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Paste extrusion of polytetrafluoroethylene fine powder resins Ariawan, Alfonsius B.

Abstract

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.

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