Paste Extrusion of Polytetrafluoroethylene (PTFE) Powders Through Tubular and Annular Dies at High Reduction Ratios Pramod D. Patil, 1 Ochoa Isaias, 1 Christos Stamboulides, 1 Savvas G. Hatzikiriakos, 1 Fabio Polastri, 2 Valerj Kapeliouchko 2 1 Department of Chemical and Biological Engineering, The University of British Columbia, Vancouver, BC, Canada 2 Solvay Solexis S.P.A, Bolate, Milan, Italy Received 19 February 2007; accepted 30 September 2007 DOI 10.1002/app.27681 Published online 22 January 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Paste extrusion experiments are reported for three different polytetrafluoroethylene (PTFE) fine powders using both capillary and annular dies having a high reduction ratio (cross-sectional area of reservoir to cross-sectional area of die at the exit) varied from 1000 up to 4000. The extrusion pressure is reported as a function of the apparent shear rate for various resins and dies. The ap- proximate mathematical model for paste extrusion through capillary dies developed by Ariawan et al. (Ariawan et al., Can Chem Eng J, 2002, 80, 1153) was used to describe the experimental data by best fitting the five material constants of the constitutive model. Using these constants, the model developed by Patil et al. (Patil et al., AIChE J., 2006, 52, 4028) was used to predict the extrusion pressure for all three resins in annular dies. The model predictions are found to be consistent with experimental results and the analogy between the rod and tube extrusion models is demonstrated. Ó 2008 Wiley Periodicals, Inc. J Appl Polym Sci 108: 1055–1063, 2008 Key words: polytetrafluoroethylene; paste flow; tube extru- sion; rod extrusion; radial flow hypothesis, annular die INTRODUCTION In polytetrafluoroethylene (PTFE) paste extrusion, fine powder of individual primary particles of diam- eter about 0.20–0.25 lm is first mixed with a lubri- cating liquid (lube) to form a paste. 1,2 The paste is consequently compacted at a typical pressure of 2 MPa to produce a preform of cylindrical shape that is nearly free of air voids. The next step involves loading the cylindrical preform into a ram extruder at a temperature slightly above 308C where PTFE particles become reasonably deformable. 2 Extrusion through annular or capillary dies produces strong extrudates due to the structure formation through fibrillation of individual particles. This is usually followed by the evaporation of the lubricant by passing the extrudates through an oven. Sintering at high temperatures (3808C) is necessary when full strength and porosity elimination are required for processes such as wire coating and tube fabrica- tion. 2–4 Numerous constitutive models have been devel- oped for flows of viscoelastic materials, such as polymer melts, 5 solids under plastic deformations, 6 and elastic–plastic materials that exhibit strain hard- ening as in the case of metal forming or wire draw- ing. 7 The empirical equation suggested by Benbow and Bridgwater 3 has been proven to be a good model; however, it cannot predict the effect of die entrance angle on the extrusion pressure of PTFE paste, although it works quite well for other pasty materials. 8–11 Because of its empirical nature, modifi- cations of any theoretical significance are also diffi- cult to incorporate. An improved analytical model for orifice extrusion of visco-plastic materials has recently been proposed. 12 Because of structure for- mation (fibrillation), strain hardening effects are obtained at high contraction angles during PTFE flow and, therefore, these models (Benbow and Bridgwater 3 and Basterfield et al. 12 ) are not suitable for PTFE paste flow through cylindrical and annular dies. Ariawan et al. 4 have proposed a visco-plastic model to predict the dependence of extrusion pres- sure on die geometrical parameters for rod extru- sion. This approximate model successfully captures the nonmonotonic dependence of extrusion pressure on die entrance angle and other geometrical charac- teristics of the cylindrical die. Its derivation is based Correspondence to: S. G. Hatzikiriakos (hatzikir@interchange. ubc.ca). Contract grant sponsor: Solvay Solexis, Bollate Milan, Italy. Journal of Applied Polymer Science, Vol. 108, 1055–1063 (2008) V V C 2008 Wiley Periodicals, Inc.