Development of an lonomer Tracer for Extruder Residence
Time Distribution Experiments
R. A. WEISS and H. STAMATO
Dept. of Chemical Engineering and
Polymer Science Program
University of Connecticut
Storrs. Connecticut 06268
A polymeric tracer was developed from the tributylamine salt of lightly sulfonated polystyrene. The residence
time distribution (RTD) measured for the extrusion of
polystyrene with this tracer was compared with that measured using conventional particulate tracers. In general, the
particulate tracers had a longer mean residence time and
a broader RTD, which were attributed to increased mixing
of the particulates.
INTRODUCTION
lasticating extrusion is commonly used by the
polymer industry to compound and to shape
plastics, elastomers, and fibers. The time the material actually spends in the extruder will be reflected
in the quality of the product. The process is relatively
simple, involving melting of a solid polymer and mixing and pumping of the melt, but the actual path
taken by the polymer molecules as they move through
the extruder can be quite complicated and difficult to
describe. This is important, however, because the
time the material spends in the extruder will often
influence the quality of the product. For example, the
degree of mixing or the extent of degradation of a
polymer will depend on how long it was exposed to
the processing conditions. A common way to describe
the history of the polymer in the extruder is by its
residence time distribution (RTD).
The problem of determining the RTD of a polymer
is one of how to distinguish between molecules that
enter the extruder at different times. In order to
accomplish this, it is necessary to provide some sort
of contrast between the material that enters at the
different times, i.e., one needs to somehow mark the
polymer entering the extruder and monitor the exit
of the extruder to determine when the marked material leaves. This has been achieved in practice by
marking the fluid with a dye (1, 2). particulate (2-4),
radioactive tracer (5).or a n immiscible second polymer (6).I t is clear, however, that if the aim of RTD
measurements is to gain information about flow patterns and mixing, the marking material, or tracer,
should not perturb these patterns. Yet one might
suspect that the introduction of foreign material to a
polymer melt, such a s a dye or particulate, will modify the actual flow behavior that one wishes to meas-
P
134
ure. This was recently shown by Lappe and Potente
(2)who reported that the RTD measured for a polyethylene melt depended on the tracer used.
The premise behind the study described in this
paper was that the best way to measure the unperturbed RTD is to use a molecular tracer that is chemically similar and miscible with the polymer and has
the identical rheological behavior. This might be accomplished, for example, by selectively exchanging
the hydrogen atoms on some of the polymer molecules with deuterium. The concentration of the deuterated tracer in the extrudate could then be measured by spectroscopic methods such as 'HNMR or
infrared spectroscopy. Alternatively, one might
chemically modify the polymer such that the modified and unmodified materials may be easily distinguished analytically, but behave the same rheologically. In the work described herein, polystyrene (PS)
was lightly sulfonated to form a n ionomer. The counterion was chosen such that the sulfonated PS ionomer (SPS)had the same viscosity function as the
starting PS. The SPS was used a s a tracer for RTD
experiments with PS, and the results are compared
with measurements made using particulate tracers.
RESIDENCE TIME DISTRIBUTION
There are two methods by which one may introduce the tracer material to the process (in this case,
the extruder), a step input or a pulse input. The
former involves introducing a continuous feed of the
marked material at some time after the process has
reached steady state and measuring the concentration of the tracer in the extrudate until it reaches a
constant value equal to the feed concentration. A
pulse input involves feeding the marked material for
a short interval, chosen to be much less than the
POLYMER ENGINEERING AND SCIENCE, JANUARY 1989, Vol. 29, No. 2
Development of a n Ionomer Tracer
average residence time. One then monitors the extrudate until the tracer concentration becomes zero
again. This was the method chosen for these experiments primarily because of the limited amount of
tracer material available.
Schematic idealized response curves plotted as
tracer concentration, C ( t ) , versus time following a
pulse input are given in Fig. l a . Alternatively, one
could plot the integrated tracer concentration, F@),
F(t) = (QIW
J
C ( t )d t
(1)
where M is the total amount of tracer added and Q is
the volumetric flow rate. These are also shown in
Fig. 1 b. For the case of no mixing, i.e., plug flow, the
C ( t ) output is identical to the pulse input with a
residence time T = V/Q, where V is the volume of the
extruder. F ( t ) is a step function at t = T . For axial
mixing, both the C ( t ) and F ( t ) curves broaden, and
with complete mixing, the response is spread out over
time.
EXPERIMENTAL SECTION
Materials
The polymer used for the RTD experiments was a
commercial general purpose polystyrene, PS, (Grade
XP6065 from Dow Chemical Company) that had
number and weight average molecular weights of
73,500 and 222,000, respectively, as determined by
gel permeation chromatography. Glass microspheres,
type pf-1 1 , were obtained from the Cataphote Division of Ferro Corporation. According to the manufacturer, these beads were made of soda-lime silica
glass, had a size distribution of 5 to 60 pm and a
specific gravity of 2.1. Carbon lampblack was obtained from J. T. Baker, and the specific gravity was
estimated to be about 2 (7). The marked polymers
were prepared by mixing a measured quantity of
glass or carbon black with PS in a Wayne Yellow
Jacket, 0.75 inch, single-screw extruder, equipped
with a general purpose, standard taper screw (LID =
25). The controllers for the three barrel heaters, as
well as for the heated strip dye, were set at 200"C,
and the screw speed was fixed at 50 rpm. The compounded extrudates were powdered in a Wiley mill
and re-extruded twice more using the same conditions in order to achieve a uniform dispersion. The
concentration of the tracer material was determined
with a DuPont 95 1 thermogravimetric analyzer. The
marked polymer was heated to 300°C under nitrogen
and the polymer was allowed to decompose, leaving
behind the filler material. The filler concentration
was measured to within 1%. The tracers prepared
contained 29% (14% by volume) glass microspheres
or 4% (2%by volume] carbon black.
Ionomer Tracer
A polymeric tracer based on a lightly sulfonated polystyrene ionomer (SPS)was developed with
the objective that it be soluble in PS and match its
rheological behavior. Sulfonic acid derivatives of SPS
containing 1.15, 1.82, and 2.65 acid groups per 100
styrene repeat units (i.e., mole %) were previously
prepared in our laboratory (8)following the method
of Makowski, e t al. (9).These were based on a commercial PS (Styron 666 from Dow Chemical Company) with a n Mn and M w of 103,000 and 288,000,
respectively. Tracer candidates based on alkyl amine
salts of these materials were prepared by dissolving
the polymer in chloroform and neutralizing the sulfonic acid groups with a stoichiometric amount of the
appropriate alkyl amine (obtained from Fisher Scientific). The neutralized SPS ionomer was precipitated in methanol, filtered, washed with methanol
and vacuum dried at 150°C for 24 hours. The salts
prepared are summarized in Table 1.
Rheological Measurements
The melt rheological behavior of PS and the tracerfilled PS's were measured with a Rheometrics System
4 mechanical spectrometer using a n oscillatory shear
deformation and a parallel plate geometry. The complex viscosity, v*, storage modulus, G', and loss modulus, G", were determined over a range of frequencies
from 0.016 Hz to 16.0 Hz and temperatures from
140°C to 2 10°C.
Master curves referenced to 150°C were constructed by conventional time-temperature superposition techniques ( 1 0).
Dye-Partition Analysis
Fourier transform infrared spectroscopy (FTIR)
was initially evaluated for quantifying the concentraTable 1. Candidate SPS lonomer Tracers Evaluated.
COI
Sample Designation
Mole 70
Sulfonation
Salt
______
TIME
TIME
Fig. 1 . Idealized residence time distributions f o r a pulse
input: (a) no miwing, (bj axial mixing, and (c) complete
mixing.
POLYMER ENGINEERINGAND SCIENCE, JANUARY 1989, Vol. 29, No. 2
1.15 HSPS
1.82 (tBuA) SPS
1.82 (TBA) SPS
1.82 (tOA) SPS
1.82 (DodA) SPS
1.82 (DBA) SPS
1.82 (BA) SPS
2.65 (TBA) SPS
1.15
1.82
1.82
1.82
1.82
1.82
1.82
2.65
free acid
t-butylamine
tributylamine
t-octylamine
dodecylamine
dibutylamine
n-butylamine
tributylamine
135
R . A. Weiss and H . Stamato
tion of the SPS tracer in PS. However, because of the
relatively low concentration of sulfonate groups used
and the fact that the IR absorbances for PS overlap
with those specific to the sulfonate groups, FTIR
characterization was not successful. An alternate
analytical technique based on a dye partition analysis (11-13) was used to measure sulfonate concentration. In this method, an aqueous phase containing
a cationic dye is contacted with a n organic solution
of the water-insoluble polymer. The dye, which is not
soluble in the organic phase, migrates into this phase
only a s the counterion to the sulfonate group. A fresh
solution of 20 mg/l methylene blue in 0.1 N HC1 was
prepared on the day of the analysis. A magnetic
stirrer was used to agitate a mixture of 3 ml of the
dye solution with 8 ml of a 0.1 % (wt) polymer solution
in chloroform for 3 hours. The organic phase was
then separated and transferred to a quartz cuvette
for spectroscopic analysis.
Spectroscopic Analysis
The absorbance at 653.4 nm of the dyed polymer
solutions was measured with a Beckman Model 25
UV/Vis spectrophotometer. This wavelength corresponds to the maximum absorbance of methylene
blue. Calibration curves were constructed by measuring the absorbance of solutions of known SPS concentration, and the calibration was repeated for each
experiment.
Residence Time Distributions (RTD)
The same Wayne extruder as described earlier and
equipped with a strip die was used to study RTDs.
Temperature was regulated by three heating controllers on the barrel and one on the die. Pressure was
measured at the end of the screw just before the
breaker plate with a Gentran, model GT-76, transducer. Volumetric flow rates were determined by collecting the extrudate for 0.5 to 1.O min and measuring the volume by displacement of water in a gradutated cylinder. Pulse RTD experiments were run a s
follows. The controllers for all heaters were set to
185°C and the extruder was allowed to warm-up for
30 minutes. The motor was adjusted to provide a
screw speed of 10.20, or 40 rpm. Granulated polymer
was hand fed to the extruder at a rate sufficient to
keep the flight of the screw covered by polymer at all
times, but so a s not to allow any build-up in the
hopper. The extruder was operated for about 15 minutes in order to insure that steady state was achieved,
and at this point a single pulse of the marked polymer
was introduced. The input pulse consisted of a five
gram batch of marker-filled pellets. This material
was added directly to the screw flight all at once. The
extruder was then starve-fed until the marked polymer was completely taken up by the screw, and then
normal operation was resumed. The duration of the
pulse depended on the flow rate and was determined
by measuring the time it took to feed a five gram
batch of polymer under identical conditions. The experimental pulse times are given in Table 2. Follow136
ing the pulse, the extrudate was collected for 10 min.
a s strips, each corresponding to 30 sec of operation.
Each of the strips were subsequently cut into eight
equally spaced samples. The time corresponding to
when each sample exited the extruder was determined by its position in the original extrudate.
RESULTS AND DISCUSSION
Sulfonate Analysis
The results of three separate calibration curves for
the analysis of SPS concentration for blends of 1.15
HSPS and PS are shown in Fig. 2. Two features of
these results point out some of the difficulties with
this procedure. First, the absorbance curves did not
extrapolate to zero at zero concentration. This is
believed to be due to a slight solubility of methylene
blue in chloroform (14),which may be due in part to
the presence of ethanol (ca. 0.75% in reagent grade
chloroform) that is used as a stabilizer. The second
problem related to differences in the calibration
curves measured on different days. This lack of reproducibility may arise from the metachromic shift
of methylene blue (15, 16).That is, the peak absorbance shifts to shorter wavelengths due to the association and interaction of the dye molecules. Such
changes in the visible spectra have been observed for
M (17) and in
molar concentrations a s low a s
the presence of sulfonated polystyrene (18).The metachromic shift is a function of concentration (15,
16, 18),pH (19, 20), temperature (17, 21). hydration
(18). and interaction with the polyanion (18).Additional complications may arise from the contamination of methylene blue with trimethyl thione (TMT).
Methylene blue slowly decomposes to TMT, particularly in acidic solutions (19).In order to eliminate the
Table 2. Pulse limes vs. Screw Speed.
Screw speed,
rPm
Flow Rate
(ml/min)
Pulse Duration, s
10
10
20
40
14
34.2
20.0
22
12.1
-R9
@/
0.4
I
I
I
I
I
I
I
I
I
WT% SPS
Fig. 2. Calibration curves f o r dye-partition analysis using 1.15H-SPSIPSblends. Each curve represents a s e p a rate calibration.
POLYMER ENGINEERING AND SCIENCE, JANUARY 1989, Yo/. 29, No. 2
Development of a n Ionomer Tracer
influence of these variables and the variations that
may arise from batch to batch of reagent, new calibration curves were generated for each set of samples
analyzed. Although a consistent routine was employed for all analyses, there was still a significant
scatter of the absorption data from any one sample.
This was presumably due to the difficulties in avoiding a small amount of included water droplets in the
organic phase. In order to minimize the uncertainty
of the analysis a large number of measurements were
made for each sample. A typical calibration curve for
blends of 1.82 (TBA)SPSand polystyrene, which were
used as the tracer in the experiments described in
this paper, is shown in Fig. 3. The non-linear behavior is believed to be due to the dependence of the
metachromic shift on concentration and/or the kinetics of the ion-exchange of the amine by methylene
blue. When a standardized routine for the analysis
was used, the calibration curves were found to be
relatively reproducible and given the other sources of
error present in the RTD determinations, the accuracy of the dye-partition analysis was judged to be
acceptable, especially in light of its good sensitivity
at low concentrations.
Tracer Development
A s discussed earlier, it was thought that a major
shortcoming of the tracers traditionally used in RTD
experiments was that they perturb the flow characteristics of the polymer being evaluated. This is demonstrated in Fig. 4, which gives the complex viscosity
master curves as a function of frequency for PS, the
glass bead-filled PS, and the carbon black-filled PS
used in these studies. Also included for comparison
are data for 1.15 HSPS. The solid fillers raise the
viscosity of PS by about a factor of two. It is also
worth noting that even at a sulfonation level as low
a s 1.15%, the viscosity of the HSPS is higher than
not only the PS, but also the two filled polymers. For
the ionomer, the increase in viscosity is due to intermolecular associations due to hydrogen bonding that
increase the effective molecular weight of the flow
a l
ob"""'
4
I
8
12
WT% sps
16
I
unit. Although the sulfonated polymer was attractive
from the standpoint that it is chemically very similar
to PS and that the sulfonate groups provide a means
to distinguish it from PS, the enhancement of the
viscosity limits its usefulness.
Weiss, e t al. (22).however, reported that when SPS
was neutralized with a sterically bulky amine such
as tributylamine, the viscosity of the ionomer was
very similar to that of the starting PS. In the present
study several different alkylamine salts of 1.15, 1.82,
and 2.65 mole % SPS were evaluated as candidates
for the tracer material. The complex viscosity master
curves of five of these are given with that of the
unfunctionalized PS in Fig. 5. There is clearly an
excellent match of q* for the 1.82 tributylamine salt
and PS over the range of frequencies covered, about
six decades. Although it is generally observed that
the viscosity of ionomers is greater than that of the
unfunctionalized polymer because of intermolecular
associations, for a tributylammonium salt the steric
hindrance of the bulky cation restricts interactions.
For the same reasons, one might presume that the
tributylamine salt of 1.82 SPS and PS are miscible
judging from the clarity of the blend. One must recognize, however, that clarity alone is not a sufficient
criterion for miscibility. Unfortunately, the usual criterion for miscibility of polymer blends, a single composition dependent Tgcannot be used for this system
because of the similarity of the Tg'sof the two components. In any event, the matching of the viscosities
and the chemical similarity to PS makes the tributylamine salt of 1.82 SPS a n ideal molecular tracer for
RTD experiments with PS.
The actual tracer used was a blend of 20% (wt) of
1.82 (TBA)SPS and 80%polystyrene. The Tg of the
blend as measured by DSC, 102°Cwas indistinguishable from that measured for PS and the rheological
behavior was identical to the curves for the blend
components as shown in Fig. 5.
Residence Time Distribution
The experimental C(t) curves are given in Fig. 6a
for three different screw speeds. The experimental
curves exhibit a long tail of low marker concentration
on the longer time side of the distribution. The curves
do not go to zero concentration as a result of termi-
I]
20
Fig. 3. Typical calibration curve f o r dye-partition analysis using 1.82 [TBA)SPS/PS blends.
POLYMER ENGINEERING AND SCIENCE, JANUARY 1989, Vol. 29, No. 2
W*)
Fig. 4 . Complex uiscosity us.frequency mastercurues at
15OCf o r (0)polystyrene, (0)PS/glass bead tracer, (0)PS/
carbon black tracer, and (A) 1.15H-SPS.
137
R . A . W e i s s a n d H . Starnato
I '
-5
I
I
1
I
I
I
-4
-3
-2
I
I
-1
logba,)
I
I
1
I
I
I
I
I
0
1
2
3
(a)
Fig. 5. Complex viscosity u s frequency mastercurves at
15OC for (0)polystyrene, (0) I.82(TBA)SPS, (0)tracer
based on 20% 1.82(TBA) + 80%P S , fa) 1.82fBAfSPS, [b)
1.82(DBAJSPS, (cf 2.65(TBAJSPS, and (d) 1. I5fTBAfSPS.
The data pointsfor curves (af-(d)were ornittedfor clarity.
nating the experiment when the marker concentration approached the detection limits of the analysis.
A s a consequence, only about 85% of the marker
material was accounted for by the C ( t )curves. Nevertheless, several trends are apparent in the data.
As expected, as the screw speed increases the
curves shift in the direction of shorter residence time.
Despite the large scatter of the data, which is typical
for these types of measurements, a couple of differences in the RTD for the three tracers can be seen.
At 10 and 20 rpm the ionomer tracer clearly has a
shorter average residence time and less tailing in the
distribution at longer times than do the two particulate tracers. The distributions of all three tracers are
similar for the highest flow rate. Comparison of these
C(t )curves to the idealized processes discussed earlier
indicates that the process under consideration here
approximates most closely that of axial mixing. The
pronounced tail in the data, however, indicates a
more complicated mechanism of mixing. The increased broadening of the C ( t )curves with decreasing
screw speed is probably due to increased mixing at
the slower extrusion rates.
The differences in the RTD of the different tracers
is more easily seen in the integral distributions, F ( t ) ,
in Fig. 6b. Like the C ( t )curves, the F ( t ) curves shift
to shorter times as screw speed increases. The broadening of the C ( t ) curve that results from increased
mixing is manifested in the F ( t ) curve by a more
gradual increase, i.e., lower slope. Thus, it appears
that at the two slower extrusion rates, the particulate
tracers are better mixed than the molecular tracer.
An important question, which unfortunately cannot
be answered by these results alone, is whether this
increased degree of mixing is representative of the
flow of the continuous RTD phase or whether it is an
artifact resulting from the different viscosities of the
particles and the polymer.
The first two moments of the C ( t )distribution, the
mean residence time ( 7 ) and the variance (u) were
calculated using E q s 1 and 2 and are given in T a b l e
138
Fig. 6. Experimental RTD curves for the extrusion of PS
at three screw speeds: (a)Cftf and (b)Fit).
3. The variance is a measure of the breadth of the
distribution.
j-E
rJ=
JtC(t ) dt
SC(t) d t
J ( t - r ) C ( t )d t
SC(t ) d t
(3)
The differences in the two types of tracers are clearly
seen in the data for 10 and 20 rpm. The particulate
tracers have a longer mean residence time and a
broader distribution, i.e., greater variance, than the
ionomer tracer. At the highest screw speed studied
all the tracers appeared to behave similarly.
These results are in general agreement with the
conclusions of Lappe and Potente (2)who found that
the measured RTD is dependent on the tracer used.
The distribution of residence times arises from the
mixing of the polymer in the extruder. Note that no
mixing would result in a pulse output corresponding
to the pulse input. The greater the mixing, the
broader should be the RTD.
While it is impossible to state unambiguously
which, if any, of the distributions given in Fig. 6
most closely represents that of the PS, intuition
might suggest that the flow of the molecular tracer
would better approximate that of the polymer. Because of the large differences in the viscosities of the
molten polymer and the solid particulates, their relative motions are expected to be different. A s a consequence, the residence time of the polymer may
actually be less than what is predicted on the basis
of the data for the particulate tracers. This, of course,
is demonstrated by the experiments reported here.
The fact that no differences were observed between the RTD's measured for the glass microPOLYMER ENGINEERING AND SCIENCE, JANUARY 1989, Vol. 29, No. 2
Development of an Zonomer Tracer
REFERENCES
Table 3. Moments of the RTD’S.
Glass
Spheres
Carbon
Black
lonomer
Screw
Speed
(rpm)
7
U
T
U
7
U
(min)
(min)
(min)
(min)
(rnin)
(min)
10
20
40
3.97
3.69
2.08
.28
37
.19
4.06
3.81
2.15
.24
.36
.ll
3.69
3.21
1.98
.14
.06
.09
spheres and the carbon black was at first surprising.
That is, one might expect the RTD to also be sensitive
to the particle size of the tracer. In fact, the two
tracers used in this study were chosen in order to
assess the dependence of the measured RTD on particle size. However, although the sizes of the individual particles of the as-received glass microspheres
and the carbon black differed by a factor of ca. 100,
agglomeration of the carbon black and fracture of
the microspheres either during compounding or in
the actual experiments resulted in approximately
equal sizes of the particles. This was confirmed by
electron microscopy of fracture surfaces of the experimental extrudates.
CONCLUSIONS
A molecular tracer developed from a lightly sulfonated polystyrene ionomer was used in residence time
distribution studies of the extrusion of polystyrene.
Compared with the results using particulate tracers,
the ionomer-marker RTD’s were characterized by
shorter mean residence times and narrower distributions. The differences between the results obtained with different tracers was attributed to a n
increased amount of axial mixing that occurs with
particulate tracers.
POLYMER ENGINEERING AND SCIENCE, JANUARY 1989, Vol. 29, No. 2
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