| Summary: | Due to its high melting point and melt viscosity, polytetrafluoroethylene (PTFE) is
processed by a number of unusual techniques. These include paste extrusion. In PTFE paste
extrusion, a free-flowing fine powder resin, having a typical individual particle diameter of
0.2 pm, is processed with the aid of a lubricating liquid to form an extrudate of considerable
strength. The process is carried out at near ambient temperatures and is usually followed by
sintering. Although PTFE paste extrusion has been commercialized, little is known of the
fundamental mechanisms underlying the process.
In this work, the fundamental theoretical and experimental aspects of PTFE paste
extrusion were studied. Five resins of different molecular structure were tested. Experiments
were conducted using Instron capillary rheometers, equipped with barrels of different
diameters and dies of various design. Analyses were performed using a differential scanning
calorimeter (DSC), scanning electon microscope (SEM) and micro-Raman spectrometer.
The tensile properties of paste extrudates were determined using a universal Instron
mechanical testing machine. In addition, visualization experiments were performed to
determine the pattern of PTFE paste flow during extrusion.
Prior to extrusion, the PTFE powder-lubricant mixture (paste) is preformed to
produce a compacted cylindrical billet. The preforming behavior of PTFE pastes was studied
in this work, in order to identify and determine the effects of important processing variables.
It was found that the minimum preforming pressure and the duration required to ensure
uniform paste compaction are dependent on the resin molecular structure. An empirical
relationship was established to illustrate this. In the rheological study, the effects of various
operating parameters were investigated. To quantitatively describe the flow behavior of
PTFE paste, a one-dimensional mathematical model was developed, based on observations
from flow visualization experiments. The model takes into account the elastic-plastic (strain
hardening) and viscous nature of the material in its non-melt state. Finally, the mechanism of
PTFE paste flow, which involves the formation of fibrils, was determined using SEM and
verified using DSC . The properties of the extrudates were also analyzed, in terms of fibril
quantity and quality (i.e. fibril orientation and continuity). A balance between fibril quantity
and quality was found to be necessary to ensure acceptable product quality, as illustrated
through the effects of various operating variables on the extrudate tensile strength. === Applied Science, Faculty of === Chemical and Biological Engineering, Department of === Graduate
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