Morphology of polymer blends in the melting section of co-rotating twin screw extruders

The properties of polymer blends are largely determined by the morphologica l structure of the polymer combinations that are involved. In terms of extr uder design, this means it is necessary to have models available for estima ting 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 fo r binary material combinations but also the initial formation and further d evelopment of the morphology in this section of the extruder. In the framew ork of this study, experimental investigations were conducted into polyprop ylene/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 Wades. The degree of melting and the development of the morph ology over the length of the screws were determined for the individual test s. The study of blend morphology in the melting section reveals key finding s that must be taken into account for modeling the initial formation and fu rther development of the morphology. It is very clear that, on the second c omponent, which melts at higher temperatures, a kind of melt film removal o ccurs at the surface of the granules as they melt. The drops of second comp onent in the melting section, which are directly adjacent to components tha t have not yet fully melted in some cases, have already assumed dimensions (in the mum range) similar to those that are seen at the end of the extrusi on process. This means that, in the melting section of the twin-screw extru der, no volumes become detached from or are worn off the already-molten gra nule surfaces. An evaluation of scanning electron micrographs also shows th at, in the melting section of co-rotating twin-screw extruders, virtually a ll the degradation mechanisms that can essentially be distinguished, such a s quasi-steady drop breakup, folding, end pinching and decomposition throug h capillary instabilities, take place in parallel.