The Transient Flow Rheology and Instabilities of Polyethylene Melts
type: CreativeWork
description: The extrusion of polymers such as polyethylenes can be inhibited by melt flow instabilities, which differ in nature from one type of polyethylene to the other. In this thesis, the rheology and flow of three polyethylenes, each having a distinct molecular structure is studied. These polyethylenes are High Densiry Polyethylene (HOPE), Polyoleftn Elastomer (POE) and Linear Low Density Polyethylene (LLDPE). The objective of this thesis is to relate the rheological and processing behaviour of these polyethylenes and establish the type of flow instabilities exhibited by each of these polyethylenes. The main experimental work has been carried out using a newly developed two-piston capillary rheomter; Mulripass Rheometer (MPR). The parameters required to rheologically characterise the linear and non-linear behaviour (for lower shear rates) of these polyethylenes were determined from dynamic (oscillatory) experiments and stepstrain experiments performed using the Rheametrics (RDS), parallel-plate rheameter. Similar measurements using a capillary geometry with the MPR were in close agreement with the data obtained using the RDS. The flow of HDPE at higher shear rates is characterised by pressure oscillations of fixed amplitude, which occur when a critical shear stress is attained. These oscillations appear to be sustained by a periodic decompression-compression cycle in the capillary, which is governed by a stick-slip transition occurring at this critical shear stress. Depending on the shear rate, four different flow regimes, including a regime of pressure oscillations could be identified. This type of flow behaviour was mathematically modelled using melt compressibility rather than complex rheological or arbitrarily specified parameters. The predictions of this model were in good agreement with the experimental data. Flaw birefringence experiments for HDPE also gave further insights into its unstable behaviour. In comparison to HDPE, the transient flaw of POE appeared to be …
abstract: The extrusion of polymers such as polyethylenes can be inhibited by melt flow instabilities, which differ in nature from one type of polyethylene to the other. In this thesis, the rheology and flow of three polyethylenes, each having a distinct molecular structure is studied. These polyethylenes are High Densiry Polyethylene (HOPE), Polyoleftn Elastomer (POE) and Linear Low Density Polyethylene (LLDPE). The objective of this thesis is to relate the rheological and processing behaviour of these polyethylenes and establish the type of flow instabilities exhibited by each of these polyethylenes. The main experimental work has been carried out using a newly developed two-piston capillary rheomter; Mulripass Rheometer (MPR). The parameters required to rheologically characterise the linear and non-linear behaviour (for lower shear rates) of these polyethylenes were determined from dynamic (oscillatory) experiments and stepstrain experiments performed using the Rheametrics (RDS), parallel-plate rheameter. Similar measurements using a capillary geometry with the MPR were in close agreement with the data obtained using the RDS. The flow of HDPE at higher shear rates is characterised by pressure oscillations of fixed amplitude, which occur when a critical shear stress is attained. These oscillations appear to be sustained by a periodic decompression-compression cycle in the capillary, which is governed by a stick-slip transition occurring at this critical shear stress. Depending on the shear rate, four different flow regimes, including a regime of pressure oscillations could be identified. This type of flow behaviour was mathematically modelled using melt compressibility rather than complex rheological or arbitrarily specified parameters. The predictions of this model were in good agreement with the experimental data. Flaw birefringence experiments for HDPE also gave further insights into its unstable behaviour. In comparison to HDPE, the transient flaw of POE appeared to be …
author: Ranganathan, Meera
language: eng
license: https://www.repository.cam.ac.uk/bitstreams/9e771f9e-fc86-460b-a4bf-ffde6bdbe8d3/download
license: http://www.rioxx.net/licenses/all-rights-reserved/
date published: 2000-02-15
date modified: 2026-07-22T22:23:57Z
provider: Cambridge University
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Cambridge

PropertyValue
property ID: DOI
value: 10.17863/CAM.58373