In-process rheometry studies of LDPE compounds - Part I: Unfilled resin

The initial part of a comprehensive study into the investigation and valida tion of in-process measurements on a low density polyethylene (LDPE) compou nd during extrusion is reported An unfilled commercial grade of low density polyethylene was processed using fully instrumented, computer monitored, s ingle and twin screw extruders in the Interdisciplinary Research Centre (IR C) laboratories, University of Bradford. Three in-process rheometers were u sed; a prototype Rosand capillary on-lint rheometer (OLR), a four sensor sl it die in-line rheometer (ILR) and a six sensor slit die ILR. Off-line comp arisons were carried out using a Rosand RH7 twin bore capillary rheometer. All devices allowed shear flow characterisation at a range of processing ra tes, and the capillary rheometers also provided a measurement of entry pres sure drop through the use of orifice dies. The six-sensor ILR also allowed measurement of entry pressure drop into the slit, although the differences in flow geometry meant these measurements were not directly comparable with the capillary, devices, and exit pressures were extrapolated from ILR pres sure drops. The in-process devices exhibited very good agreement with measu rements made off-line, in both shear flow and entry pressures. In-line slit die measurements were very close to those made using capillary dies, despi te the in-line tests being non-isothermal and employing a different geometr y of measurement section. Differences in melt rheology between single and t win extrusion were observed, due to the amount of mechanical work input by each extruder, Overall, good agreement was observed from the four different routes to measurement, for the LDPE examined.