Extrusion Flow Instability Of Polythene And Its Blends

Flow instability of polymer melts extruded at high speed is very complicated, so it has a practical and theoretical significance to investigate these phenomena.A set of linear, branched polythene and their blends were investigated by double barrels capillary rheometer of constant type. Sharkskin and pressure oscillation were described quantitatively. Flow curves of linear and branched polythene were compared, and the change of sharkskin and pressure oscillation at different condition was discussed. At last, the mechanisms of the flow instabilities were analyzed.It is indicated that the melt slide at wall and the extrusion pressure oscillation occurred easily with linear macromolecule melts and more difficult with polymer melts which have bigger side-group or broader molecule weight distribution. By describing the amplitude, cycle, critical shear stress, wall slip velocity and critical slide length, it can be found that the more the extrusion speed, the higher the frequency of extrusion pressure oscillation, the shorter the period, the amplitude varied slight. With the extrusion temperature rising, the critical shear rate rise and the critical shear stress almost keep the same. When another kind of polymer was added into HDPE, the amplitude of all blends decreases and it can decrease obviously when the extrusion temperature rises. It also indicates that the pressure oscillation of LDPE mainly originates at the die exit. The amplitudes of the pressure oscillation are small, and the flow curve doesn't exhibit discontinuity.With the sharkskin photos, the wavelength and depth of sharkskin were quantitative described. It shows that with the shear rate rising, the average wavelength and average depth of sharkskin both increased, so the sharkskin became more obvious; whereas, with the temperature rising, the average wavelength and average depth of sharkskin both reduced, i.e. rising temperature can improve sharkskin effectively. Generally, blending can reduce the uniformity of melts, which induces the sharkskin to become more seriously. Sharkskin also has close relation to the interfacial condition between melt and die wall.