Pramod D. Patil, James J. Feng & Savvas G. Hatzikiriakos

*J. Non-Newtonian Fluid Mech. ***139***, *44-53* *(2006).

**Abstract** - A constitutive rheological
equation is proposed for the paste extrusion of
polytetrafluoroethylene (PTFE) that takes into account the continuous
change of the microstructure during flow, through fibril
formation. The mechanism of fibrillation is captured through a
microscopic model for a structural parameter,ξ. This model
essentially represents a balance of fibrillated and unfibrillated
domains in the PTFE paste through a first order kinetic
differential equation. The rate of fibril formation is assumed to
be a function of the strain rate and a flow type parameter, which
describes the relative strength of straining and rotation in
mixed type flows. The proposed constitutive equation consists of
a shear-thinning and a shear-thickening terms, the relative
contribution of the two being a function of ξ. Finite element
simulations using the proposed constitutive relation predict
accurately the variations of the extrusion pressure with the apparent
shear rate and die geometrical parameters.