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      Paste Extrusion of Polytetrafluoroethylene: Temperature, Blending and Processing Aid Effects


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          Blends of different PTFE fine powder resins have been prepared and extruded in an attempt of improving the PTFE paste extrusion process and/or the extrudates. The extrusion pressure and the mechanical properties of the extrudates were found to fall between those exhibited by the pure components. While this is an expectable behavior, it offers opportunities for controlling the degree of fibrillation needed in the final products by mixing highly fibrillated with low fibrillated PTFE resins. Boron nitride and organically modified montmorillonite clays (solid lubricants) are also tested as additives to the lubricants used in the PTFE paste extrusion in order to identify enhanced processing aids. It was found that the addition of boron nitride and clays increase the extrusion pressure but at the same time improve the mechanical properties of the final extrudates in most cases. This again offers possibilities for controlling the final mechanical properties by controlling the degree of fibrillation, i.e., by adding a small amount of solid lubricants to adjust pressure, fibrillation, and thus the final mechanical properties. Finally, it is shown that fibrillation depends on temperature. Moreover, the mechanical properties of extrudates increase as the temperature goes through its two transition temperatures of 19°C and 30°C that render PTFE particles more deformable and prone to fibrillation.

          Most cited references23

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          Dense packing and symmetry in small clusters of microspheres.

          When small numbers of colloidal microspheres are attached to the surfaces of liquid emulsion droplets, removing fluid from the droplets leads to packings of spheres that minimize the second moment of the mass distribution. The structures of the packings range from sphere doublets, triangles, and tetrahedra to exotic polyhedra not found in infinite lattice packings, molecules, or minimum-potential energy clusters. The emulsion system presents a route to produce new colloidal structures and a means to study how different physical constraints affect symmetry in small parcels of matter.
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            Is Open Access

            Jamming, two-fluid behaviour and 'self-filtration' in concentrated particulate suspensions

            M. Haw (2003)
            We study the flow of model experimental hard sphere colloidal suspensions at high volume fraction \(\Phi\) driven through a constriction by a pressure gradient. Above a particle-size dependent limit \(\Phi_0\), direct microscopic observations demonstrate jamming and unjamming--conversion of fluid to solid and vice versa--during flow. We show that such a jamming flow produces a reduction in colloid concentration \(\Phi_{x}\) downstream of the constriction. We propose that this `self-filtration' effect is the consequence of a combination of jamming of the particulate part of the system and continuing flow of the liquid part, i.e. the solvent, through the pores of the jammed solid. Thus we link the concept of jamming in colloidal and granular media with a 'two-fluid'-like picture of the flow of concentrated suspensions. Results are also discussed in the light of Osborne Reynolds' original experiments on dilation in granular materials.
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              Role of processing aids in the extrusion of molten polymers


                Author and article information

                International Polymer Processing
                Carl Hanser Verlag
                : 21
                : 5
                : 497-503
                1 Department of Chemical Engineering, The University of British Columbia, Vancouver, Canada
                2 School of Mining Engineering and Metallurgy, National Technical University of Athens, Athens, Greece
                Author notes
                Mail address: S. G. Hatzikiriakos, Department of Chemical Engineering, The University of British Columbia, Vancouver, Canada. E-mail: hatzikir@ 123456interchange.ubc.ca
                © 2006, Hanser Publishers, Munich
                : 16 March 2006
                : 21 June 2006
                Page count
                References: 23, Pages: 7
                Self URI (journal page): http://www.hanser-elibrary.com/loi/ipp
                Regular Contributed Articles

                Polymer science,Materials technology,Materials characterization,General engineering,Polymer chemistry


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