Nakagawa Tomokazu, Kazuo Yamaguchi
Papers # 2016 Berlin
Bi-modal high density polyethylene (HDPE) has become more popular in recent years, because it has both easier processability and higher mechanical strength. The easier processability is brought from low molecular weight component and the higher mechanical strength is brought from high molecular weight component. The cluster of high molecular weight component is called “gel” in this study. When the gels are not eliminated sufficiently, quality problems in products take place such as fish eyes in films and white spots in black pipes. However, it is difficult to eliminate gels by low molecular weight component. In this study, we compared gel elimination mixing performance of two types of our mixers, which are typically applied to high capacity processing of the bimodal HDPE. One is counter-rotating twin screw continuous mixer LCM-H and the other is co-rotating twin screw extruder KTX.
In mixing experiments, bi-modal HDPE for pipe grade with carbon black was used. From the results of white spot area rate (WSA, %) against specific energy input (SEI, kWh/kg), it is found that LCM-H brought about better gel elimination performance than KTX.
Furthermore, we performed partially filled numerical analyses around molten mixing zones to understand the physical meanings of experimental results. The residence time in the molten zone of KTX was longer than that of LCM-H. The fluid in the molten zone in LCM-H had larger region where fluid receives high stress than that of KTX.
As a conclusion, LCM-H has better gel elimination mixing performance than KTX because LCM-H gives gels larger stress and appropriate residence time than KTX.