Chemistry Reference
In-Depth Information
where
n
is the power law index mentioned earlier. Application of the Bagley and
Rabinowitsch corrections (in that order) converts the apparent flow curve from
capillary rheometer measurements to a true viscous flow curve such as would be
obtained from a cone-and-plate rheometer. However, this manipulation has sacri-
ficed all information on elastic properties of the polymer fluid and is not useful
for prediction of the onset of many processing phenomena, which we will now
consider.
Measurements made under standardized temperature and pressure conditions
from a simple capillary rheometer and orifice of stipulated dimensions provide
melt flow index
(MFI) or
melt index
characteristics of many thermoplastics. The
units of MFI are grams output/10 min extrusion time. The procedure, which
amounts to a measurement of flow rate at a standardized value of
τ
a
, is very widely
used for quality and production control of polyolefins, styrenics, and other com-
modity polymers. A lower MFI shows that the polymer is more viscous under the
conditions of the measurement. This parameter can be shown to be inversely
related to a power of an average molecular weight of
the material
[26]
Þ
2
1
½
~
M
3
:
4
2
4
:
7
w
. MFI, which is easy to measure, is often taken to be an inverse
token of polymer molecular size. The problem with this assumption is that MFI, or
η
a
for that matter, scales with average molecular weight only so long as the molec-
ular weight distribution shape is invariant. This assumption is useful then for con-
sideration of the effects of variations in a particular polymerization process but
may be prone to error when comparing products from different sources.
A more serious deficiency resides in reliance on MFI to characterize different
polymers. No single rheological property can be expected to provide a complete
prediction of the properties of a complex material like a thermoplastic polymer.
Figure 4.32
shows log
MFI
τ
a
flow curves for polymers having the same melt
index, at the intersection of the curves, but very different viscosities at higher shear
stress where the materials are extruded or molded. This is the main reason why
MFI is repeatedly condemned by purer practitioners of our profession. The parame-
ter is locked into industrial practice, however, and is unlikely to be displaced.
η
a
2
log
common
τ
a
Log shear stress
FIGURE 4.32
Apparent flow curves of different polymers with the same MFI (at the intersection point).
