SLIP-FLOW OF POLYBUTADIENE THROUGH FLUORINATED DIES

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