On Silt- and Capillary-Die Rheometry

Abstract

A revised theory is presented of slit- and capillary-die rheometry, which, contrary to the earlier ones, predicts that the ratio of the second normal stress difference to the first, (Srr−Sθθ)/(Szz−Srr), is negative in sign and its magnitudes is always less than unity. In order to test the theory, use is made of exit-pressure measurements for five polymer melts and six polymer solutions. Exit pressures of polymer melts were obtained from both slit and capillary dies, and exit pressures of polymer solutions were obtained from slit dies with flush-mounted pressure transducers. The values of first normal stress differences determined from exit-pressure measurements show good agreement with the ones measured with a Weissenberg rheogoniometer. It has been found that (Srr−Sθθ)/(Szz—Srr) lies between −0.4 and −0.6 under the conditions investigated for five polymer melts. This is in line with recent measurements of the normal stress ratio of various polymer solutions.

A slit viscometer for the measurement of the viscosity of aqueous systems at high temperatures

“…For the 2 mm slit the ratio of the entrance length to the pressure port is 12.5 times the slit height whereas for the 0.5 mm slit this ratio increases to 50. Comparing these ratios with those of 4 and 16 obtained for fully developed flow by Han [7] and Blum et al [8] respectively, it appears that for the 2 mm slit the entrance length may be suspect at the highest limit of the shear rate attainable whilst for the 0.5 mm slit the entrance length seems adequate at all shear rates. As previously mentioned the assumption of one-dimensional flow will lead to errors of the order of 10% for the 2 mm slit.…”

An experimental study of exit pressures for polymer melts

“…We have a firm advocator of the method in Han, who did an abundant amount of work on the subject in the 1970s. On the whole, Han and his coworkers [6,12,13] presented attractive data in which the axial pressure profiles are linear and the extrapolated exit pressures are all positive, increasing with increasing shear stress, and with minimal scatter. The first normal stress difference values calculated according to Eq.…”

“…The nature of this velocity profile rearrangement is not clear and quantitative measurement is not easy, if at all possible. Neglecting this term by assuming that the flow in the die remains viscometric right up to the die exit, as was done in earlier derivations [5,6], one has for the flow in a slit in the absence of inertial effects d log Pc× …”

“…First, the wrong equations were used [3,4] to relate the exit pressure to the normal stress differences; these were later corrected [5,6]. The most comprehensive analysis of the exit pressure method is by Boger and Denn [7].…”

Shear viscosity measurement of highly filled polycarbonate melts using a slit-die rheometer

“…They are mostly used in connection with conventional extruders in measuring the shear viscosity in low shear rate regions (Han, 1974). In this study, a slit-die rheometer mounted on an injection molding machine is employed to measure the shear-dependent viscosity of highly filled short-fiber reinforced polycarbonate.…”