In this work, we have studied the effects of extrusion die design, resin molecular structure, and lubricant concentration on the properties of FTFE paste extrudates by performing macroscopic extrusion pressure measurements, Raman spectrwcopy, differential scanning calorimetry and mechanical testing on the exhudates. Five resins of Werent 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 extrudmg 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 entmnce angle, the extent of fibrillation is also increased, a l t h o m 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..Corresponding author. E-mail address: hatzildr@interchange.ubc.ca IS. G. HatAkwakw). 25 wt?! (4, 6). The paste is then compacted at a typical pressure of 2 MPa to produce a cylindrical billet (preform) that is h e of air void (5-7). The next step involves the extrusion of the preform using a ram extruder at a temperature slightly higher than 30°C (4). This is usually followed by the evaporation of the lube through an oven and then sintering, for processes such as wire coating and tube fabrication. In the production of FTFE tapes, the extrudate is calendered before passing through the oven, with no sintering to follow. More detailed descriptions of the processes can be found elsewhere (4.6,8).The fabrication of extrudates of considerable mechanical strength, even without sintering, from a free flowing fine powder resin is possible with PTFE because of its unique phase transition properties. PTFE has two transition temperatures of approximately 19°C and 30°C which are particularly important because of their proximity to ambient temperature (2-5). Below 19°C. shearing will cause PTFE crystals to slide past each other, retaining their identity. Above 19°C. FTFE molecules are packed more loosely, and shearing Reduction Ratio = 56:l. Vol. Flowrate = 75.4 mm3/s Reduction Ratio = 336: 1 Vol. Flowrate = 124.9 mm3/s UD = 20, a = 30°, m Melt Creep Viscosity * 1 O -' ' (Pas) Ffg. 12. 'Ihe effect of resin melt creep viscosity (molecular &ht) on extmdak tensile strength Lines are drawn to guide the eye. Fiued symbols represent copolymer series and Unmd symbols represent hornpolymer series.

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The influence of molecular structure on the rheology and processability of blow‐molding grade high‐density polyethylene (HDPE) resins is studied using capillary and extensional rheometers, a melt indexer, and a blow‐molder unit. Twenty‐four commercial HDPE resins were analyzed in terms of their shear and extensional flow properties, extrudate swell characteristics, and melt strength. The resins had varying molecular weight characteristics and were produced using a variety of polymerization technologies (gas, slurry, and solution phase). It was found that shear viscosity is not only influenced by the weight average molecular weight (Mw) and polydispersity index (PI), but also technology dependent, irrespective of molecular characteristics. Increasing Mw was found to increase both shear and extensional viscosity, while increasing PI by increasing the concentration of smaller molecules increases the tendency of the resin to shear thin. In relating melt strength and temperature sensitivity of shear viscosity to molecular parameters, resins had to be grouped according to ranges of PI < 8, 8. < PI < 10, and PI > 10. Moreover, it was possible to relate melt strength to the Hencky strain obtained from creep experiments and to the melt index of the resins. Finally, it was found that extrudate swell behavior and melt strength are important parameters to be considered during parison formation, as observed during blow‐molding experiments. © 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 1–13, 2001

The influence of molecular structure on the rheology and processability of blow-molding grade high-density polyethylene (HDPE) resins is studied using capillary and extensional rheometers, a melt indexer, and a blow-molder unit. Twenty-four commercial HDPE resins were analyzed in terms of their shear and extensional flow properties, extrudate swell characteristics, and melt strength. The resins had varying molecular weight characteristics and were produced using a variety of polymerization technologies (gas, slurry, and solution phase). It was found that shear viscosity is not only influenced by the weight average molecular weight (M ) and w polydispersity index (PI), but also technology dependent, irrespective of molecular characteristics. Increasing M was found to increase both shear and w extensional viscosity, while increasing PI by increasing the concentration of smaller molecules increases the tendency of the resin to shear thin. In relating melt strength and temperature sensitivity of shear viscosity to molecular parameters, resins had to be grouped according to ranges of PI Ͻ 8, 8. Ͻ PI Ͻ 10, and PI Ͼ 10. Moreover, it was possible to relate melt strength to the Hencky

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.

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