Paste extrusion of polytetrafluoroethylene (PTFE): Surface tension and viscosity effects

The effects of the lubricant physical properties on the processing of polytetrafluoroethylene (PTFE) fine powder resins are studied. Lubricants having different surface tension and viscosity were used; the two properties changed independently. These effects were studied by using dies of various contraction angle and reduction ratio for 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 of the physical properties on the extrusion pressure influence significantly 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. Finally, the effects of die geometry on extrusion pressure and mechanical properties of extrudates were also assessed in order to determine the geometrical characteristics and operation conditions for the optimization of the process.