Chemistry Reference
In-Depth Information
Engmann
et al.
, 2005). Squeeze flow is encountered in various pro-
cesses, for example, filtration, pressing, rolling compression moulding
and rheometry (Steffe, 1996).
In squeeze flow test equipment, the material is placed between rigid
parallel plates, which are then moved together. The material exits radi-
ally, and the radial fluid velocity is a function of the position
z
between
the plates at any radial position,
r
(Sherwood
et al.
, 1991). The plates
can be driven together at constant velocity, constant load or constant
strain rate. As deformation proceeds, a pressure distribution develops
within the tested material, which depends on the material properties
and the prevailing wall boundary conditions (Gibson and Toll, 1999;
Engmann
et al.
, 2005). The force required to squeeze a thin cylindrical
shaped sample between two approaching parallel circular plates depends
on the material rheology and also on the friction characteristics at the
material-plate interface (Laun
et al.
, 1999). As the deformation energy
is transmitted to the bulk of the material via the wall, the interfacial
boundary environment must be described as accurately as possible; in
practice, this may be as important, if not more so, than the bulk behaviour
(Corfield, 1996).
It should be noted that due to the complex nature of the flow fields
produced by using this technique, there is no universal model available
for application to all materials.
2.8
MEASURING INSTRUMENTS - PRACTICAL ASPECTS
2.8.1
Choosing the right measuring system
The choice of instrumentation and the measuring system requires care-
ful consideration of the particular fluid being analysed and the rheolog-
ical properties being studied. Without the knowledge of the parameters
influencing the rheology of a material, measurements can easily be
misleading and sometimes completely incorrect.
There are two main types of rotary rheometers, namely controlled
stress and control strain rheometers (Fig. 2.8a). Here, a set of commonly
used rheometers are described and compared. More detail can be found
in Macosko (1994).
2
.
8
.
1
.
1
Cone-and-plate rheometry
Cone-and-plate rheometry has the benefit of having a well-defined flow,
and hence accurate interpretation is possible (Fig. 2.8). It requires a
small sample volume, and it is easy to clean. The main advantage is
that the shear rate applied is uniform throughout the sample. However,
