Measurement and modelling of friction in polymer melts during macroscopic slip at the wall

Abstract

This article examines the flow of two polydimethylsiloxanes, a polybutadiene and a polyethylene, in axisymmetrical capillaries over the entire range of possible flow rates. Measuring and plotting flow curves has shown that macroscopic slip at the wall occurs with these highly entangled polymers as soon as a sufficient level of stress is reached. For each capillary and each flow rate considered, the entry pressure losses at the die inlet was also estimated. A simple method is proposed for determining the polymer's friction curve and it is shown that this curve is scarcely dependent on the dimensions of the dies used. The results obtained for each polymer enables stress variations at the wall to be represented as a function of slip velocity and their general shape to be deduced. The variations are distinctly non-linear, as they introduce a threshold, a maximum and two minimum levels of stress. Finally, on the basis of the experimental measurements, a procedure is proposed for modelling polymer slip at the wall during steady flow. Taking into account the existence of a static friction stress, it can be used to represent the general shape of stress variations at the wall as a function of slip velocity. Combined with the compressibility of the fluid, the law used gives reasonable access to variations in pressure as a function of time during extrusion regimes corresponding to cork flow.