Measurement of apparent extensional viscosities of polyolefin melts from
process contraction flows
Martyn, M. T., D. of Mechanical and Medical Eng., IRC in Polymer Sci. and Tech., U. of Bradford, England
Charoen Nakason, Asst. Prof., D. of Rubber Tech. and Polymer Sci., F.of Sci. and Tech., PSU.
Coates, P. D., D. of Mechanical and Medical Eng., IRC in Polymer Sci. and Tech., U. of Bradford, England
Corresponding e-mail : ncharoen@bunga.pn.psu.ac.th
Grant : Government Budget, PSU., ESRC Council and University of Bradford
Presented : J of Non-Newtonian Fluid Mechanics 2000, 92 : 203-226
Key words : extensional viscosity, extensional strain rate, continuum mechanics,
flow visualisation
The extensional characteristics of branched and linear polyolefins melts have been evaluated
in slit flow cells with abrupt contraction ratio of 4 : 1 and 15 : 1 on a single screw extruder. Apparent extensional viscosities of the melts in planar flow have been obtained by two routes, the first employ-ing extensional strain rate data measured from particle velocimetry, the second using continuum mechanics analysis based on the entry flow profile. The influence of flow geometry on apparent visco-sities of the barnched and linear polyolefin melts are found to be in good agreement, dispite difference in the strain histories imposed by two contraction geometries (centre line extensional stress and corresponding average axial strain rates differ). Particle velocimetry was found to be better that the continuum mechanics approach in obtaining extensional strain rate due to a lower susceptibility to experimental errors. It appears that the in-process methods of assessing apparent extensional visco-
sity, with an abrupy 180o entry slit die, are geometry independent for the range of materials, strain and strain rates covered and provide a useful technique for ranking process-typical extensional behaviour of melts.
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