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Paste extrusion of polytetrafluoroethylene (PTFE) fine powder resin : the effects of processing aid physical properties Ochoa, Isaias

Abstract

The rheological properties of a number of polytetrafluoroethylene (PTFE) pastes were studied relevant to paste extrusion process. The effects of the physical properties of lubricants and the geometrical characteristics of the extrusion die on the preforming, extrusion pressure and mechanical properties of the final extrudates were studied. A number of lubricants were identified as suitable for the paste extrusion of polytetrafluoroethylene (PTFE). They were characterized in terms of both flow and surface properties. It was found that it is possible to alter the flow and surface properties of these lubricants independently and thus it became possible to study their relative effects on preforming and extrusion of PTFE paste. Based on this study, it was concluded that preforming quality increases with increase of lubricant viscosity and with improvement in the wetting characteristics of the lubricant with PTFE. The effects of the lubricant physical properties on the processing of polytetrafluoroethylene (PTFE) fine powder resins were also studied by using dies of various geometrical characteristics and resins having a variety of molecular architecture. It was found that the wettability (surface tension) of the lubricant strongly affects the pressure needed to extrude the PTFE pastes. The viscosity of the lubricant was also found to play a significant role in the process since a lubricant with a low viscosity causes the paste to be extruded at a lower pressure. These effects significantly influence the mechanical properties of the final extrudates. The latter are functions of the degree of fibrillation which is significantly influenced by the wettability and viscosity of lubricants. The effects of die geometry on extrusion pressure and mechanical properties of extrudates were assessed in order to determine the geometrical characteristics and operation conditions for the optimization of the process. A rheological equation was developed and proposed for flow simulations of the paste extrusion of PTFE pastes, aiming at optimizing this process. Finally, various additives were tested in order to determine possible processing aids for the paste extrusion of PTFE. It was found that moderate amount of boron nitride and organically modified clays can have beneficial effects on mechanical properties of the final extrudates.

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