Evaluating the performance of processing aids in eliminating melt fracture

This study presents a comprehensive experimental protocol to evaluate the effectiveness of a polymer processing aid (PPA) in eliminating melt fracture of a metallocene linear low-density polyethylene using both capillary rheometry and single-screw extrusion. The prime effects addressed are those of the die length-to-diameter ratio (L/D), the concentration of the PPA, and the temperature on the pressure transients in startup flow and wall slip, while monitoring extrudate appearance. Results show that increasing the temperature or the PPA concentration helps eliminate melt fracture. We also found that the effect of the L/D ratio on the melt fracture differs depending on whether PPA is present. In detail, using longer dies worsens melt fracture in the absence of PPA, while the opposite is true in the presence of PPA. This is because longer coated dies allow more stress relaxation of the melt before exiting the die. Results obtained from a capillary rheometer are compared with those obtained from a single-screw extruder to relate rheometry with real processing. Excellent agreement is found pointing to the significance of using capillary rheometry to evaluate the performance of PPAs in polymer processing operations.