Wall Slip in Viscous Fluids and Influence of Materials of Construction

TLDR: In this paper, the question of slip and the influence of materials of construction on the observed extrudate irregularities are examined for high viscosity molten polyethylenes. But the assumption of "no slip at the rigid boundary" is generally not valid for polyethylene above a critical shear stress of approximately 0.1-0.14 MPa, when either surface or gross irregularities are present in the extrudates.

Abstract: The question of slip and the influence of materials of construction on the observed extrudate irregularities are examined for high viscosity molten polyethylenes. Capillary rheometer studies were conducted for several linear (LLDPE and HDPE) and branched (HP‐LDPE) polyethylenes, Viton A, and Barex‐210. Extensive blown film fabrication studies were conducted for narrow MWD UNIPOL process LLDPE resins. The results indicate that the assumption of “no‐slip at the rigid boundary” is generally not valid for polyethylenes above a critical shear stress of approximately 0.1–0.14 MPa, when either surface or gross irregularities are present in the extrudate. Loss of extrudate gloss at the critical shear stress defines the onset of melt fracture. Within the range of variables examined, the critical stress is relatively insensitive to molecular characteristics (molecular weight, MWD, and chain branching), melt temperature, and the detailed design of the capillary. Contrary to capillary rheometer observations, blown fi...