A study of time dependence of polyethylene flow in capillary viscometry

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Abstract
It is known that polyethylene extruding from a capillary viscometer does not attain a steady‐state output rate immediately following a pressure imposition. The time dependence of output rate was studied quantitatively, and under some extrusion conditions was found to be of surprisingly large magnitude. The most notable effect is an overall rise in the output rate; its magnitude and duration depends on the extrusion pressure and temperature and on the entry geometry of the die employed. The magnitude of the effect is also characteristic of particular polyethylene samples and seems associated with smooth filament extrusion of the polymer. The steady‐state output rate can exceed the initial rate by as much as 40%. The rising output rate characteristic has been associated with a decreasing melt viscosity. This is taken to be the result of configurational changes occurring in the extruding melt. The data are shown to be qualitatively consistent with this postulate.