Study On Processing Of Microcellular Foam Polystyrene By Twin-screw Extrusion

Microcellular foam plastics (MCP), typically defined as foamed plastics with a cell size in 10μm magnitude and a cell density more than 10⁸ cells/cm³, have aroused tremendous research interest because of their unique properties and outstanding performance. In this research, polystyrene (PS) or PS/MPPO blend was added to a twin-screw extruder, supercritical carbon dioxide was injected into the extruder as a foaming agent. A homogeneous polymer/gas solution will be formed in downstream of screws for its high capacity of mixing and shearing. After that the pressure of the solution has been successfully stepped up by using a gear pump system. A thermodynamic instability in polymer/gas system is induced by a rapid pressure drop in the die, the gas separated out from the polymer/gas solution, and the microcellular foam structure were formed.1. In the aspect of equipment, a supercritical fluid injection system was designed. The system can be used to control the injected pressure temperature and flux of the supercritical CO₂, Some improvement for the extruder barrels, screw configuration, and gas injection inlet has been done. During the research some novelty screw element were specially designed for the foaming process. A gear pump system was constructed and mounted directly at downstream of twin screw extruder. Three type of molding die were also designed and tested for microcellular foaming process.2. In the aspect of the foaming process, the effects of the material composition, temperature, pressure and gas concentration on the microscopic structure of the sample were preliminary studied. Based on our experimental study, an optimized extrusion processing route for PS microcellular foam were established.By reconstructing the conventional extrusion equipment and optimizing process control, the microcellular PS foam was successfully extruded in this study.