Abstract
In the processing of thermoplastic polymers, materials are to be deformed in the very state of melts. From this point of view, thorough studies on polymer melts are necessary to understand the flow phenomena of melts in the practical processing. We have been investigating the capillary flow phenomena of polymer melts by means of a kind of rheometer which is a screw extruder modified to measure the relation between pressure and flow rate in wide range.Each polymer has its distinctive features in the flow phenomena differing from those in other polymers, such as flow properties, entrance pressure loss or end effects correction factor, melt fracture and Barus effect. For instance, the flow property of high-density polyethylene is not in a single curve but in two seperated curves, one corresponding to the lower shear rate region with smooth extrudates, and the other corresponding to the higher shear rate region with melt fracture.In Fig. 1 is shown the typical relation between the pressure and the flow rate obtained in the case of various length nozzles with the same diameter. With increasing pressure in the experiment, a little wavy roughness is recognized on the surface of extrudates from S point on figure. Further increase of pressure causes a sudden jump of flow rate, and at the same point (F point in Fig. 1) melt fracture is recognized and some decrease of pressure follows this discontinuity. Consequently, two values of flow rate exist under the same pressure in the transient region.Figs. 2, 3, 4 show the flow properties of high-density polyethylene which are independent of nozzle dimensions below F point but dependent on nozzle diameter above F point.If we presume that the jump of flow rate of high-density polyethylene is caused by slippage at capillary wall, the equation (4) gives successfully the slippage velocity at wall which is independent of nozzle diameter and the function of only shear stress at wall as shown in Fig. 7 and equation (10).Not so many reports on the flow properties of high-density polyethylene have yet been published, but they will be important along the development of the resin. And there remain now many unsolved problems in the field of the flow phenomena of polymer melts for investigation.
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