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Article
Synthesis of Ultrahigh Molecular Weight Polyethylene Using
Traditional Heterogeneous Ziegler#Natta Catalyst Systems
Sudhakar Padmanabhan, Krishna R. Sarma, and Shashikant Sharma
Ind. Eng. Chem. Res., 2009, 48 (10), 4866-4871• Publication Date (Web): 14 April 2009
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2. Synthesis of Ultrahigh Molecular Weight Polyethylene Using Traditional
Heterogeneous Ziegler-Natta Catalyst Systems
Sudhakar Padmanabhan,* Krishna R. Sarma, and Shashikant Sharma
Research Centre, Vadodara Manufacturing DiVision, Reliance Industries Limited, Vadodara, India, 391 346
Ultrahigh molecular weight polyethylene was synthesized from traditional Ziegler-Natta type catalysts (ZN),
namely, TiCl4 anchored on MgCl2 support. This, upon activation with AlRR′2 (where R, R′ ) isoprenyl or
isobutyl), gave precatalysts (C-2 to C-5) having 16, 21, 25, and 32% trivalent titanium, respectively. The
reduction in oxidation states also accompanies the reduction in particle size of the catalysts, which in turn
gets reflected in the resulting polymer properties under specified operating conditions. We have demonstrated
the effect of process conditions that can surmount the catalyst dependency over the polymer characteristics,
and hence, it can result in polymer with consistent polymer properties, which is an important need of the
polymer industries. The polymer characteristics such as particle size distribution, average particle size, bulk
density, reduced specific viscosity, and concentration of fine and coarse particles were determined and were
dependent on various process parameters. Under identical reaction conditions, the polymerization with larger
scale yield polymer with different characteristics. The fine-tuning of process conditions yielded polymer with
consistent quality.
Introduction
Ultrahigh molecular weight polyethylene (UHMWPE) be-
longs to the specialty polymer grade, having unique properties
and hence finding applications in areas requiring less abrasion,
excellent impact strength, good chemical resistance, etc.1–3
UHMWPE has excellent wear resistance, outstanding impact
strength, and very good chemical resistance. Consequently, it
finds applications in diversified areas with unique requirements.3
More than two-thirds of the commercial processes involved
are based on Hostalen’s continuous stirred tank using conven-
tional ZN catalysts.4
A couple of processes are also based on
metallocene catalyst systems with very limited capacities.5
The
initial patents on catalysts relating to UHMWPE date back to
the early 1970s and still continue to dominate the scene, even
after a span of 4 decades. The concept of anchoring TiCl4 on
supports like Mg(OR)2/MgCl2 followed by treatment with
aluminum alkyls has been fully exploited through diverse
process variations.6
Major players in this field arranged chro-
nologically include Ruhrchemie, Hoechst, Himont, and Ticona.4
Petrobras aimed at improved morphology of the polymer
through spherical catalyst systems involving supporting and
spray-drying techniques.7
Phillips’ novelty was in the use of
modified alumina and silica supports to immobilize metals like
Ti, V, Cr, Zr, and Hf.8
Equistar derived their strengths through
the use of quinolinoxy-containing single site catalysts through
a nonalumoxane route.9
Besides, there are numerous examples
available in the literature pertaining to the use of homogeneous
single-site catalysts involving metals like Ti, V, and Zr for the
synthesis of UHMWPE.10
Among the various grades of UHMWPE, the grade with
molecular weight 4-5 million g/mol is unique because of its
optimum abrasion resistance, impact strength, chemical resis-
tance, etc.3
Hence the 4-5 million molecular weight grade has
maximum business volume. At higher molecular weights, though
the abrasion resistance was slightly better than that of the lower
molecular weight polymers, the impact strength dropped down
considerably. Considering this, it is imperative that we have
special grades with unique properties for unique applications.2
Most of the polyethylene produced based on the market needs
are manufactured using traditional Ziegler-Natta catalysts,
which typically comprise titanium halides (TiX4 where X is
generally Cl) supported on magnesium chloride (MgCl2) through
various chemical modifications.6
Olefin polymerizations involv-
ing such ZN catalysts involve a catalyst preactivation step
involving aluminum alkyls, aluminoxanes, or borate compounds
(generally known as cocatalysts) wherein, apart from reduction
of oxidation states of the titanium, there is also a vacant
coordination site created on the titanium. It is on this vacant
site that the olefin coordinates, and through a series of
transformations, the polymer chain grows. The activity of these
catalysts not only depends on the total titanium present in the
system but also depends on the percentage of the reduced
titanium. The production of UHMWPE using these catalyst
systems is again a big task, taking in to account of the possible
termination reactions that can kill the propagating active species.
The presence of excess aluminum alkyls can bring about the
termination via transfer of polymer chain to aluminum. This
can reduce the length/molecular weight of the polymer chain
and also broaden the molecular weight distribution.11
Experi-
ments on a slightly larger scale in a 5 L laboratory-scale reactor
poses a vigorous threat because of the usage of less catalyst,
which can easily be killed by the presence of a small amount
of impurities in the reaction medium. Hence, process optimiza-
tion studies play a bigger role in these reactions.
In this paper we have demonstrated the capability of using
hydrocarbon as a polymerization solvent for producing UHM-
WPE having desired bulk density (BD), average molecular
weight, and average particle size (APS) with controlled fine
(<10 µm) and coarse (>250 µm) material and also developed
laboratory process for making UHMWPE of 4-6 million g/mol
molecular weight with consistent productivities.
Experimetal Section
The required catalyst, C-1 with 20% titanium content (80%
magnesium and chlorides), was synthesized by adopting a well-
* To whom correspondence should be addressed. Tel.: +91 265 669
6260. Fax: +91 265 669 3934. E-mail: Sudhakar.padmanabhan@
zmail.ril.com.
Ind. Eng. Chem. Res. 2009, 48, 4866–48714866
10.1021/ie802000n CCC: $40.75  2009 American Chemical Society
Published on Web 04/14/2009

3. known procedure:6
10 g of magnesium ethoxide is added to
120 mL of varsol, a high boiling kerosene fraction, under an
atmosphere of nitrogen and mechanical stirring. The temperature
is increased to 85 °C and maintained. Subsequently, about 35 g
(20 mL) of TiCl4 is added to the magnesium ethoxide suspension
under a gentle atmosphere of nitrogen slowly over a period of
5-6 h. The molar ratio of Mg:Ti is about 1:2. After the
completion of TiCl4 addition, the temperature is increased to
120 °C and maintained for about 60 h to temper the catalyst.
The solvent contains the precatalyst as a pale yellow to white
suspension. The catalysts were stored under nitrogen atmosphere
as a slurry in hexane. The slurry concentration was maintained
at 12-15% for easy handling. The slurry was homogenized
completely and was transferred using standard syringe tech-
niques. The slurry concentration of the catalysts was determined
before each experiment to calculate the amount of the catalyst
added. All manipulations like handling and transfer of catalysts
and pyrophoric aluminum alkyls were carried out in a nitrogen
glovebox/bag.
The actual catalyst for UHMWPE is prepared from C-1 (white
catalyst) by reducing the same using AlRR′2. The molar ratio
employed between the titanium catalyst and the aluminum alkyl
varied on the basis of the Ti3+
content intended. The aluminum
alkyl is gently added at about 25 °C to the white catalyst slurry
under a stream of nitrogen and with mechanical agitation over
a period of 3-5 h. The color of the slurry changes to grayish
black, and hence, the catalyst is also referred to as the “black
catalyst”. Here the titanium is present as a mixture of quadriva-
lent and trivalent titanium (predominantly) with traces of
divalent titanium.
Polymerizations were carried out in laboratory Buchi reactors
of 1, 5, and 19 L capacity using well-established and validated
procedures in hexane as the medium. The hexane used in the
runs is dry distilled under a nitrogen atmosphere after refluxing
it over sodium hydride as the desiccant, and the moisture content
was around 5-8 ppm. The prereduced catalyst slurry in hexane
was homogenized and a suitable amount was transferred out
for a run. The agitation has been standardized around 500 rpm
and the temperature was maintained at 75 °C over the period
of 2 h. Hydrogen dosing was done through a precalibrated bomb
hooked to the reactor for controlling the molecular weight. The
polymer was then filtered, washed with acetone, and then dried
in an air oven at about 75 °C. The weight of polymer was
recorded to calculate the productivity of the catalyst in terms
of grams of polymer/grams of catalyst and grams of polymer/
millimole of Ti. The productivity was based on a 2 h period.
Catalyst characterization was carried out by measuring
parameters like slurry concentration for the solid content;
compositional analysis for Ti, Mg, and Cl by UV-vis spec-
trophotometry and EDTA and argentometric titrations, respec-
tively; oxidation states of Ti (quadrivalent, trivalent) by
cerimetry; and the particle size distribution (PSD) for APS by
a Malvern Mastersizer-E, a laser diffraction based particle size
analyzer. The viscosity-based average molecular weight was
calculated using Margolie’s equation. The reduced specific
viscosity (RSV) was determined at 135 °C in decaline as solvent
in an Ubbelohde viscometer with constant ) 0.01 by measuring
the flow times for solvent and subsequently a 0.02% solution
of the polymer.
Results and Discussion
Titanium supported on MgCl2 (C-1) upon activation with
AlRR′2 (an equal mixture of triisobutylaluminum and isopre-
nylaluminum) and hydrogen as the molecular weight regulator
is being used for the generation of UHMWPE. There is enough
literature precedence for the use of such catalysts with triethyl
aluminum (TEAL) as an activator for the production of high
density polyethylene (HDPE).12
Tailoring such catalyst to
produce UHMWPE through process optimization in hydrocar-
bon media meeting rigid polymer specifications has been a
challenge in the industrial arena. The necessary precatalyst C-1
has been prepared with 20% titanium loading on MgCl2 support
and activated with AlRR′
2, which yield catalysts with active
titanium center. The catalyst batches with different Ti 3+
contents
were synthesized by adjusting the AlRR′2 quantity and are C-2
(16% Ti3+
), C-3 (21% Ti3+
), C-4 (25% Ti3+
), and C-5 (32%
Ti3+
). The process overview is given in Scheme 1.
The role of aluminum alkyls in olefin polymerization is of
paramount importance and consequently today we have a diverse
range of such Lewis acids, each with a unique role to play in a
polymerization. The crux of the earlier statements is that the
correct aluminum alkyl has to be primarily identified for a
polymerization process and then subsequently its amount with
respect to the catalyst needs to be optimized to arrive at the
desired productivity and polymer characteristics, namely, mo-
lecular weight, average particle size, bulk density, etc. The use
of AlRR′2 as an activator yields the required polyethylene with
ultrahigh molecular weight. For a particular ethylene pressure
and catalyst system (C-3), we carried out the Al/Ti optimization
experiments and we observed that the optimum value of Al/Ti
is around 7-8 under the specified operating conditions, namely,
2 atm of ethylene pressure (PC2 2 atm). This exercise needs to
be optimized when the conditions are changed. Thus, at an
ethylene pressure above 5 atm we found the optimized Al/Ti
ratio was around 4-5. By operating at a different Al/Ti value,
besides yield, the other polymer properties like BD and average
molecular weight also change, thus providing a lever to alter
the polymer characteristics at the cost of yield. At 2 atm PC2,
we have evaluated the polymerization with C-2 to C-5 and found
that there is a close agreement between the polymer particle
Scheme 1. Process Overview
Figure 1. Comparison of PSD of polymer with catalyst nature.
Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009 4867

4. size distribution (PSD) and catalyst PSD (Figure 1). As we
increase the % Ti3+
, there is a reduction in catalyst PSD which
in turn gets reflected in the APS of the polymer obtained with
increased fines. The optimum value of Ti3+
content was found
to be 20-25% under PC2 2 atm conditions (Figure 1). When
we increased the pressure from 2 to 7.5 atm, we found there is
not much difference in PSD of the polymer obtained among
C-2 to C-5 catalyst systems, clearly revealing the importance
of process conditions over polymer properties.
Besides AlRR′2, we have also evaluated TEAL as the
activator12
for selected catalyst batches for obtaining UHMWPE.
At 2 atm PC2 employed, though molecular weight between 4
and 10 million g/mol could be achieved through H2 mediation,
it was observed that BD was always around 0.25 g/cm3
, and
the fines generated were also on the higher side. Extensive
process optimization studies need to be performed for better
polymer characteristics.
During the course of our investigation with a view to generate
UHMWPE with the desired characteristics (BD, PSD/APS,
RSV), we have carried out polymerizations with ethylene
pressures ranging from 2 to 7.5 atm. With typical catalyst and
process conditions, we could achieve productivity of ∼2.5 (
0.5 kg of UHMWPE/g of catalyst at 7.5 atm ethylene pressure
over 2 h (Figure 2a). Nonetheless, besides productivity, the other
polymer characteristics could be fine-tuned by playing with the
pressure. BD improved considerably at enhanced pressures,
which is highly desirable. Changing the ethylene pressure was
a convenient way to change the partial pressure of hydrogen
during molecular weight control experiments, thus providing
leverage for producing UHMWPE with desired average mo-
lecular weight.
We have observed that the temperature at which the polym-
erization was performed had an effect on the average molecular
weight of UHMWPE, akin to what has been observed by other
groups.13
Thus, keeping all other parameters constant and
carrying out polymerizations at 70, 75, and 80 °C resulted in
UHMWPE with progressive reduction in average molecular
Figure 2. (a) Effect of PC2 on polymer productivity. (b) Effect of Ti3+
content on molecular weight of the polymer obtained.
Figure 3. (a) H2 dosing bomb calibration. (b) Effect of H2 pressure on polymer RSV.
Table 1. Effect of H2 Pressure on Molecular Weight of UHMWPE
at PC2 5 atma
run PH2 (atm) productivity (g PE/g cat.) BD (g/cm3
) APS (µ)b
Mη
c
1 3 1442 0.36 94 4.0
2 2 1282 0.36 94 4.4
3 1 1359 0.35 97 4.3
4 0.5 1195 0.35 108 5.3
5 2 1049 0.33 97 4.6
6 2 1344 0.36 100 4.4
7 0.17 1282 0.36 103 5.1
8 0.34 1303 0.36 98 4.2
a
General conditions: PC2 5 atm, Al/Ti ) 5, 75 °C, 500 rpm; 0.34 g
of C-4; PH2 in 100 mL bomb. b
Analyzed by both Malvern PSA and
traditional sieve shaker methods. c
Viscosity-based average molecular
weight (million g/mol) calculated using Margolie’s equation.
Table 2. Effect of H2 Pressure on Molecular Weight of UHMWPE
at PC2 7.5 atma
run PH2 (atm) productivity (g PE/g cat.) BD (g/cm3
) APS (µ)b
Mη
c
9 0 3235 0.41 124 8.3
10 3.0 3468 0.40 115 2.1
11 1.0 3439 0.41 120 3.1
12 0.65 3453 0.40 121 4.2
13 0.60 3147 0.41 116 4.3
14 0.55 3246 0.41 118 4.2
15 0.34 3235 0.42 114 4.5
16 0.08 3351 0.41 125 4.2
a
General conditions: PC2 7.5 atm, Al/Ti ) 5, 75 °C, 500 rpm; 0.34 g
of C-4; PH2 in 100 mL bomb. b
Analyzed by both Malvern PSA and
traditional sieve shaker methods. c
Viscosity-based average molecular
weight (million g/mol) calculated using Margolie’s equation.
4868 Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009

5. weight, though not on a major scale because of the small
difference in temperature. It has to be realized that carrying out
polymerization at much lower temperatures is not economical
from the commercial angle, since the reaction rate drops down
drastically for even a drop in temperature of about 10 °C.
Under more or less similar operating conditions (within limits
of experimental error) catalyst systems C-2, C-3, C-4, and C-5
containing 16, 21, 25, and 32% Ti3+
were screened for
UHMWPE polymerization. The trend when ethylene pressure
was 7.5 atm is shown in Figure 2b. We could see that the
ethylene pressure predominates over the trivalent Ti content in
the catalysts to alter the kinetics of the process.
Hydrogen has been in regular use as a molecular weight
regulator in ethylene and propylene polymerizations. It is
convenient to use due to various practical reasons, since an
extensive amount of data pertaining to its solubility in various
solvents is available at different temperatures.14
Research groups
have also determined the Henry’s constant for hydrogen and
ethylene in hexane at different temperatures.15
We have also
studied the solubility characteristics of ethylene, hydrogen, and
their mixtures in solvents like varsol and hexane.16
Hydrogen
is one such gas where its solubility increases with temperature,
unlike the expected reverse trend. For UHMWPE systems, this
can have far reaching implications since a proper combination
of solvent, temperature, and hydrogen partial pressure can result
in unique molecular weight control.
With an objective of controlling the molecular weight of
UHMWPE with hydrogen,17
we have calibrated the hydrogen
dosing bomb hooked to the Buchi reactor. The bomb was
pressurized at ambient temperature, 30 °C, with hydrogen at
different pressures, and the volume of hydrogen was mea-
sured using a gas flow meter. The results are given in Figure
3a. This gave a method to measure the volume of hydrogen
dosed based on the pressure employed in the bomb. The
calibration results are quite linear, with an excellent regres-
sion constant of almost 1.
During the course of our investigation for regulating the
molecular weight of UHMWPE in polymerization, we realized
that there is a threshold limit for hydrogen using the specified
bomb under the employed conditions. This is essentially the
threshold or saturated solubility of hydrogen in 3 L hexane at
the specified operating conditions based on the partial pressures
of hydrogen, ethylene, and hexane.18
We could not go down to
lower hydrogen pressures than this due to the bomb limiting
capacity. The size of the dosing bomb was approximately 100
mL at atmospheric pressure. The approach available to us was
to hook up another bomb of smaller size, say 50 mL in capacity,
or to reduce the hydrogen partial pressure by significantly
Table 3. Scale up Studiesa
run reactor size (L) solvent vol (L) catalyst concn (mmol Ti) Al/Ti productivity (g PE/g cat.) Mη
b
17 5 3 1.00 8 2760 10.0
18 1 0.5 0.2 8 3000 12.6
19 19 10 4.00 8 2250 5.3
20 19 10 4.00 5 1950 7.2
a
General conditions: PC2 7.5 atm, PH2 0 atm, 75 °C, 500 rpm with C-4. b
Viscosity-based average molecular weight (million g/mol) calculated using
Margolie’s equation.
Figure 4. SEM images of the polymers produced from (a) C-2, (b) C-3, (c) C-4, and (d) C-5.
Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009 4869

6. increasing the ethylene pressure. We have used the second
approach. Here also we could not go beyond 7.5 atm ethylene
pressure due to system configurations. The results are depicted
in Figure 3b.
It can be observed how effectively the partial pressure of
hydrogen is controlled at two different ethylene pressures, viz.,
5 and 7.5 atm. Obviously, as expected the line at 5 atm ethylene
pressure controls molecular weight regulation in a higher region
than the 7.5 atm ethylene pressure, again verifying Henry’s law
for the solubility of gases. We can observe from Figure 3b that
achieving an average molecular weight of ∼4.5 million is
statistically more favored at hydrogen pressures from 0.1 to 0.5
atm, since the partial pressure of hydrogen is not lowered down
significantly at these lower hydrogen pressures. Molecular
weight control with hydrogen pressure of 1, 2, and 3 atm reflects
a linear response, with the molecular weight progressively
dropping down since the partial pressure of hydrogen now
becomes significant (Figure 3b).
In case we wanted molecular weight control in a still higher
region compared to 5 atm ethylene pressure, the approach would
be to operate at lower ethylene pressures; this would lower the
partial pressure of hydrogen, thus increasing the molecular
weight. In doing so, we might realize that other vantage
properties like productivity, BD and APS might get affected.
In a nut shell, the overall game is optimization of all parameters
such that we get all the desired properties.19
Thus, experiments at 5 atm ethylene pressure gave us good
productivity, except that the bulk density was below 0.4 g/cm3
and APS was low. The results are shown in Table 1. Experi-
ments at 7.5 atm ethylene pressure gave us most of the desired
polymer properties. We found that it was an excellent recipe
for making the 4.5 million molecular weight grade with
enhanced productivity, desired BD, and PSD/APS (Table 2).
UHMWPE produced using different catalyst batches with
different Ti3+
contents (Figure 4) hardly showed any variation
in morphology. The SEM images of several other batches mimic
the same kind of images, confirming the consistent quality of
the polymer obtained in different grades synthesized.7
After thorough examination of the 5 L scale laboratory
experiments, we did scale up the same reaction to 19 L scale.
The productivity and quality of the polymer in terms of other
polymer characteristics were found to be comparable, but the
molecular weight of the polymer obtained came down drastically
(Table 3). This led us to do the reaction in smaller scale also
and we indeed found that at 1 L scale the molecular weight
was higher. The experiment with lesser aluminum alkyl, i.e.
Al/Ti ratio of 5 in 19 L, gave polymer with comparable yield
and increased molecular weight. It is worth noting that the
aluminum alkyl, which is in excess, plays the role of a chain
terminator, thereby reducing the molecular weight. The reaction
in 1 L scale, having fewer aluminum alkyls available for chain
termination, gave higher molecular weight polymer.11
Thus,
controlling the effective alkyl aluminum concentration is an
important parameter, especially while synthesizing polymers
having ultrahigh molecular weight.
Conclusion
The production of UHMWPE having molecular weight of
4-6 million g/mol under specified operating conditions was
established on a scale of 5 L. The polymer obtained had defined
product characteristics, which is highly desirable from an
industrial point of view. The study further emphasizes the
importance of the proper concentration of catalyst and cocatalyst
and other process conditions for achieving the desired polymer
characteristics.
Acknowledgment
We thank Mr. Viral Kumar Patel for his technical and
analytical assistance throughout the course of the work. Sincere
thanks are due to Dr. R. Char and his team for the pilot plant
studies. We also sincerely thank Dr. Ajit Mathur and Dr. Rakh
V. Jasra for their continuous encouragement to carry out this
work.
Supporting Information Available: The detailed procedure
for estimating total titanium content and different oxidation
states present in the catalyst systems is given in detail. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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ReceiVed for reView December 29, 2008
ReVised manuscript receiVed February 25, 2009
Accepted March 17, 2009
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Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009 4871