The Transient Flow Rheology and Instabilities of Polyethylene Melts
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 strongly dependent on its shear history and this effect was named as the Delayed Pressure Build-up (DPB) effect. In DPB effect, the increase in apparent viscosity with apparent shear rate was delayed during the initial period of build-up of pressure across the capillary. Flow visualisation experiments for POE revealed the presence of layer near the walls of the capillary, associated with the development of the DPB effect. This layer appears to be a direct result of the flow of POE through the capillary and is independent of shear rate. LLDPE was found to exhibit flow characteristics observed for both POE and HDPE. The MPR was capable of providing an unique insight into the transient flow behaviour and instabilities for the three polyethylenes, because of its ability of repeatedly shear a given sample volume. In general, the flow of all three polyethylenes appears to be affected by polymer /wall interactions and/ or polymer/ polymer interactions near the wall.
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