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Evaluation on extrudate swell and melt fracture of polypropylene/kaolin composites at high shear stress

Kaolin-filled PP composites were compounded using a heated two-roll mill prior to extrusion process in order to improve the rheological properties of the samples. The extrudate swell and flow instability (melt fracture) behavior of kaolin-filled PP composite melts were investigated using a constant rate type of capillary rheometer at high extrusion rates and test temperatures varied from 165°C to 220°C. The extrudate swell studies demonstrate that the die swell ratio decreases with an increase in the processing temperature and filler loading at fixed shear stress. Nonetheless, the study on the effect of shear stress and shear rate showed that the die swell ratio (B) of the melts decreases linearly with increasing apparent shear stresses and shear rates for both systems, unfilled and filled PP only up to one point above the critical shear rate (1.8 × 10 s-1). The B values remained almost constant when the shear rate is greater than 1.8 × 10 s -1 at a constant temperature. Kaolin-filled PP composites exhibit a characteristic of flow instability and extrudate distortions upon exiting from extrusion dies at low filler loading and at low processing temperature of 165°C. This type of defect is characterized by highly rough, fractured, and distorted extrudate surface. It was also found that decreasing the shear stress improved the surface appearance of all samples. Finally, the melt fracture tended to be suppressed when a capillary die with a high L/D ratio, which is 10 was used.