A TEMPERATURE WINDOW OF REDUCED FLOW RESISTANCE IN POLYETHYLENE WITH IMPLICATIONS FOR MELT FLOW RHEOLOGY .2. RHEOLOGICAL INVESTIGATIONS IN THE EXTRUSION WINDOW

The present part of this series on the temperature window of reduced flow resistance in polyethylene confirms and consolidates the inference from the first paper in this series that the source of the theological singularity resides in the capillary, specifically at the capillary wall, in contrast to the previously held view that the elongational flow at the entrance is responsible for the effect. The experimental material leading to this conclusion comprises the varying of the angle of the entrance orifice and of the capillary dimensions (length and diameter), the use of a constrictionless parallel barrel and also of an orifice without a capillary in the various extrusion experiments, together with the use of different materials of construction for the capillary. The results of each of these examinations are detailed in this paper. In addition to leading to the above conclusion, these studies extended the theological examination of polyethylene melts, specificaly to the unusual theological behaviour associated with the temperature window of reduced flow resistance which has been explored as a function of the above variables. Amongst others, the effects in the capillary have been separated from those in the entry orifice, providing an unexpected new approach to the exploration of flow instabilities to be pursued in the next paper of this series. The source of the temperature window is discussed in the light of the above findings with reaffirmation of its origin as a phase transformation induced by chain extension along the capillary wall. Attention is drawn to the wider implications beyond the confines of the rather special circumstances of the present experiments.