Extrusion Blending And Foaming Of Polypropylene/Waste Ground Rubber Tire: Relationship Among Flow Field, Structure, And Properties

Blending waste ground rubber tire （WGRT） with polypropylene （PP） can recycle thewaste rubber and improve the toughness of the PP. The dispersion of the WGRT in the PPmatrix is mainly influenced by the flow field in processing. In this dissertation, the80/20PP/WGRT blends were prepared using the single screw extruder with shear and chaotic flowfields, respectively, which were induced by different screw geometries. The80/20PP/WGRTblends were also prepared using a tandem single screw extruder with chaotic flow field and atwin screw extruder. The aforementioned tandem single screw extruder was also used toprepare the PP/WGRT blends with different WGRT contents and PP viscosities, respectively.Phase morphologies, rheological properties, and mechanical properties of the blend samplestaken at different locations along the extruder were analyzed. The flow field-phasemorphology-property relationship and the effects of the WGRT content and PP viscosity onthe phase morphology and properties were investigated. Furthermore, extrusion foaming wascarried out on the80/20PP/WGRT blend using supercritical carbon dioxide （Sc-CO₂） as thephysical foaming agent. The effects of the die temperature and Sc-CO₂content on the cellularstructure of the foamed PP/WGRT blend samples were investigated.Compared with the shear flow field, the chaotic flow field can provide effectivedispersive and distributive mixing, which facilitated to reduce the size of the WGRTagglomerates and to disperse them in the PP matrix. The tandem single screw extruderprolonged the time of the chaotic mixing, and then facilitated to refine and well disperse theWGRT agglomerates in the PP matrix. The reduction in the size of the WGRT agglomeratesincreased the complex viscosity and storage modulus of the PP/WGRT blend in lowfrequency region. The blend sample prepared using the tandem single screw extruder had thehighest impact strength （7.4kJ/m²ï¼‰, which was increased by54%compared with that of thepure PP sample.The increase of the WGRT content led to larger size of the WGRT agglomerates.Moreover, this increased the complex viscosity, storage modulus, and loss modulus of thePP/WGRT blend samples in the low frequency region and their impact strengths. The effectof the PP viscosity on the phase morphology and properties of the blend samples wasinvestigated. The result showed that the PP with a melt flow index of2.7g/10min wasfavorable to reduce the size of the WGRT agglomerates, which was attributed to the fact thatthis PP matrix exerted a strong shear force on the WGRT agglomerates. Smaller WGRTagglomerates resulted in larger interfacial area and interfacial interaction between the PP and WGRT phases, and then higher complex viscosity and storage modulus of the PP/WGRTblend samples in the low frequency region and higher impact strengths of the blends. TheWGRT enhanced the crystallization of the PP, which was attributed to the fact that the WGRThad the function of heterogeneous nucleation for the PP.The foamability of the PP/WGRT blend was improved upon the addition of compatilizerowing to the improvement in the compatibility between the PP and WGRT. The dietemperature of140°C resulted in higher die pressure and pressure drop rate, which facilitatedthe cell nucleation and led more uniform cell structure in the foamed blend sample. Thefoamed PP/WGRT blend sample prepared at a CO₂content of3%exhibited the minimum celldiameter （13μm）, maximum cell density （6.07×10⁵cells/cm³ï¼‰, lowest foam density （0.26g/cm³ï¼‰, and maximum expansion ratio （3.8）. The PP/WGRT blend displayed improvedfoamability than the PP, which was attributed to the fact that the WGRT acts as aheterogeneous nucleation agent.