Abstract
The capillary flow of polymer melts is often accompanied by flow instabilities in the high shear stress region during polymer processing. In order to investigate these behaviors, a new capillary rheometer was developed by modifying an injection molding machine. The experimental results were as follows: (1) With an increase in shear stress, stable steady flow changed into unstable sharkskin flow at a critical shear stress σs·s., and then, into gross melt fracture flow at σm·f. At these critical shear stresses, both apparent viscosities and elasticities showed discontinuous changes. (2) In the case of sharkskin flow, the flow patterns in the longitudinal sections of the extrudates indicated that the cyclic turbulence in flow was limited only at the thin polymer-layer contacting the capillary wall, and stable laminar flow was maintained in the inner part. (3) During sharkskin flow in the capillary, slippage at the wall was observed. The slip-velocities decreased remarkably with an increase in the adhesive strength at the wall/polymer interface. (4) Considering these results, a model for the flow-mechanism of unstable sharkskin flow was discussed, and the calculated values of critical shear stresses agreed approximately with the observed values.
