Modeling the molecular weight loss during single screw extrusion

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The description of the processing of high speed single screw extrusion is possible in many fields. However, today there is no calculable variable which describes the quality of the melt at the die opening and the problems of the molecular weight loss caused by thermal and mechanical stress. The aim of this research project is to calculate the molecular weight reduction for an exemplary chosen polymer in the real extrusion process at various screw geometries and operating points. For reaching this aim, at first a material law has to be found which provides the context of pressure, temperature and shear stress. In order to achieve this, experimental investigations has to be executed to parameterize the known material laws from the literature. The results must be compared to experimental data. In the first both years of the project, loaded samples have been successfully generated. Because of an overlapping in the extruder of the parameters shear stress, temperature and residence time, a self-designed machine was used. With this machine one parameter could be varied, while the other parameters were not changed. The loaded samples were analyzed and evaluated. The influence of shear stress, temperature and residence time could be successfully described by a mathematical equation. This mathematical model was implemented in the software environment REX (computer aided extruder design). Experimental investigations were made on a lab extruder and samples were taken at the die opening. The project it is planned to expand the data base of two more polymers. Loaded samples of polystyrene (PS) and polybutylene terephthalate (PBT) shall be generated and analyzed. By the presence of water molecules PBT reacts and breaks down, therefore the influence of water on the molecular weight loss will be investigated. Furthermore, the influence of pressure on the molecular weight shall be taken into account for polypropylene and shall be implemented to the software REX. Experimental investigations will carry out on a single screw extruder by varying the pressure on the screw tip. Under the condition of a successful development it provides the opportunity to design the extrusion process more material gently.