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Materials Science and Engineering B 168 (2010) 132–135

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Materials Science and Engineering B

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Synthesis of ultra high molecular weight polyethylene: A differentiate materialfor specialty applications

Sudhakar Padmanabhan ∗, Krishna R. Sarma, Kishor Rupak, Shashikant SharmaResearch 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 2009Received in revised form 15 October 2009Accepted 16 October 2009

Keywords:UHMWPEMg-Ti catalystParticle size distributionBulk densityAverage particle sizeEntanglementMorphology

a b s t r a c t

Tailoring the synthesis of a suitable Ziegler-Natta (ZN) catalyst coupled with optimized polymerizationconditions using a suitable activator holds the key for an array of differentiated polymers with diverseand unique properties. Ultra high molecular weight polyethylene (UHMWPE) is one such polymer whichwe have synthesized using TiCl4 anchored on MgCl2 as the support and activated using AlRR′

2 (whereR, R′ = iso-prenyl or isobutyl) under specific conditions. Here in we have accomplished a process forsynthesizing 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 amountof fine and coarse particles. Scanning electron micrographs (SEM) reflected the material to have uniformparticle size distribution with a spherical morphology. The extent of entanglement was determined fromthermal 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 differentorganic frameworks and metals [1]. The new generation metal-locene and non-metallocene based catalysts in combination withaluminum alkyls or borates have dominated the area of olefin poly-merization over two–three decades [2,3]. The fine tuning of thecatalysts was mainly stressed for generating the stereo regular-ity in the polyolefin synthesized and also for making a variety ofcopolymers, as the environment of the metal center determines thesame [4,5]. The novel grades of polymers which can be synthesizedfrom these new generation catalysts hold higher price material orrather specialty materials. In the commodity grade polymers likeHDPE, commercial plants are still highly dependent on traditionalheterogeneous 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 highmolecular weight polyethylene (UHMWPE) as most of the poly-mers are produced from the Hostelen’s stirred tank process usingtraditional ZN catalysts and alkyl aluminums [9–19]. The differencebetween HDPE and UHMWPE processes hovers around optimizedprocess 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 catalyststransform to UHMWPE is the nature of the activator coupled withthe extent of trivalent titanium present in the system. The poly-merization media governs the monomer and hydrogen solubility inisolation or in conjunction leading to the desired molecular weightcharacteristics which is more significant for UHMWPE. To makethis entire process of controlling the hydrogen and ethylene dosagesimpler, we have already demonstrated by tuning the catalystdosing with fixed ethylene and hydrogen pressure to synthesizeUHMPWE of required molecular weights [18]. In the present study,we have explored the feasibility of using varsol as the medium forthe polymerization of ethylene using similar catalyst systems. Wehave studied the binary solubility of ethylene and hydrogen in var-sol and found that the solubility of ethylene increased unlike inhexane [20]. In view of this it is obvious that the monomer sol-ubility holds the key to the overall polymerization kinetics. Wehave proved the same concept through polymerization studies bycapturing the difference in polymerization behavior in hexane andvarsol.

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 nitrogenbefore an experiment. All manipulations like handling and transfer

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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 anitrogen glove bag as far as possible.

2.2. Synthesis of a typical black catalyst

A 2-L double jacketed glass reactor vessel having three standardjoints at the top and with provision for water circulation was assem-bled after pre-heating same in the oven followed by cooling underdry nitrogen. The same was equipped with a variable speed stirrermotor for stirring at the centre joint followed by an addition funneland a leibig condenser in the side joint through a Y bend. The thirdjoint was kept stoppered and it served the function for additionor removal of material. The assembly was purged and maintainedunder a gentle nitrogen atmosphere throughout by connecting theoutlet to an oil trap.

Transferred under a dry nitrogen atmosphere 250 mL of homog-enized white catalyst (designated as C-1) slurry in hexane having10% slurry concentration (g/mL) and 610 mmol/L of Ti into theabove reactor and started the agitation gently (150–200 rpm).Added 80 mL of a 20% (w/v) AlR3 solution in hexane drops fromthe addition funnel under dry nitrogen atmosphere over 45–60 minafter 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 thenblackish. Continued the reaction for 5 h and transferred the blackcatalyst dispersion (designated as C-2) into a 500 mL catalyst stor-age conical flask with side arm and Teflon stop-cock for nitrogenatmosphere. Stoppered the flask under nitrogen and preserved ina 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 thebasis of the hydrolyzed acid layer (for Ti3+) and subsequently afterreducing 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 wasestimated 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 reactorsof 0.5 L capacity. The solvent used in the runs is dry distilled undera nitrogen atmosphere after refluxing it over sodium hydride asthe desiccant. The moisture content was typically around 5–8 ppm.Ethylene used was of polymerizable grade. Ethylene pressure usedhas been varied and was maintained depending on the molecularweight and product characteristics. AlR3 used was diluted in var-sol and its concentration was 20% (w/v). The black catalyst slurryin hexane was homogenized and a suitable amount was trans-ferred out for a run such that one could have the amount of catalystcharged 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 inthe 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. Temperaturewas 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 thereactor for controlling the molecular weight.

The isolated polymer slurry in hexane was treated withmethanolic HCl to destroy any unreacted catalyst and aluminumalkyl. The polymer was then filtered on a Buckner funnel, washedwith acetone and then dried in an air oven at about 75 ◦C. Theweight of polymer was recorded to calculate the productivity ofthe 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 bymeasuring parameters like yield, bulk density (BD), PSD for APS,amount of fines and coarse material (by standard test sieves usinga mechanical sieve shaker) and reduced specific viscosity (RSV)at 135 ◦C in decalin as solvent in an Ubbelohde viscometer withconstant = 0.01 by measuring the flow times for solvent and subse-quently a 0.02% solution of the polymer. The viscosity based averagemolecular weight was calculated using Margolie’s equation.

3. Results and discussion

UHMWPE was synthesized using saturated hydrocarbon solventas the medium and a typical Ti supported on MgCl2 Ziegler cata-lyst employing AlR3 (an equal mixture of tri-isobutyl aluminumand iso-prenyl aluminum) as the activator and hydrogen as themolecular weight regulator. The use of this catalyst for makingHDPE after activation with TEAL is routine even on a commer-cial scale. Tailoring this catalyst to produce UHMWPE equivalentto 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 mediumof polymerization and compared its performance with literaturereported results of hexane based process. The polymerizationstudies 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 polymerizationconditions in varsol and compared the same with hexane basedprocess. Our earlier studies optimizing the Al/Ti ratio in hexanewere extended to varsol to check for any departure, if any. For aparticular 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 alsoobserved that Al/Ti ratio needs to be optimized when the conditionsare changed. Thus, at an ethylene pressure of about <2 bar we foundthat 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 thedesired polymer characteristics. By operating at a different Al/Tivalues, besides yield, the other polymer properties like bulk den-sity and average molecular weight also changes, thus providing alever 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)/averageparticle size distribution (APS), reduced specific viscosity(RSV)/average molecular weight) ethylene polymerization wasperformed with pressures ranging from 2 to 8 atm. We realizedthat the productivity was directly related to the ethylene pres-sure, a phenomenon which is nothing new in the area of olefinpolymerization. Typical catalyst and process conditions yielded a

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Table 1Optimized 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 82 0.22 4 5 0.7 140 1.9 0.34 ND ND ND3 0.22 4 5 0.1 176 5.1 0.34 156 15.6 2.74 0.22 4 5 0.1 166 5.3 0.35 149 14.4 1.85 0.22 4 7 0.1 225 5.5 0.37 151 20.5 1.66 0.11 4 8 0.1 65 8.7 0.30 ND ND ND7 0.22 2 8 0.1 163 8.7 0.35 177 2.3 10.78 0.22 4 2.5 0.7 25 ND ND ND ND ND9 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 6.811 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 productivitythe other polymer characteristics could be fine tuned by playingwith the pressure. The polymerization temperature had an effecton the average molecular weight of UHMWPE, akin to what hasbeen observed by other groups [9–17]. Polymerizations performedbelow 70 ◦C is not economical from the commercial angle sincethe reaction rate drops down drastically for even a drop of about10 ◦C in the temperature.

For regulating the molecular weight of UHMWPE using the spec-ified hydrogen bomb hooked to the polymerization reactor therewas a threshold limit for hydrogen. This is essentially the thresh-old or saturation solubility of hydrogen at the specified operatingconditions based on the partial pressure of hydrogen, ethylene andvarsol. It can be observed how effectively the partial pressure ofhydrogen is controlled at two different ethylene pressures, viz. 5and 7 atm. Obviously as expected the line at 5 atm pressure ethy-lene controls molecular weight regulation in the higher regionthan the 7 atm ethylene pressure again verifying Henry’s law forthe solubility of gases. From Table 1 it can be seen that achiev-ing an average molecular weight of ∼4.5 million is statisticallymore favored at hydrogen pressures 0.1–0.5 atm since the partialpressure of hydrogen is not lowered down significantly at theselower 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 hydrogennow becomes significant.

In case molecular weight control in a still higher region cf. to5 atm ethylene pressure is required, the approach would be tooperate at still lower ethylene pressures—this would lower the par-tial pressure of hydrogen thus increasing the molecular weight. Indoing so, the other vantage properties like productivity, BD and APSmight get affected. The option of changing the reactor dimensionin total to achieve this objective under the experimental condi-tions employed would be the other alternative. Thus the overallreaction kinetics involving the concentrations of monomer, cata-lyst and regulator is governed as per the situation coupled with thereactor configuration. This approach resulted in different grades ofUHMWPE with desired molecular weights of 4–10 million g/molhaving unique and diverse applications, making it a differentiatedpolymer.

Use of aromatic solvents was detrimental in synthesizingUHMWPE using traditional Ziegler-Natta catalysts. The use oftoluene yielded low molecular weight HDPE type polymers withless productivity. With 0.22 mmol Ti (Al/Ti ratio of 4, PC2 2 barwith out any hydrogen) the productivity was 62 g/mmol of Tiwith the molecular weight of 0.5 million g/mol. Hence use of purealiphatic hydrocarbons (dearomatized samples) as the medium forthis type of polymerizations was imperative. It is well known thatthe 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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Fig. 3. DSC of the UHMWPE produced (recorded with the heating rate of 10 ◦C/minin three cycles) (insets: melting points during (a) first, (b) second, (c) third heatingcycles 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 polymerproduced. In our earlier study, gas liquid behavior of ethylene, inthe presence and absence of hydrogen, was studied in two processsolvents namely, hexane and varsol at various process pressuresand temperatures. Solubility of ethylene increases with increase inpressure and decreases with increase in temperature in both thesolvents [20]. Ethylene solubility decreases with increase in car-bon number of solvent under identical conditions. The presence ofhydrogen strongly influences the solubility of ethylene in hexaneand varsol. The solubility of ethylene in hexane decreases in thepresence of hydrogen compared to its binary solubility, while thepresence of hydrogen increases the solubility of ethylene in varsolcompared to its binary solubility. Thus it was obvious that use ofvarsol as a polymerization medium to produce UHMWPE was pre-ferred compared to hexane [18,19]. Varsol was having the desiredkinetics profile providing the leverage for controlling molecularweight 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 comparableSEM confirming consistent quality of the polymer obtained in dif-ferent grades synthesized. The polymer obtained was of porousnature as seen from the SEM images (Fig. 2).

From DSC studies the initial Tm was found to be in the range of143–144 ◦C and during the second and third heating cycles it gotshifted to 133–134 ◦C which is the typical pattern for UHMWPE.The crystallization temperature was found to be 121 ◦C for the firstcooling which got shifted to 119–120 ◦C during the second andthird cooling also supported the formation of UHMWPE (Fig. 3). Themelting temperature during the first cycle was higher compared tothat of the second and third indicating the change in morphology

from nascent to melt crystallized form. The polyethylene formedin a solvent during the process precipitates well below the meltingtemperature and hence its nascent morphology was considerablyinfluenced by the polymerization processes. Depending on thenature of the catalyst, cocatalyst and other polymerization condi-tions, variety of morphologies have been reported for the polymerproduced from Ziegler-Natta catalysts. The nascent polyethylenehaving a uniform morphology upon heating and cooling during thefirst cycle of DSC measurement undergoes a change and forms amelt crystallized sample. The nascent UHMWPE crystals have ahigher melting point than the melt crystallized samples [21,22].From the DSC studies pertaining to the nature of UHMWPE forits 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 andpolymerization conditions to synthesize UHMWPE of diverse char-acteristics classifying it as a differentiated polymer. The solventchange from hexane to varsol for the UHMWPE synthesis is verylogical based on our solubility and experimental data. The polymersobtained have uniform morphology and are porous in nature. Fromthermal analysis it is seen that the polymer produced was of highlyentangled nature having capability to function as a differentiatedmaterial.

Acknowledgements

We thank Mr. Viralkumar Patel for his technical and analyti-cal assistance throughout the course of the work. We also sincerelythank Dr. A.B. Mathur and Dr. R.V. Jasra for their continuous encour-agement to carry out this work.

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