Pospone Polymer Processing Instabilities: the Sharkskin Extrusion Instability and its Minimisation in Polyethylene Processing
At high rates of polymer processing, irregularities or defects appear on the surface of the polymer product whenever the wall shear stress exceeds a critical value [1,2,3]. These instabilities cause a serious problem in the polymer processing industry, limiting the maximum production rates on commercial applications, such as in blown and cast film and in wire coating. The first surface distortion that appears above the critical stress level is known as sharkskin and is characterised by fine scale high frequency small amplitude surface defects with varying levels of surface roughness (see Fig. 1). Sharkskin gives rise to an opaque extrudate and also has a detrimental effect on the mechanical properties of the polymer product. Extrusion is one of the main methods of processing polymers. It involves forcing molten polymer (melt) through a shaping device called a die. Dawn's work has involved extrusion of polymer melts through dies and looking at ways to minimise sharkskin extrusion instabilities.
|Figure 1. SEM of sharkskin surface on high-density polyethylene.|
The aim of Dawn's PhD project was to further the understanding of the sharkskin extrusion instability and to carry out an experimental and simulation study to investigate various methods to minimise its effects. This work is part of a larger project titled 3PI or "Postpone Polymer Processing Instabilities", which was being undertaken by a consortium of European companies and academic centres. Its objective was to link the viscoelastic properties of polymers, the boundary conditions at the die wall and the processing conditions to the appearance and development of instabilities.
Polymer Characterisation: Five polymers were characterised in this study. The three that were found to exhibit sharkskin also displayed limiting stresses at high shear rates. This supported the idea favoured by a growing number of researchers [4,5,6] that the instability is caused by a failure of the polymer to sustain high shear stresses and rupturing above a critical stress.
Gas-Assisted Extrusion: Since it had been observed in the literature that melt slip at the die wall eliminates sharkskin [7,8], Dawn performed experiments to observe the effect of full slip due to a gas boundary layer between the melt and die wall. She discovered that sharkskin was not reduced with gas. Numerical simulations in Polyflow showed that the stresses believed to cause sharkskin are very dependent on the level of slip. Therefore slip alone is not sufficient to reduce sharkskin but a controlled level of partial slip is necessary.
Decreasing Die Wall Roughness: Extrusion pressure drops and flow birefringence observations indicated that decreasing die wall roughness promotes partial slip of the melt along the die wall. This is possibly the reason that sharkskin was found to be reduced for very smooth dies.
Die Exit Curvature: Conventional dies have 90o exits. Flow visualisation experiments showed that in curved exit dies, separation of the melt from the die is delayed, allowing relaxation of the melt before leaving the die. Polyflow simulations confirmed smaller stress concentrations for curved exit dies. These results verified those of .
Addition of fluoropolymer (FP) to high-density polyethylene (HDPE): This was found to completely eliminate sharkskin confirming previous results . The FP is believed to migrate to the die wall during extrusion forming a slip boundary layer.
The results from this study support the belief that sharkskin is caused by local die exit, melt stress concentrations. Gas-assisted extrusion experiments and Polyflow numerical simulations showed that slip alone is not sufficient to reduce sharkskin but a controlled level of partial slip (~ 30%) is required. Decreasing the die wall roughness was dicovered to reduce the magnitude of sharkskin by 40%, while curvature of the die exit reduced sharkskin by 45%. The addition of fluoropolymer to high-density polyethylene was found to be the most effective method to minimise sharkskin as it was found to completely eliminate the instability.
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