The Phenomenon And Improvement Measures Of Polypropylene' Helical Melt Fracture In High-Speed Extrusion

This subject was supported by the National Natural Science Funds. The overall objective of this study is to summarize complicated abnormal rheological behavior of polymer melt (mainly polyolefin) occurred in high-speed extrusion process, the purpose is to analyze mechanism of abnormal rheological behavior and find out some countermeasures of improvement.In this work, rheological behaviors and distortion characteristics of polypropylene random copolymer (PP-R) was systematically studied by a double-capillary rheometer. The stress concentration effect, inlet press drop and flow curves were analysised to certify the head stream and mechanism of instable flow. In our work, the PP-R/TLCP (thermotropic liquid crystalline polymer) in-situ fiberized composites and PP/CNTs (Carbon Nanotubes) composites were prepared to alleviate and restrain instable flow. The head stream and the mechanism of instable flow was verified by mixing trace polymer processing aid - fluoroelastomer (PPA) into PP-R melt.The results suggest that the extrudate of PP-R obtained by the short die exhibited a peculiar helical melt fracture. The stress concentration effect, inlet press drop and extrudate helical melt fracture became more serious with higher molecular weight, elasticity and extrusion speed. Raising temperature could eliminate the helical melt fracture. Analysis showed that the headstream of distortion is located at the die entrance where the stress concentration effect and the second flow– the helical flow was serious which caused the helical melt fracture. This helical flow was verified in the experiment by adding little plastics coloration into PP-R melt.The instable flow occurred at the die inlet during melt extrusion process could be relieved and restrained by adding a certain amount of anisotropic materials. The method of in-situ fibrillation was taken to prepare PP-R/TLCP composites. The result showed that, a spot of TLCP with a fiber structure could relieve the helical melt fracture of PP-R. The fibrous TLCP can effectively weaken the instable flow, stress concentration effect and inlet press drop occurred at the die inlet. It will also could decreased the interaction of molecular chain and weaken the melt viscoelasticity, which were benefit to the extrusion process. PP-R/CNTs composites prepared by master method were to improve the inlet unstable flow. In this paper, CNTs were modified by physical methods. The results showed that, a spot of CNTs could weaken the stress concentration effect, inlet press drop and the second flow occurred at the die inlet.The PP-R/PPA composites were prepared by master method, the experimental results showed that, the extrudate appearance of PP-R obtained by the long die were get improved by adding a small amount of PPA, the critical shear rate for the melt fracture was delayed form 353 s-1 to 503 s-1 by adding 200 ppm PPA. It is concluded that the head stream of the instable flow and flow stress concentration effect during the melt extrusion process mainly occurred at the die inlet. The melt/wall interfacial effect could influence the instable flow, but not the main reason for the melt fracture of the extrudate obtained by the long die.By summing up and making quantitative description of the instable flow of different polymer melt (mainly polyolefin) in high speed-extrusion, the database for recognizing the polymer process behavior was established, which designated the mechanism of different instable flow and presented improvement measures.