Study Of PS/Talc Composites Extrusion Foaming With Supercritical CO₂

Microcellular plastics are characterized by cell sizes less than10μm, and cell densitieshigher than109cells/cm3. The initial concept for microcellular plastics was conceived inresponse to an industrial need for reducing the material cost for certain polymer productswithout compromising the mechanical properties. Foamed polymer products possess manyunique characteristics such as higher specific tensile strength, higher toughness and betterthermal and sound insulation properties compared to the solid counterparts. Due to its strongrigidity of molecular chain, polystyrene (PS) has poor impact strength and toughness. In orderto solve this problem, PS/Talc composites with different Talc contents were prepared and theirmechanical and rheological properties and thermal stability were investigated. The extrusionfoaming of the composite was carried out and meanwhile, the effects of Talc content, diepressure and die temperature on cell morphology in the microcellular extrusion foamingprocess using supercritical CO2as the foaming agent were studied.Firstly, a slit die and a filament die with high depressurization rate were designedthrough analyzing the melt flow behavior in the die and calculating using the pressure dropequation and pressure drop rate equation. A new screw used in our experiment was alsoredesigned and manufactured on the basis of traditional two-stage exhaust screw to make itsuitable for foaming with supercritical CO2.Secondly, Mechanical and rheological properties and thermal stability of the compositeswere characterized. The results showed that the mechanical properties of PS/Talc compositeswere improved with the addition of Talc in the PS matrix. When the content of the Talc was8wt%, the optimum comprehensive mechanical properties of composites were obtained, thatis, the tensile strength was32.139MPa, Elongation at break was6.94%, the impact strengthwas3.154KJ/m2, the bending strength was60.90MPa, and the flexural modulus was3.73GPa.The impact fracture surfaces of the samples were observed using SEM. It was found that thephenomenon of agglomeration became more obvious with the increasing content of the Talc.Also, the melt flow rate (MFR) decreased, showing poor fluidity. However, Thermal stabilityand heat resistance temperature of the composites became better with the increasing concentration of the Talc. When the Talc was10wt%, the samples had the highestextrapolated starting decomposition temperature and vicat softening temperature. Thetemperatures increased by4.3%and3.75%respectively compared with the pure PS.In the end, we adopted orthogonal experimental method to investigate the influence ofthe content of Talc, die pressure and die temperature on the foam morphology. The resultsshowed that the cell diameter increased slightly at the beginning and then decreased rapidlywhile the cell density increased when increasing the die pressure. The cell size tended tobecome smaller as the die temperature increased and the cell density decreased at the sametime. A higher concentration of Talc is beneficial for decreasing the cell size and increasingthe cell density. When the content of Talc was10wt%, the cell density was2.02×108cells/cm3and the cell diameter was12.8μm. However, the phenomenon of cell coalescence wasnoticeable.