Determination of melt viscosity as a function of hydrostatic pressure in an extrusion rheometer

Polymer melt viscosity was determined as a function of hydrostatic pressure in an extrusion rheometer. An extrusion rheometer consisting of a barrel and a capillary can be looked upon as two capillaries in series. The barrel itself was used as the main viscometer and several different capillaries having length/diameter ratios of 3.5–100 were used as pressure boosters. For a Phillips type polyethylene of 0.95 g/cc density and 0.4 melt index, the apparent viscosity at 190°C and 7.12 sec−1 was increased nearly fourfold as the average pressure in the barrel was raised from atmospheric to 24000 psi. With colored layers of polyethylene as tracers the flow pattern in the barrel was examined. The velocity profiles in the barrel were well developed and parabolic, and thus indicated regular laminar flow under these conditions.