibre spinning, without damaging the glass flow cells in which the planar contraction geometries were etched. The contraction flow of a nematic aramid solution was compared with the behaviour of a PEG-PEO Boger fluid in the same geometry, using flow visualisation and Particle Image Velocimetry (PIV). It was shown that, under fibre spinning conditions, a nematic aramid solution shows viscoelastic vortex growth. Like in contraction flows of isotropic polymeric fluids, the vortex size in the aramid solution increases with increasing flow rate, and decreases when the contraction entrance is made more gradual (e.g. tapered, or rounded). The velocity field in the aramid solution was demonstrated to be characteristic of its shear-thinning behaviour. The influence of the defect structure in the aramid solution was visible in a wavy instability in the upstream channel, and in the occurrence of regions with a higher velocity than the surrounding flow, in the first minutes after starting the flow. The oscillation of the extruded jet was shown to be coupled to asymmetric velocity fluctuations in the upstream channel. Although no extrusion instability was encountered in the experiments, the existence of a relation between the jet behaviour and the upstream velocity field implies that the stability of the upstream velocity field is important for the stability of the jet. The similarity between the contraction flows of a nematic aramid solution and an isotropic viscoelastic fluid justifies the use of experiments with model fluids in the study of para-aramid fibre spinning. Therefore, the study of the influence of the presence of multiple outlets was carried out with a model fluid. Experiments using a PEG-PEO Boger fluid, and numerical simulations using a FENE-CR model (Finitely Extensible Non-linear Elastic, Chilcott-Rallinson closure), were performed in contraction geometries with one or three outlets, and a large or small distance between the outlets in the three-outlet geometries. The experiments and simulations show that in the three-outlet geometries, the curvature of the streamlines towards the outlets causes a horizontal pressure gradient in the upstream channel, resulting in the flow rate being distributed unequally over the outlets. Because the streamline curvature changes with increasing lip vortex size, the distribution of the flow rate over the outlets depends on the Weissenberg number of the flow. Furthermore, the vortex size was observed to decrease due to the presence of multiple outlets, with a smaller distance between the outlets leading to smaller lip vortices. This was demonstrated to result in a higher maximum elongation rate, a higher pressure drop over the geometry, and a decreased stability of the flow. The fluctuations in vortex height in what is classified here as unstable flow seems to lead to fluctuations in flow rate and outflow direction in the outlets. The results presented in this thesis are relevant for fibre spinning pro- cesses, but also for other production processes featuring multi-outlet extrusion of viscoelastic fluids. A logical next step would be to do experiments with a transparent model fluid in a more complex, three-dimensional extrusion geometry, to study extrusion instability in a multi-outlet geometry and to optimise the efficiency of such extrusion processes.