A NEW EXTRUSIONAL RHEOMETER FOR ASSESSMENT OF VISCOELASTICAL PROPERTIES
ZITNY R., SESTAK J., AMBROS F.
ABSTRACT:
A new concept of an extrusional rheometer is based on flow of investigated sample in an annular slit
between circular tube and a central pin with constant or variable cross section. Strain gauges installed into
grooves milled into the outer cylinder serve for monitoring of axial pressure profile, allowing thus to evaluate
some effects ascribed to viscoelastical properties, e.g. so called exit pressure, or differences of pressure drop in
convergent / divergent slit. Routines for data processing and corrections to compressibility are briefly discussed.
Rheograms and exit pressures for collagen are presented.
1. INTRODUCTION
The new extrusion rheometer developed for rheological studies of highly viscous
heterogeneous materials (e.g. collagen, minced meat, curds, dough, plastic plasters) is an
evolution of the capillary rheometer, described in AMBROS 1974. Because now it is possible
to measure axial pressure profiles, the rheometer enables evaluation of some phenomena
ascribed to viscoelastical properties of tested samples, e.g. of the so called exit pressure (HAN
1971).
2. RHEOMETER DESIGN
The experimental setup of the extrusion rheometer is shown in Fig.[70K]
. The sample of
tested material is pushed by hydraulically driven piston from container to measuring section,
where the material flows either in circular capillary or in an annular slit between outer tube
(diameter 28.5 mm, length 250 mm) and inner core, replaceable pin. Standard geometry of the
pin is cylindrical (with gaps 2,3,5 mm) or conical (cone or doublecone). These alternatives
enables assessment of elastic effects in flows through converging slit (JAMES 1990), or
converging / diverging slit (PAVLOVEC 1991). Five pressure transducers described in the
following paragraph are installed along the axis and on the outer surface of measuring tube
(max. allowable pressures 1.7, 1.9, 2.1, 2.3, 2.8 MPa, distances 25, 75, 125, 177, 226 mm
from exit cross-section). Three Kulite silicon membrane pressure transducers are used for the
capillary measuring section (inner diameter 7 mm, length 300 mm). The pressure transducers
are connected to strain gauge bridge and its outputs, along with the output of piston
displacement potentiometric transducer, are evaluated in the PC computer with help of a
special software for on-line data sampling and processing. Some algorithms used in this
program will be described later. Manually adjusted hydraulic drive enables continuous
variation of piston velocity and therefore flowrate in rather broad range (from 10 -7 to 4.10
-4
m 3.s -1); the range of shear rates depends upon the measuring section geometry - for wide
annular gap (h=5 mm) it is approximately from 2 to 9000 s -1 .
3. EXPERIMENTAL RESULTS
As an example of application some results obtained with collagen will be presented.
Collagen is an organic matter, which constitutes approximately 30% of albumen in any animal
organism. This material is used e.g. for manufacturing of artificial intestines, and rheological
properties are important for its processing in technological lines (e.g. dosing of glycerol into
homogenizer according to consistency). The following Fig.4 contains diagrams of consistency
variables of three collagen samples differing with dry matter content (9.58%, 10.03%,
10.62%); measurement was done with cylindrical pin (gap H=2 mm), pressure sampling rate
was 10 Hz and temperature 19 C. It should be stressed that each diagram was obtained just
in one run, which is an obvious advantage in comparison with time demanding drying
methods.
Collagen threads form rather complicated nested structures, which explain its
viscoelastic behavior. It displays e.g. exit pressure effect: pressure transducers at the wall
measure in fact superposition of isotropic pressure and normal stresses Srr which correspond
to wall shear rate. Reading of the transducer, which is located just at the exit, therefore will
not be the ambient pressure (as it would be for purely viscous liquids), it will be higher by
Srr. It was shown that the exit pressure dependence upon wall shear stress
enables evaluation the primary difference of normal stresses Srr-Sxx.
REFERENCES
Han D.C.: Measurement of the rheological properties of polymer melts with slit rheometer.
Journal of Appl.Polym.Sci., 15, 1971, pp. 2567-2577
James D.F., Chandler G.M.: A converging channel rheometer for the measurement of
extensional viscosity. Journal of Non-Newtonian Fluid Mech., 35, 1990, pp. 421-443
Pavlovec J.: Extrusional rheometer for viscoelastic liquids. Dipl. thesis, CTU Faculty of
Mech. Eng., Prague, 1991
Ambros F., Zitny R.: A capillary extrusion rheometer for the determination of rheological
properties of food stuffs. Dechema-Monographien Nr. 1505-1536, Band 77, 1974, Verlag
Chemistry Frankfurt, pp. 65-72.
Ofoli R.Y., Steffe J.F.: Some observations on the use of slit rheometry for characterizing
the primary normal stress difference of extrudates. Journal of Food Engineering, 18, 1993,
pp. 145-157
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