Abstract
The objective of the present study is to compare experimentally the capillary flow behavior determined by using a KOKA-type flow tester with the concentric cylindrical flow behavior by using a rotational viscometer at the critical condition of the irregular flow of high-density polyethylene and polypropylene melts.
In general, the yield shear stress τy obtained by the rotational viscometer test is lower than the critical shear stress τw* obtained by the capillary flow test. This is because the yield shear stress decreases considerably with increasing temperature while the critical stress increases slightly with temperature in the higher temperature region.
The characteristic curves, or the relationship between the shear rate and shear stress, for the concentric cylindrical and capillary flow tests under a constant temperature showed discontinuity in the temperature range of the present study, and it is reasonably considered that one of the reasons for such difference between the flow curves is the effect of the hydrostatic pressure in the reservoir of the capillary flow test.
