Relation between kneading behavior and flow instability of a high molecular weight high density polyethylene, applications to extrusion

Non‐monotonic continuous curves of torque as a function of shaft speed, M(N), have been obtained for a high molecular weight high density polyethylene (HDPE) from measurements obtained with a torque rheometer (Haake Rheocord). Previous papers have given theoretical demonstration of the non‐monotonic character of the shear stress‐shear rate function, s(\documentclass{article}\pagestyle{empty}$ {\rm \dot \gamma } $ ), which makes it possible to explain the extrusion behavior of a high molecular weight HDPE. In capillary rheometry, it is not possible to obtain the values of s(\documentclass{article}\pagestyle{empty}$ {\rm \dot \gamma } $ ) into the “well zone” of this function because the compressibility of the polymer creates a phenomenon of oscillation in the barrel affecting the die output flow rate and the pressure loss. The M(N) function measured by the Haake Rheocord is a complete representation of the s(\documentclass{article}\pagestyle{empty}$ {\rm \dot \gamma } $ ) function, although the capillary rheometer only gives a partial representation of this function. The transformation of the M(N)function into s(\documentclass{article}\pagestyle{empty}$ {\rm \dot \gamma } $ ) is quite difficult because of the complex geometry of the Haake Rheocord measuring head. The “critical points” of the s(\documentclass{article}\pagestyle{empty}$ {\rm \dot \gamma } $ ) function in the capillary rheometer (appearance of oscillations), can be correlated to the maximum points of the M(N) function in the Haake Rheocord at constant temperature. The non‐monotonic aspect of the s(\documentclass{article}\pagestyle{empty}$ {\rm \dot \gamma } $ ) function provides an important technological application: extrusion of a high molecular weight HDPE at an increased flow rate at low temperatures.