SLIP-FLOW OF POLYBUTADIENE THROUGH FLUORINATED DIES

Wall surface energy affects the flow of polymer melts in extrusion dies. The aim of the present study is to demonstrate and characterise the flow regimes with slip that are specific to low-energy walls. The polymer used is a polybutadiene (PB) of high molar mass. The dies considered are two-dimensional with either steel walls or walls covered with a fluorinated (PTFE) deposit. Flow curves are drawn for each of the dies and compared. The appearance of the extrudate corresponding to the various flow regimes is observed. Additional results are obtained by various techniques such as flow visualisation and local velocity measurement with a Doppler velocimeter, and stress determination by means of birefringence. For PTFE dies, two stable slip regimes are observed, for low and high levels of stress, separated by a transition zone. The second slip regime, corresponding to higher levels of stress, is characterised by extremely low friction al the wall. The occurrence of slip produces major modifications in the velocity and stress fields, as well as in the appearance of the extrudate, which shows a crack-free surface.