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Materials Science and Engineering B 168 (2010) 132–135
Contents lists available at ScienceDirect
Materials Science and Engineering B
journal homepage: www.elsevier.com/locate/mseb
Synthesis of ultra high molecular weight polyethylene: A differentiate material
for specialty applications
Sudhakar Padmanabhan∗
, Krishna R. Sarma, Kishor Rupak, Shashikant Sharma
Research Centre, Vadodara Manufacturing Division, Reliance Industries Limited, Vadodara, 391 346, Gujarat, India
a r t i c l e i n f o
Article history:
Received 30 July 2009
Received in revised form 15 October 2009
Accepted 16 October 2009
Keywords:
UHMWPE
Mg-Ti catalyst
Particle size distribution
Bulk density
Average particle size
Entanglement
Morphology
a b s t r a c t
Tailoring the synthesis of a suitable Ziegler-Natta (ZN) catalyst coupled with optimized polymerization
conditions using a suitable activator holds the key for an array of differentiated polymers with diverse
and unique properties. Ultra high molecular weight polyethylene (UHMWPE) is one such polymer which
we have synthesized using TiCl4 anchored on MgCl2 as the support and activated using AlRR 2 (where
R, R = iso-prenyl or isobutyl) under specific conditions. Here in we have accomplished a process for
synthesizing UHMWPE in hydrocarbon as the medium with molecular weights ranging from 5 to 10 mil-
lion g/mole. The differentiated polymers exhibited the desired properties such as particle size distribution
(PSD), average particle size (APS), bulk density (BD) and molecular weight (MW) with controlled amount
of fine and coarse particles. Scanning electron micrographs (SEM) reflected the material to have uniform
particle size distribution with a spherical morphology. The extent of entanglement was determined from
thermal studies and it was found to be highly entangled.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
The search for new generation catalysts for olefin polymeriza-
tion has resulted in a variety of novel catalysts having different
organic frameworks and metals [1]. The new generation metal-
locene and non-metallocene based catalysts in combination with
aluminum alkyls or borates have dominated the area of olefin poly-
merization over two–three decades [2,3]. The fine tuning of the
catalysts was mainly stressed for generating the stereo regular-
ity in the polyolefin synthesized and also for making a variety of
copolymers, as the environment of the metal center determines the
same [4,5]. The novel grades of polymers which can be synthesized
from these new generation catalysts hold higher price material or
rather specialty materials. In the commodity grade polymers like
HDPE, commercial plants are still highly dependent on traditional
heterogeneous Ziegler-Natta type catalysts having titanium sup-
ported on magnesium chloride along with aluminum alkyls [6–8].
This is true with even some special grade polymers like ultra high
molecular weight polyethylene (UHMWPE) as most of the poly-
mers are produced from the Hostelen’s stirred tank process using
traditional ZN catalysts and alkyl aluminums [9–19]. The difference
between HDPE and UHMWPE processes hovers around optimized
process conditions, besides having the tailored catalyst composi-
tion. Proper optimization studies can transform HDPE catalysts in
∗ Corresponding author. Tel.: +91 265 669 6000x2216; fax: +91 265 669 3934.
E-mail address: sudhakar.padmanabhan@ril.com (S. Padmanabhan).
to UHMWPE catalysts which have been clearly demonstrated ear-
lier from our group [19]. The key factor which makes HDPE catalysts
transform to UHMWPE is the nature of the activator coupled with
the extent of trivalent titanium present in the system. The poly-
merization media governs the monomer and hydrogen solubility in
isolation or in conjunction leading to the desired molecular weight
characteristics which is more significant for UHMWPE. To make
this entire process of controlling the hydrogen and ethylene dosage
simpler, we have already demonstrated by tuning the catalyst
dosing with fixed ethylene and hydrogen pressure to synthesize
UHMPWE of required molecular weights [18]. In the present study,
we have explored the feasibility of using varsol as the medium for
the polymerization of ethylene using similar catalyst systems. We
have studied the binary solubility of ethylene and hydrogen in var-
sol and found that the solubility of ethylene increased unlike in
hexane [20]. In view of this it is obvious that the monomer sol-
ubility holds the key to the overall polymerization kinetics. We
have proved the same concept through polymerization studies by
capturing the difference in polymerization behavior in hexane and
varsol.
2. Experimental
2.1. General experimental techniques
All glass wares used were thoroughly cleaned and oven dried.
The glass wares were cooled under an atmosphere of dry nitrogen
before an experiment. All manipulations like handling and transfer
0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.mseb.2009.10.026

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S. Padmanabhan et al. / Materials Science and Engineering B 168 (2010) 132–135 133
of catalysts and pyrophoric aluminum alkyls were carried out in a
nitrogen glove bag as far as possible.
2.2. Synthesis of a typical black catalyst
A 2-L double jacketed glass reactor vessel having three standard
joints at the top and with provision for water circulation was assem-
bled after pre-heating same in the oven followed by cooling under
dry nitrogen. The same was equipped with a variable speed stirrer
motor for stirring at the centre joint followed by an addition funnel
and a leibig condenser in the side joint through a Y bend. The third
joint was kept stoppered and it served the function for addition
or removal of material. The assembly was purged and maintained
under a gentle nitrogen atmosphere throughout by connecting the
outlet to an oil trap.
Transferred under a dry nitrogen atmosphere 250 mL of homog-
enized white catalyst (designated as C-1) slurry in hexane having
10% slurry concentration (g/mL) and 610 mmol/L of Ti into the
above reactor and started the agitation gently (150–200 rpm).
Added 80 mL of a 20% (w/v) AlR3 solution in hexane drops from
the addition funnel under dry nitrogen atmosphere over 45–60 min
after maintaining the reaction mixture at ambient temperature
(∼28 ◦C) to drain away the exothermicity generated during acti-
vation. The pale yellow coloured slurry turns grayish and then
blackish. Continued the reaction for 5 h and transferred the black
catalyst dispersion (designated as C-2) into a 500 mL catalyst stor-
age conical flask with side arm and Teflon stop-cock for nitrogen
atmosphere. Stoppered the flask under nitrogen and preserved in
a nitrogen glove box.
2.3. Characterization of black catalyst batch for UHMWPE
The above catalyst batch was characterized for its slurry concen-
tration and also the Ti oxidation state content by cerimetry on the
basis of the hydrolyzed acid layer (for Ti3+) and subsequently after
reducing an aliquot of the hydrolyzed acid layer by Zn/Hg amal-
gam (for total Ti). The Ti3+ was found to be 25%. The total Ti was
estimated by UV–vis. The Ti2+ was found to be about 10–20 mmol/L.
2.4. Typical polymerization procedure
Polymerizations were carried out in laboratory Buchi reactors
of 0.5 L capacity. The solvent used in the runs is dry distilled under
a nitrogen atmosphere after refluxing it over sodium hydride as
the desiccant. The moisture content was typically around 5–8 ppm.
Ethylene used was of polymerizable grade. Ethylene pressure used
has been varied and was maintained depending on the molecular
weight and product characteristics. AlR3 used was diluted in var-
sol and its concentration was 20% (w/v). The black catalyst slurry
in hexane was homogenized and a suitable amount was trans-
ferred out for a run such that one could have the amount of catalyst
charged in g as well as in terms of mmole of Ti, based on the cata-
lyst slurry concentration (solid content) and the total Ti content in
the catalyst slurry. The molar ratio of the activator and the catalyst
(Ti from catalyst) was maintained around 4–5 for most of the runs.
The same was arrived at after carrying out optimization studies.
The agitation has been standardized around 500 rpm. Temperature
was maintained at 75 ◦C and the duration was 2 h for a run. Hydro-
gen dosing was done through a pre-calibrated bomb hooked to the
reactor for controlling the molecular weight.
The isolated polymer slurry in hexane was treated with
methanolic HCl to destroy any unreacted catalyst and aluminum
alkyl. The polymer was then filtered on a Buckner funnel, 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. The productivity was based on a 2 h period.
Fig. 1. Optimization of Al/Ti ratio in varsol (PC2: 2.5 bar).
Polymer characterization was carried out in the laboratory by
measuring parameters like yield, bulk density (BD), PSD for APS,
amount of fines and coarse material (by standard test sieves using
a mechanical sieve shaker) and reduced specific viscosity (RSV)
at 135 ◦C in decalin as solvent in an Ubbelohde viscometer with
constant = 0.01 by measuring the flow times for solvent and subse-
quently a 0.02% solution of the polymer. The viscosity based average
molecular weight was calculated using Margolie’s equation.
3. Results and discussion
UHMWPE was synthesized using saturated hydrocarbon solvent
as the medium and a typical Ti supported on MgCl2 Ziegler cata-
lyst employing AlR3 (an equal mixture of tri-isobutyl aluminum
and iso-prenyl aluminum) as the activator and hydrogen as the
molecular weight regulator. The use of this catalyst for making
HDPE after activation with TEAL is routine even on a commer-
cial scale. Tailoring this catalyst to produce UHMWPE equivalent
to bench marked grades through process optimization in hex-
ane was already demonstrated in our earlier communications
[18]. Presently we have shown the feasibility of using a mix-
ture of hydrocarbons (commercially called as varsol) as a medium
of polymerization and compared its performance with literature
reported results of hexane based process. The polymerization
studies in different solvents are in alignment with our earlier sol-
ubility studies of the monomer in different hydrocarbon solvents
[20].
To begin with, we have established the optimum polymerization
conditions in varsol and compared the same with hexane based
process. Our earlier studies optimizing the Al/Ti ratio in hexane
were extended to varsol to check for any departure, if any. For a
particular ethylene pressure and catalyst system (containing 25%
Ti3+) the optimum value of Al/Ti was found to be ∼4 under the spec-
ified operating conditions (Fig. 1). During these studies we have also
observed that Al/Ti ratio needs to be optimized when the conditions
are changed. Thus, at an ethylene pressure of about <2 bar we found
that optimum Al/Ti turned out to be 8 where as for ethylene pres-
sure of 7.5 atm we found the Al/Ti ratio is around 4 retaining the
desired polymer characteristics. By operating at a different Al/Ti
values, besides yield, the other polymer properties like bulk den-
sity and average molecular weight also changes, thus providing a
lever to alter the polymer characteristics at the cost of yield.
With a view to generate UHMWPE having desired character-
istics (bulk density (BD), particle size distribution (PSD)/average
particle size distribution (APS), reduced specific viscosity
(RSV)/average molecular weight) ethylene polymerization was
performed with pressures ranging from 2 to 8 atm. We realized
that the productivity was directly related to the ethylene pres-
sure, a phenomenon which is nothing new in the area of olefin
polymerization. Typical catalyst and process conditions yielded a

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134 S. Padmanabhan et al. / Materials Science and Engineering B 168 (2010) 132–135
Table 1
Optimized ethylene polymerization results in varsol medium.a
.
Run Cat (mmol Ti) Al/Ti molar ratio P C2 (atm) P H2 (atm) Yield (g) M␩ (Million g/mol)b
BD (g/mL)c
APS (␮)d
% <63 (␮)d
% >250 (␮)d
1 0.24 5 5 0 150 > 10 0.40 124 17 8
2 0.22 4 5 0.7 140 1.9 0.34 ND ND ND
3 0.22 4 5 0.1 176 5.1 0.34 156 15.6 2.7
4 0.22 4 5 0.1 166 5.3 0.35 149 14.4 1.8
5 0.22 4 7 0.1 225 5.5 0.37 151 20.5 1.6
6 0.11 4 8 0.1 65 8.7 0.30 ND ND ND
7 0.22 2 8 0.1 163 8.7 0.35 177 2.3 10.7
8 0.22 4 2.5 0.7 25 ND ND ND ND ND
9 0.22 4 2.5 0 154 7.8 0.31 164 3.3 3.9
10 0.22 4 2.5 0.1 153 3.8 0.33 159 0.14 6.8
11 0.22 8 2.5 0.1 170 1.5 0.33 156 4.6 2.9
a
General reaction conditions: activator used AlRR 2 (an equal mixture of tri-isobutyl aluminum and iso-prenyl aluminum), 75 ◦
C, 500 rpm and different catalyst concen-
tration, PH2 in 1 L Buchi; ND: not determined.
b
Viscosity based average molecular weight (Million g/mole) calculated using Margolie’s equation [(5.37 × 104
× RSV1.49
)/106
].
c
Bulk density was measured as per standard methods.
d
Analyzed by both Malvern PSA and traditional sieve shaker methods.
productivity of ∼2 ± 0.5 kg of UHMWPE/g of catalyst at 7 atm ethy-
lene pressure over 2 h (Table 1). Nonetheless, besides productivity
the other polymer characteristics could be fine tuned by playing
with the pressure. The polymerization temperature had an effect
on the average molecular weight of UHMWPE, akin to what has
been observed by other groups [9–17]. Polymerizations performed
below 70 ◦C is not economical from the commercial angle since
the reaction rate drops down drastically for even a drop of about
10 ◦C in the temperature.
For regulating the molecular weight of UHMWPE using the spec-
ified hydrogen bomb hooked to the polymerization reactor there
was a threshold limit for hydrogen. This is essentially the thresh-
old or saturation solubility of hydrogen at the specified operating
conditions based on the partial pressure of hydrogen, ethylene and
varsol. It can be observed how effectively the partial pressure of
hydrogen is controlled at two different ethylene pressures, viz. 5
and 7 atm. Obviously as expected the line at 5 atm pressure ethy-
lene controls molecular weight regulation in the higher region
than the 7 atm ethylene pressure again verifying Henry’s law for
the solubility of gases. From Table 1 it can be seen that achiev-
ing an average molecular weight of ∼4.5 million is statistically
more favored at hydrogen pressures 0.1–0.5 atm since the partial
pressure of hydrogen is not lowered down significantly at these
lower hydrogen pressures. Molecular weight control with hydro-
gen pressure 1–3 atm reflects in a linear response with the RSV
progressively dropping down since the partial pressure of hydrogen
now becomes significant.
In case molecular weight control in a still higher region cf. to
5 atm ethylene pressure is required, the approach would be to
operate at still lower ethylene pressures—this would lower the par-
tial pressure of hydrogen thus increasing the molecular weight. In
doing so, the other vantage properties like productivity, BD and APS
might get affected. The option of changing the reactor dimension
in total to achieve this objective under the experimental condi-
tions employed would be the other alternative. Thus the overall
reaction kinetics involving the concentrations of monomer, cata-
lyst and regulator is governed as per the situation coupled with the
reactor configuration. This approach resulted in different grades of
UHMWPE with desired molecular weights of 4–10 million g/mol
having unique and diverse applications, making it a differentiated
polymer.
Use of aromatic solvents was detrimental in synthesizing
UHMWPE using traditional Ziegler-Natta catalysts. The use of
toluene yielded low molecular weight HDPE type polymers with
less productivity. With 0.22 mmol Ti (Al/Ti ratio of 4, PC2 2 bar
with out any hydrogen) the productivity was 62 g/mmol of Ti
with the molecular weight of 0.5 million g/mol. Hence use of pure
aliphatic hydrocarbons (dearomatized samples) as the medium for
this type of polymerizations was imperative. It is well known that
the concentration of ethylene in the solvent of slurry polymeriza-
Fig. 2. SEM images of the UHMWPE produced in different resolutions revealing the particle size and porous nature (a) in varsol medium and (b) hexane medium.

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S. Padmanabhan et al. / Materials Science and Engineering B 168 (2010) 132–135 135
Fig. 3. DSC of the UHMWPE produced (recorded with the heating rate of 10 ◦
C/min
in three cycles) (insets: melting points during (a) first, (b) second, (c) third heating
cycles and freezing points during (d) first, (e) second, and (f) third cooling cycles.
tion process is important as it determines the extent of reaction,
reaction temperature, and the molecular weight of the polymer
produced. In our earlier study, gas liquid behavior of ethylene, in
the presence and absence of hydrogen, was studied in two process
solvents namely, hexane and varsol at various process pressures
and temperatures. Solubility of ethylene increases with increase in
pressure and decreases with increase in temperature in both the
solvents [20]. Ethylene solubility decreases with increase in car-
bon number of solvent under identical conditions. The presence of
hydrogen strongly influences the solubility of ethylene in hexane
and varsol. The solubility of ethylene in hexane decreases in the
presence of hydrogen compared to its binary solubility, while the
presence of hydrogen increases the solubility of ethylene in varsol
compared to its binary solubility. Thus it was obvious that use of
varsol as a polymerization medium to produce UHMWPE was pre-
ferred compared to hexane [18,19]. Varsol was having the desired
kinetics profile providing the leverage for controlling molecular
weight profiles at designated hydrogen dosage (Table 1). The exper-
imental observations in Table 1 also support the solubility pattern.
Different batches of UHMWPE synthesized exhibit comparable
SEM confirming consistent quality of the polymer obtained in dif-
ferent grades synthesized. The polymer obtained was of porous
nature as seen from the SEM images (Fig. 2).
From DSC studies the initial Tm was found to be in the range of
143–144 ◦C and during the second and third heating cycles it got
shifted to 133–134 ◦C which is the typical pattern for UHMWPE.
The crystallization temperature was found to be 121 ◦C for the first
cooling which got shifted to 119–120 ◦C during the second and
third cooling also supported the formation of UHMWPE (Fig. 3). The
melting temperature during the first cycle was higher compared to
that of the second and third indicating the change in morphology
from nascent to melt crystallized form. The polyethylene formed
in a solvent during the process precipitates well below the melting
temperature and hence its nascent morphology was considerably
influenced by the polymerization processes. Depending on the
nature of the catalyst, cocatalyst and other polymerization condi-
tions, variety of morphologies have been reported for the polymer
produced from Ziegler-Natta catalysts. The nascent polyethylene
having a uniform morphology upon heating and cooling during the
first cycle of DSC measurement undergoes a change and forms a
melt crystallized sample. The nascent UHMWPE crystals have a
higher melting point than the melt crystallized samples [21,22].
From the DSC studies pertaining to the nature of UHMWPE for
its extent of entanglement as reported by Rastogi et al. [23–25]
revealed that these materials are highly entangled.
4. Summary
To summarize we have tailored the catalyst, activator and
polymerization conditions to synthesize UHMWPE of diverse char-
acteristics classifying it as a differentiated polymer. The solvent
change from hexane to varsol for the UHMWPE synthesis is very
logical based on our solubility and experimental data. The polymers
obtained have uniform morphology and are porous in nature. From
thermal analysis it is seen that the polymer produced was of highly
entangled nature having capability to function as a differentiated
material.
Acknowledgements
We thank Mr. Viralkumar Patel for his technical and analyti-
cal assistance throughout the course of the work. We also sincerely
thank Dr. A.B. Mathur and Dr. R.V. Jasra for their continuous encour-
agement to carry out this work.
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