Paste Extrusion of Polytetrafluoroethylene (PTFE) Fine Powder Resins

The rheology of non‐melt processible polytetrafluoroethylene (PTFE) pastes has been studied using a capillary rheometer. It was found that fibrils of submicron dimensions are created during PTFE paste processing, which are responsible for the final strength of the extrudate. The mechanism of fibrillation is explained in terms of the unwinding of crystallites. To describe the effects of die design, a simple mathematical model has been developed. The model takes into account the elastic‐plastic (strain hardening) and viscous nature of the material in its non‐melt state. The model predictions are found to be consistent with experimental results obtained from macroscopic pressure drop measurements and flow visualization experiments.     

Properties of polytetrafluoroethylene (PTFE) paste extrudates

In this work, we have studied the effects of extrusion die design, resin molecular structure, and lubricant concentration on the properties of PTFE paste extrudates by performing macroscopic extrusion pressure measurements, Raman spectroscopy, differential scanning calorimetry and mechanical testing on the extrudates. Five resins of different molecular structures were tested. We have found that a balance between fibril quantity and quality (in terms of fibril orientation and continuousness) is necessary to ensure acceptable products, as illustrated through the effects of the operating variables on the extrudate tensile strength. The number of fibrils formed during extrusion can be increased by extruding the paste through a die of larger reduction ratio or by decreasing the lubricant content in the paste, thereby increasing the extrusion pressure. However, excessive pressure will cause fibril breakage. By using a die of larger entrance angle, the extent of fibrillation is also increased, although the quality of the fibrils is somewhat compromised. Increasing the die aspect (L/D) ratio does not increase the extent of fibrillation. However, it increases the degree of fibril orientation and ensures smoother extrudate. Finally, we have found that extrudates obtained using a paste of higher molecular weight are mechanically superior.