Morphology of Polymer Blends in the Melting Section of Co-Rotating Twin Screw Extruders

摘要

The properties of polymer blends are largely determined by the morphological structure of the polymer combinations that are involved. In terms of extruder design. this means it is necessary to have models available for estimating the development of the morphology over the length of the screws. Since significant morphological changes are observed in the melting section. in particular .is it necessary to analyze not only the plasticizing process for binary material combinations but also the initial formation and further development of the morphology in this section of the extruder. In the framework of this study. experimental investigations were conducted into polypropylene/polyamide 6 (PP /PA6) blends with small components (by weight) of the disperse PA phase. Apart from varying the process conditions of screw speed and throughput. the viscosity ratio was also varied through the use of two different PP grades. The degree of melting and the development of the morphology over the length of the screws were determined for the individual tests. The study of blend morphology in the melting section reveals key findings that must be taken into account for modeling the initial formation and further development of the morphol- ogy .It is very clear that. on the second component. which melts at &gher tempera- tures. a kind of melt film removal occurs at the surface of the granules as they melt. The drops of second component in the melting section. which are directly adjacent to components that have not yet fully melted in some cases. have already assumed dimensions (in the IJ.m range) similar to those that are seen at the end of the extrusion process. This means that. in the melting section of the twin-screw extruder. no volumes become detached from or are worn off the already-molten granule surfaces. An evaluation of scanning electron micrographs also shows that. in the melting section of co-rotating twin-screw extruders. virtually all the degrada- lion mechanisms that can essentially be distinguished. such as quasi-steady drop oreakup, folding, end pinching and decomposition through capillary instabilities. take place in parallel.