用双毛细管流变仪对HDPE与LLDPE挤出压力振荡的研究

Pressure Oscillation of HDPE and LLDPE Studied by Double Capillary Rheometer

用RH2000恒速型双毛细管流变仪研究了3种HDPE和3种LLDPE的挤出压力振荡现象.通过压力振荡图、流动曲线和挤出物表观3方面,对两种类型聚乙烯压力振荡的差异进行了分析.发现HDPE的压力振荡很明显,振幅在2～3MPa,而LLDPE的振幅很小,甚至在压力-时间图上看不出来.6种聚合物的流动曲线都发生了断裂.在柱塞下降速度恒定下发生压力振荡时,流速并不恒定,粘界面条件下流速较小,滑界面条件下流速较大,并通过计算获得了3种HDPE发生压力振荡时对应于粘界面与滑界面的流速.HDPE和LLDPE在粘界面条件下挤出物都是鲨鱼皮,而滑界面条件下HDPE的挤出物类似于无规破裂,LLDPE挤出物表观较光滑,且没有鲨鱼皮.

The pressure oscillations of three kinds of HDPE and three kinds of LLDPE were studied by double capillary rheometer constant speed type. The differences between these two sorts of polyethylene were investigated by analyzing pressure oscillations, flow curves and extrudate appearances. It was indicated that the pressure oscillations of HDPE were more obvious than those of LLDPE. The pressure oscillation amplitude of HDPE was about 2 -3MPa, while it was less than 0.2MPa for LLDPE. The flow curves of all materials were discontinuous. When pressure oscillation occured at a constant piston rate of descent, the flow rate of melt at the die was not constant. The flow rate at the stick interface was bigger than that at the slip interface, and the flow rates of HDPE at different interface were calculated. The extrudate appearances of HDPE and LLDPE at stick interface were all sharkskin. At slip interface, the appearences of HDPE were a little like ruleless fracture, while that of LLDPE were comparative smooth and sharkskin disappeared.