The Melt Flow Instability Of LLDPE, LDPE And Their Blends At High Speed Extrusion

In modern polymer processing, the high productivity is receiving more and more attention. However, the high productivity usually accompanies the flow instability that debases the quality of products. To address this issue, in this work, the extrusion characteristics of LLDPE, LDPE and their blends at high extrusion speed were typically concerned and systematically studied through a double-capillary rheometer working under constant speed, Instable flow phenomena such as sharkskin melt fracture, extrndate pressure oscillation and the second smooth section were described quantitatively. In addition, the viscoelasticities, flow curves, pressure oscillation, and the extrudate appearances of LLDPE, LDPE and their blends were compared. Also, on the understanding of rheological character of the related polymer, some strategies such as changing blend ratio, grafting and adding F-containing lubricant have been adopted to reduce the unexpected extrudate aberrance. Finally, the mechanisms of the flow instabilities were primarily explored.The results suggest that, in high speed flow field, the appearance of LLDPE presents in turn the sharkskin melt fracture, stick-slip, second smooth section and gross melt fracture with the extrusion speed increase, while for LDPE the appearance develops directly from smooth to irregular fracture; the extrusion pressure oscillation of LLDPE occurs along the long die with a broken flow curve, while for LDPE the pressure oscillation takes place at the die entrance with a continuous flow curve. Therefore, it can be concluded that the flow stress of LLDPE concentrates on the wall of the capillary while that of LDPE takes place at the entrance of the die.In the various efforts for the improvement of the melt fracture, the addition of a little LDPE into LLDPE was found having significantly restrained the extrusion pressure oscillation of LLDPE, but adding a little LLDPE into LDPE could not show any positive effects on the counterpart. Both nonlinear viscosity and elasticity of the blends increase in comparison with the singular component. The extrudate surface of the blends becomes deterioration. In another effort, the grafted LLDPE with higher molecular weight, viscosity flow activation energy and melt elasticity was tested. The results suggest that the extrusion pressure of the grafted LLDPE tends to be stable and the appearance of the extrudate is also improved obviously. Additionally, the application of the lubricant of F-containing elastomer can decrease the extrusion pressure and extrusion swelling ratio, and increase the critical extrusion speed related to the stable extrusion and thus broadening the available processing area. In the case of sufficient usage of the lubricant, the sharkskin melt frature disappears completely. Increasing the temperature or decreasing the shear rate can also amend the extrudate appearance effectively. Finally, it should be pointed out that the extension of the die length is helpful for the improvement of the extrudate appearance of LLDPE but not for LDPE.The second smooth section was noticed while LLDPE was extruded at high speed. The appearance of the extrdate in the second smooth section is similar with that at slipping interface when pressure oscillation occurs. Therefore, it can be concluded that in the second smooth second the slipping takes place on the capillary wall. Since the extrusion speed in the second smooth section is much higher than that in the first stable extrusion zone as well as the extrusion press in the second smooth section is not higher than the critical pressure related to pressure oscillation, the discovery of the second smooth section are so interesting that it may be used for the fast, safe, stable, and economic processing of the polymers.