Practical Rheology and its Role in Polymer Processing
Timothy W. Womer, CPlasT
TWWomer & Associates, LLC
Introduction
Rheology is the science of material flow
behavior, which is a very complex and
multi-dimensional science. Even though it
is complex, it also is quintessential to
understand in order to optimize the
processing of polymers.
Knowing the
difference
between
amorphous
and
crystalline polymers, what Melt Index really
tells, and the effects of melt temperature on
melt fracture are all important elements in
the understanding of rheology. A simple
understanding of what polymer rheology is
and how shear and temperature can affect
the flow characteristic of a polymer may
make a big difference in the P & L of a
company.
What is Rheology?
Webster’s definition of rheology is “a
science dealing with the deformation and
flow of matter.” But Tim’s definition for
rheology is “the flow characteristic of a
polymer at various shear and temperature
conditions.” Basically, everything flows!
I have always believed that everything is
relative to something that we already know
about. In reference to rheology, a good
example to help in the understanding of
how the flow characteristic of a polymer
changes as temperature changes would be
to think how molasses flows if it is kept in a
refrigerator at 38°F versus how it flows if it
is brought to a boil on a stove top. At 38°F,
molasses flows very slowly out of any
container in which it is stored; but at 225°F
at which it boils, it will flow from the
container like water.
Plastics typically respond to temperatures
in the same way. As the temperature of a
polymer is increased, the flow ability of the
polymer increases.
Also, during the
plasticizing process, depending on the
geometry of a screw more or less shear
upon the polymer can be induced.
Generally, higher compression screws will
apply more shear on the polymer than a low
compression screw. The shear rate in the
channel of the screw is defined as follows:

 Dn
hm
(where)
 = Shear Rate (1/sec)
D =Screw Diameter (inches)
n = Screw Speed (rev./sec.)
hm = Channel depth in metering section of
screw
Therefore, since hm is in the denominator of
the equation, it has an inverse relationship
with the shear rate on the polymer being
processed. A deeper screw will have a
lower shear rate than another screw with a
shallower metering section run at the same
screw speed.
The viscosity of the
shallower screw will be lower than a deeper
screw with the same barrel zone
temperatures; hence, changing the flow
characteristics of the polymer as it enters
the die.
Whether a resin is amorphous, crystalline
or semi-crystalline does not really effect
what the layman needs to know to improve
the processing conditions in order to
improve the final end product.
The
following “Resin Pyramid” is very helpful in
understanding how resins match up in the
overall scheme of things in the plastics
world. (Figure 1)
Figure 2
Figure 1
Viscosity
Viscosity is where the rubber meets the
road when it comes to processing plastic
materials. If you understand what the
viscosity versus shear rate curves are
conveying, they will help you determine the
best processing conditions that can be
obtained from the screw and die design that
is in the extrusion system.
The shape of the rheological curves will tell
the engineer if the material is shear
sensitive or temperature sensitive as shown
in the following figures. In Figure 2, the
curves are spaced fairly far apart. This tells
the engineer that the resin is more
temperature sensitive than shear sensitive.
Therefore, if the process requires that the
plastic needs to have better flow through
the die, then the melt temperature of the
plastic needs to be increased. An example
of this type of problem would be if die
fracture is evident in the product being
extruded. Die fracture is referred to in
many different ways. Some processors
might call it “apple sauce,” “chicken tracks,”
“nerve,” “haze,” or any other type of
imperfection in the surface of the product.
Typically
these
types
of
surface
imperfections are due to trying to process
the resin too cold.
During a typical rheological study, the
plastic that is being tested will have various
imperfections as shown in Figure 3.
Figure 3
The first example pictured on the left side of
Figure 3 shows a nice smooth extrudate.
This is a material that has been processed
at the proper melt temperature. All of the
other examples in Figure 3 show some type
of surface imperfection that needs to be
improved. Therefore, increasing the melt
temperature from 216°C to 227°C, as
determined from the rheological curve
shown in Figure 2, might be all that is
needed to make a difference in producing a
poor surface finish product to producing a
nice smooth finish product.
A plastic that is not temperature sensitive
but instead is shear sensitive can be seen
in Figure 4.
Figure 4
As can be observed in this rheological
curve, the lines are very close together.
This means the viscosity changes very little
with temperature. The curves are steeper
than the lines of the curve in Figure 2. This
indicates that the resin is more sensitive to
shear than temperature in the change of its
flow characteristic.
So how does this apply to everyday
applications?
In Figure 5, there are
examples of two extrudates, one with unmelts and one without.
Figure 5
The extrudate coming out of the die with the
unmelts as in Figure 5 needs to be
corrected. Based on what we know about
the flow characteristic of the resin,
increasing the melt temperature will have
little effect on improving the unmelts but will
definitely increase the melt temperature of
the polymer. Instead, if the screw has a
mixer as part of its geometry or if it is a
barrier type screw, then the barrier gaps
can be built up and recut with reduced
clearances which in turn will increase the
shear upon the polymer at these points and
in turn eliminate the unmelts because the
shear on the polymer has been increased,
just as the rheological curves have shown.
Many times, companies do not always have
dedicated lines where they process the
same resin 24/7. The extruder will be fitted
with a screw which typically has been
designed for a special resin or group of
similar resins; but as time passes and new
opportunities for the company arise, they
must try to process a different resin on the
equipment that they have at hand.
The best way to determine if the existing
screw can process the new material is to
compare the new resin with the materials
that have been processed in the past. In
Figure 6, it can easily be noticed that the
four different resins, all at 500°F, have very
different viscosities. Occasionally, by either
Figure 6
increasing or decreasing the melt
temperature of the various resins the flow
characteristic of the different polymers can
be altered to behave like the other resins.
As long as the changes in the melt
temperature do not affect the end product,
the changes to the viscosity sometimes can
be done by modifying the barrel
temperature profile.
Melt Density
Melt density is the density of a polymer
when it is in a molten state. The melt
density is totally dependent to the
temperature at which it is measured or
processed.
Melt density can be much different than the
specific gravity of the polymer.
For
example, the specific gravity of HDPE is
typically .95 to .96 gm/cc at room
temperature, but at its processing
temperature, HDPE will have a Melt Density
in the area of .74 to .76 gm/cc depending
on the temperature that is measured.
In all mathematical calculations which
involve the flow of a polymer, the melt
density if the polymer is at the process
temperature must be used in order to have
accurate results. The melt density of a
polymer is a value that is very difficult to
obtain but can be easily measured using a
capillary rheometer. Figure 7, shows a
typical method used to measure the melt
density of a HDPE resin.
AVERAGE MELT DENSITY :
0.739
Melt Index (MI, MFR, MFI)
The terms “Melt Index” (MI), “Melt Flow
Rate” (MFR) and “Melt Flow Index” (MFI)
refer to the same test. MFR was introduced
to replace MFI. The term MFI is used to
refer to the flow rate of PE obtained under
Condition 190/2.16 (formally know as
Condition E). The use of such terms is not
encouraged for other materials.
It is
suggested, by ASTM D 1238 that the term
melt flow rate (MFR) be used for other
plastic materials.** (**obtained from other
references)
The Melt Index of a resin can be very
deceiving, because it is measured at zero
shear rate (0 1/sec.), whereas in the typical
extrusion process the shear rate in the
metering section of a screw usually ranges
between 85 to 115 1/sec. Two resins can
have the same Melt Index, but at their
processing conditions will react totally
different. Figure 8, is a schematic which
shows what this means. Melt Index is only
one point on a rheological curve and
depending on the molecular structure of the
polymer can be very different between two
resins of the same Melt Index.
VI
S
C
O
SI
T
Y
.
app = 2.4
MFI
0 sec-1
10 sec-1
SHEAR RATE
Figure 8
5.3
10
7.163
5.3
0.739913
SAMPLE WEIGHT IN GRAMS
PLUNGER TRAVEL IN CM
VOLUME IN CM^3
AVERAGE SAMPLE WEIGHT
MELT DENSITY
Figure 7
5.29
10
7.163
5.29
0.738517
If someone tells you that it is a “drop-in”
resin, have them show you the rheological
curves of the two resins to prove it. Too
many times the new “drop-in” resin
processes nothing like the original resin and
barrel zone temperatures have to be
changed so that the new resin will flow like
the original resin.
Moisture Content
The amount of moisture in a material
determined under prescribed conditions
and expressed as a percentage of the
weight of the moist specimen, that is, the
original weight comprising the dry
substance plus any moisture present.
Resins that hold moisture are said to be
hygroscopic, which means the resin has a
tendency to absorb moisture from the air. If
a resin is processed in a “wet” condition, it
can have dramatic effects on the properties
of the final product. Typical hygroscopic
resins are HIPS, ABS, Nylon, PET, PETG.
Therefore, when performing rheological
studies on these resins, they must be dried
properly in either a desiccant or vacuum
drier before testing. The moisture content
needs to be reduced to the proper level
which can range between .005% to .18%
depending on the resin being tested.
Conclusion
The study of rheology can be a very
complex science, but for the everyday “Joe”
or “Josephine” who has to operate the
extrusion equipment on a daily basis, a
simple understanding of the cause and
effects of shear and temperature can
determine if “shipping pounds out the door
or putting scrap on the floor.”
A basic understanding of the flow
characteristics of the resin being processed
can have a major effect on a company’s
profits at the end of the day.