effects of the catalyst system on the crystallization of polypropylene*

Effects of the Catalyst System on the Crystallizationof Polypropylene*

MARKUS GAHLEITNER,1 CLAUDIA BACHNER,2 EWA RATAJSKI,2 GERALD ROHACZEK,1 WOLFGANG NEIßL1

1 Borealis AG, St. Peterstr. 25, A-4021 Linz, Austria

2 Johannes Kepler University Linz, Institute of Chemistry, Altenbergerstr. 69, A-4040 Linz, Austria

Received 2 July 1998; accepted 20 January 1999

ABSTRACT: In search of a better understanding of catalyst effects on final productqualities of polypropylene, an attempt was made to establish a correlation betweencatalyst type, polymer chain structure of homopolymers, crystallization behavior, aswell as final morphology and mechanical properties. Conventional Ziegler–Natta cat-alyst systems as well as novel metallocene catalysts were investigated, and influencesof molar mass distribution and chain regularity were investigated as separate factors.Metallocene-based isotactic polypropylene shows some special effects regarding thenucleation density and the correlation between stereoregularity and mechanics that donot fit into the general picture for Ziegler–Natta catalyst based products. © 1999 JohnWiley & Sons, Inc. J Appl Polym Sci 73: 2507–2515, 1999

Key words: polypropylene; crystallization; metallocene catalyst; molecular structure

INTRODUCTION

Since 40 years ago, when polypropylene (PP) wasfirst put on the market, it has undergone a rapidand successful development, which will make itthe most used thermoplastic material before theend of this decade. The wide application range ofPP results from its flexibility and the variety ofpossible modifications of the basic material,which already starts in the polymerization re-actor.

Tailoring polymer properties in the polymer-ization stage requires a profound insight into thecorrelation chain from catalyst via the polymerstructure to crystallization in processing and thefinal product properties.1 Being a polymer with

stereospecific chain structure (in contrast to poly-ethylene), it offers various structural possibilitiesalready in the case of a homopolymer,2 not tomention the wide range of copolymerization pos-sibilities or the area of postreactor modification.

THEORETICAL BACKGROUND

The modification of PP in the polymerization stepcan in principle be done by regulating the molarmass (normally by feeding hydrogen as a termi-nation agent into the process), by copolymeriza-tion with ethylene and/or other higher a-olefins,and by changing the catalyst system. Subse-quently, postpolymerization modification is possi-ble via degradation,3 compounding with fillersand reinforcements,4 grafting with reactive mono-mers, and blending with other polymers5 to arriveat a wide range of various material properties.

Catalyst Systems and Polymer Structure

A conventional state-of-the-art catalyst system forisotactic PP of the Ziegler–Natta type (ZN) basically

* Dedicated to Prof. Manfred Ratzsch on the occasion of his65th birthday.

Correspondence to: M. Gahleitner.Contract grant sponsor: Austrian Forschungsforderungs-

fonds fur die gewerbliche Wirtschaft; contract grant number:5/570/809.Journal of Applied Polymer Science, Vol. 73, 2507–2515 (1999)© 1999 John Wiley & Sons, Inc. CCC 0021-8995/99/122507-09

2507

consists of a supported catalyst (Ti-chloride onMgCl2) with an internal electron donor (Lewisbase), an external donor (normally an organosilan),and a cocatalyst (Al-alkyl or Al-halogen-alkyl),which also acts as a scavenger to improve processstability; for a general treatment of such catalystsystems, see, e.g., the review by Albizzatti et al.6

This way, even with one catalyst, product variationis possible by donor selection and adjusting the ratiobetween monomer, catalyst, donor, and cocatalystquantity. The tacticity of the PP chain can be variedwithin a wide range, thus influencing crystallinityand mechanics of the product.7 However, the molarmass distribution (MMD) is always rather wide(Mw/Mn $ 5) in case of ZN products and narrowMMD types are only accessible via degradation(controlled rheology process3).

The novel generation of metallocene (MC) cata-lysts8 are generally available in a wider structuralvariety and are additionally influenced by the sup-porting step as well as type and amount of cocata-lyst (e.g., methyl-alumoxan (MAO)). For the poly-merization of propylene, this means that besidesisotactic PP, also atactic, syndiotactic, and other,more complex stereospecific structures become ac-cessible.9 In contrast to ZN catalysts, MC catalystsproduce polymers with a more homogenous struc-ture (narrow MMD, constant tacticity, purely ran-dom insertion of comonomers), resulting from thedifferent nature of catalytic sites: While ZN cata-lysts normally show a variety of different activesites on one type of catalyst, this is restricted to onetype of site in case of MC catalysts (single-site type).On the other hand, MC catalysts produce chaindefects resulting from head–head or tail–tail link-age of monomer units that do not occur in the caseof ZN catalysts because of sterical restrictions. Thisleads to a different behavior in processing and finalapplication of these polymers.

Crystalline Morphology and Mechanics ofPolypropylene

Solid polypropylene consists of three phases10,11

(see also ref. 12; cf. ref. 13):

● crystalline (a monoclinic; b, hexagonal; or g,centered orthorhombic modification),

● mesomorphic (also termed “smectic” or“paracrystalline,” consisting of small a- andb-crystallites without superstructure), and

● amorphous (between and within crystallineregions, strongly influenced by those).

Generally, the mechanical properties of semi-crystalline polymers like PP are significantly in-fluenced by their crystalline structure. Apartfrom the overall crystallinity, the morphology,e.g., the dimension of spherulites or layered struc-tures developed by shear-induced crystallization,also plays an important role here. These struc-tures are in turn influenced by the molecularstructure of the material on one hand and thesolidification (or processing) conditions on theother hand.

In the past, effects of MMD and chain structureon the crystallization behavior and product prop-erties have been mainly studied for the case ofZN-PP,2,14–19 because until recently these werethe only products of technical and commercialrelevance. Here, stiffness20 as well as toughness21

effects have been observed. However, with therise of the “metallocene revolution,” polypro-pylenes produced with this new family of cata-lysts have gained substantial attention regardingtheir behavior in crystallization22–24 as well asactual processing.25 The wider structural varietyof the polymers mentioned before makes the crys-tallinity and morphology of such systems a veryinteresting field of academic work with importantimplications for actual product development.26–30

Earlier studies of our group2,17 were focused onmelt flow rate (MFR) MMD, and tacticity effectsfor ZN-PP. The separation of nucleation andgrowth effects proved essential and offered a bigadvantage over the “standardized” crystallizationexperiments like differential scanning calorime-try (DSC)-based Avrami plots. Through this, wecould conclude that the first of these processes isdominated by the MMD, while the latter ismainly controlled by the stereoregularity of thechain. Additionally, some interesting features ofshear-induced structure formation31,32 were dis-covered. The work presented here was carried outto gain an overview about the possible structuralvariations of isotactic PP (iPP) with technicallyrelevant catalyst systems as well as the conse-quences of these changes in the structure. Specialattention was paid to the positioning of MC-PP inrelation to “conventional” products.

EXPERIMENTAL

Investigated Materials

Fourteen different propylene-homopolymers wereincluded in the study, the basic characteristics ofwhich have been summarized in Table I:

2508 GAHLEITNER ET AL.

● 5 Reactor-iPPs (REPPs) with different aver-age molar mass (MFR) and standard tac-ticity.

● 3 “Controlled rheology”-iPPs (CRPPs) pro-duced from a standard tacticity reactor gradewith MFR (230°C/2,16 kg) of 0.5 g/10 min ina twin screw extruder at 210°C with appro-priate amounts of peroxide.

● One ZN-based highly amorphous PP with re-duced tacticity (HAPPs).

● 4 MC-iPPs (MCPPs) with different averagemolar mass (MFR) and based on two differ-ent catalysts.

● 3 “High-crystallinity”-reactor-iPPs (HCPPs)with different average molar mass (MFR)and increased tacticity.

For the ZN-iPPs, fourth-generation commer-cial catalysts were used. The catalyst system(mainly the external donor) was identical for theREPP and CRPP grades, but different in case ofthe HCPP grades and the HAPP. For the MC-iPPs, two different types of experimental metal-locene catalysts in combination with MAO as co-catalyst and supported on silica were applied.

All products were produced in a commercial orpilot scale plant at Borealis AG, Schwechat, Aus-tria. In all cases, liquid-pool polymerization wasapplied.

Experimental Techniques

The MMD of all investigated products was deter-mined using gas permeable chromatography (Wa-ters 150 C, at 135°C in trichlorobenzene). Table Ialso gives the MFR values (International Stan-dards Organization 1133, 230°C/2,16 kg), whichshow a rough correlation to the weight average ofthe MMD, Mw. Additional information about thechain structure was gained from IR spectra,where a method developed by CHISSO33 wasused to determine the tacticity (ratio between 998and 973 cm21 bands of the spectrum determinedon specially compression-molded film; propor-tional to the isotactic decade ratio). Melting be-havior and overall crystallinity were determinedin DSC (TA 512 C, DIN 53765) in the second heatof a heat/cool/heat cycle between 123 and 1250°Cat 10 K/min. The overall crystallinity was calcu-lated from the melting enthalpy using the for-mula

Table I Basic Characteristics of Investigated Polymers; MFR, Crystallinity Data from DSC,Stereoregularity from IR Spectroscopy, Morphology from Polarizing Light Microscopy,Mechanical Properties Determined on Injection-Molded Specimens at 123°C

MaterialMFRa

g/10 minTm

(°C)Hm

(J/g)Tc

(°C) Tacticityb

SkinThickness

(mm)

Avg.Spher.

Size (mm)

Flex.Modulus

(MPa)

NotchedImpactc

(kJ/m2)

REPP 1 3.2 162 110 113 0.969 20 ; 40 1470 4REPP 2 28 162 113 115 0.973 10 ; 20 1630 1.9REPP 3 8.4 163 111 115 0.971 14 ; 50 1600 3.4REPP 4 150 162 114 115 0.962 ; 1 ; 10 1890 1.3REPP 5 300 162 114 116 0.965 0 nd 1880 1.2CRPP 1 8 165 101 110 0.952 ; 2 ; 65 1300 3.4CRPP 2 20 162 102 109 0.966 0 ; 30 1300 1.9CRPP 3 8.6 164 101 110 0.959 ; 1 nd 1210 3.9HAPP 0.5 157 55 109 0.565 ndd nd 280 ndMCPP 1 28 154 100 110 0.897 2 ; 10 1360 2.3MCPP 2 3.5 156 101 112 0.914 10 ; 25 1350 3.5MCPP 3 21 149 89 107 0.921 4 ; 15 1385 2.0MCPP 4 21 151 99 109 0.942 5 nd 1425 2.0HCPP 1 8 165 115 114 0.978 2 nd 1640 2.7HCPP 2 8 166 117 116 0.983 3 ; 60 1790 2.7HCPP 3 8.4 166 117 115 0.981 4 nd 1750 3.3

a 230°C/2, 16 kg.b Chisso method.c International Standards Organization 179 1 eA.d nd: Not determined.

EFFECTS OF THE CATALYST SYSTEM 2509

XD 5 Hm/Hp

where Hp, the enthalpy of a totally crystalline PP,was assumed to be 209 J/g.10,12 The reason forchoosing this value rather than the 165 J/g pub-lished before by Wunderlich13 was the better ac-cordance of the resulting crystallinity level tocrystallinities calculated from density and wideangle X-ray scattering (WAXS) measurements,which was experienced in our group in severalearlier studies.

In most of the papers on polymer crystalliza-tion, Avrami equations are used to describe thecrystallization process.34 This, however, does notreally reflect the reality in polymer solidification,where one can find a significant difference in tem-perature dependence between the formation ofnuclei and the subsequent growth of crystallites.Like in our previous work,17 the number of nucleiand the spherulitic growth rate were determinedseparately in special techniques at Linz Univer-sity. For the nucleation density, again mostlyDSC was used, but in some cases this proved notto be successful because of the high number ofnuclei. For these samples, a special setup wasbuilt to allow rapid quenching from the melt tothe desired crystallization temperature (a paperdescribing this method more precisely is beingprepared). To determine the spherulitic growthrate, the same procedure as before35 was applied.

As the previous literature points to a certaininfluence of the polymer type on the developmentof special crystal modifications27,36 four samples(REPP2, CRPP2, MCPP1, and HCPP2) were ad-ditionally characterized in WAXS on compres-sion-molded plates of 2 mm thickness. The mainpurpose was to quantify overall crystallinity andthe relative content of different crystal modifica-tions. The samples were investigated in transmis-sion on a Siemens D500 diffractometer, recordingthe wide angle scatter in the region of 5° # 2u #45°. After the usual corrections, the curves weresplit up into their crystalline and amorphouspart, using the reference curve of an atacticpolypropylene (aPP) sample as amorphous mas-ter curve.

The crystallinity (XC) was determined from thepeak areas in the region of 10° # 2u # 30°; theconcentrations of the b and g modifications wereexpressed by the Turner–Jones index B and the gconcentration parameter G:

B 5Ib2300

I1101Ib23001I0401I130

G 5Ig2130

I1301Ig2130

All mechanical properties—as listed in TableI—were determined on injection-molded speci-mens of 80 3 10 3 4 mm according to standardprocedures (3-point flexural test — ISO 178,Charpy notched impact test — ISO 179 1eA) at123°C.

The actual morphology of all materials waschecked directly on cross-sections microtomed outof the injection-molded samples in polarizinglight microscopy as in earlier studies.17 Apartfrom a general judgment of the molding quality(homogeneity, absence of flow lines), this alloweda determination of the highly oriented and bire-fringent skin layer32 and a rough estimation ofthe spherulite size in the nonoriented core (seeTable I). No full statistical evaluation was madefor the latter quantity, as especially for the finermorphologies (high MFR values) the borders be-tween different spherulites were increasingly dif-ficult to distinguish.

RESULTS AND DISCUSSION

Crystallization and Crystallinity

Structural effects on the melting point appearonly in case of HAPP and MCPP; smaller varia-tions in the tacticity do not seem to influence Tmsignificantly (see Table I). In previous internalstudies a generally good correlation between Tmand the average isotactic length in the polymerchain as determined via 13C-NMR could be found.A stronger variation is seen in the overall crystal-linity XD as calculated from the melting enthalpy(see Table II), where a rough classification can bemade as follows:

HCPP.REPP.CRPP.MCPP..HAPP

The possibility of relating these values to finalmechanical properties is limited, however, as theactual structure of the injection-molded parts iscomplex and also determines the performance to agreat extent.

Even more differences appear if the parame-ters for quiescent crystallization are investigatedseparately (for a summary of the relevant param-eters, see Table II). In case of the nucleation den-sity, a previous study comparing reactor and CR


grades based on ZN catalysts2 showed the widthof the MMD to be the defining factor. Therefore itwas of interest to see whether metallocene-basediPPs with their inherently narrow MMD fit intothis picture. It turned out that this is not the case;as Figure 1 shows, the number of nuclei per unitvolume is rather in the same range as for a ZN-based reactor grade or even higher. A reason forthis effect may be the higher ash content (catalyst

residues) in some cases as compared to ZNgrades: While for the ZN-based products theamount of ash (catalyst residues; none of the sam-ples contained incombustible additives like SiO2)is in the range of 150–250 ppm, for MCPPs anamount of 300–500 ppm is found. Additionally,the temperature dependency in the investigatedrange is quite the same. Apart from CR grades, asignificantly reduced number of nuclei could onlybe found for the HAPP sample.

The second parameter for quiescent crystalli-zation—namely, the spherulithic growth rate—has been found to be dominated by the isotacticityof the chain for PP-homopolymers in previousstudies.17,37 In the present work, a significantlywider range of chain tacticity was investigated,resulting in a variation of the growth speed ofnearly one decade (see Fig. 2). Metallocene-basediPPs fit into the general picture in this case; theirgrowth rates are slightly lower than ZN-basedgrades with standard isotacticity. In fact, a rathergeneral correlation appears possible between thetacticity (IT) and the logarithm of the growth rateG9 at a given temperature (110°C in the case ofFig. 3).

If products with similar stereoregularity(REPP and CRPP) are investigated separately, anadditional molar mass effect becomes obvious (seeFig. 4). The effect, however, is much less pro-nounced!

Table II Quiescent Crystallization Characteristics and Overall Crystallinity of InvestigatedPolymers; Molar Mass Distribution Data from GPC, Number of Nuclei (N), and Spherulithic GrowthRate G* from Special Measurements (a at 110°C), Overall Crystallinity XD from DSC Results

MaterialMw

(kg/mol)Mw/Mn

(kg/mol)Na

(1/m3)G9a

(m/s)XD

(%)

REPP 1 426 5.5 4.8E 1 13 1.8E 2 06 52.6REPP 2 222 4.5 5.9E 1 13 1.9E 2 06 54.1REPP 3 357 5.0 5.0E 1 13 2.0E 2 06 53.1REPP 4 161 7.4 1.3E 1 14 3.3E 2 06 54.5REPP 5 129 7.4 6.1E 1 13 3.7E 2 06 54.5CRPP 1 353 3.2 3.2E 1 12 1.7E 2 06 48.3CRPP 2 231 3.0 6.1E 1 12 1.8E 2 06 48.8CRPP 3 318 3.1 2.5E 1 12 2.1E 2 06 48.3HAPP 1168 9.7 8.5E 1 12 8.2E 2 07 26.3MCPP 1 200 2.3 8.0E 1 13 1.1E 2 06 47.8MCPP 2 403 3.0 3.1E 1 13 1.2E 2 06 48.3MCPP 3 219 2.5 3.5E 1 13 1.1E 2 06 42.6MCPP 4 224 3.3 2.8E 1 13 1.5E 2 06 47.4HCPP 1 384 5.3 2.2E 1 13 2.2E 2 06 55.0HCPP 2 409 5.1 3.5E 1 13 3.1E 2 06 56.0HCPP 3 384 5.1 2.0E 1 13 2.9E 2 06 56.0

Figure 1 Temperature dependence of nucleation den-sity under quiescent conditions; (E) REPP1 (ZN reactorgrade, MFR 3,2), (1) MCPP2 (MC reactor grade, MFR3,5), (l) CRPP1 (ZN CR grade, MFR 8).


Another significant difference was found in theWAXS investigations concerning polymorphism11

(see Table III). While the ZN-based reactor grade,even in the complete absence of an external b-nu-cleating agent shows a detectable amount of bmodification, the development of the g modifica-tion can be observed in case of the metallocene-based grade and only the a modification is found

for the CRPP and HCPP samples. In the litera-ture, several possible reasons for the developmentof g modification are given: special processingconditions like the application of extremely highpressure36 or high shear rates38,39 on one handand special structure of the polymer on the otherhand. For the latter case, a significant g contentwas found for low molar mass materials,11 ran-dom copolymers with ethylene or low isotacticityproducts40 and metallocene-based products.24,27,28

In all cases, disturbances of the chain whethercaused by a higher content of chain ends (lowmolar mass), comonomer (random copolymers), orstereoirregularities seem to favor this modifica-tion. Interestingly enough, this is parallel to anegative effect on the spherulitic growth rate [in-vestigations on random copolymers with ethylene(C2) show similar effects here; results of respec-

Figure 2 Temperature dependence of spheruliticgrowth rate under quiescent conditions; (F) HCPP1(ZN high cryst. reactor grade, MFR 8), (h) CRPP1 (ZNCR grade, MFR 8), (1) MCPP1 (MC reactor grade,MFR 28), (l) HAPP (ZN low cryst. reactor grade, MFR0,5).

Figure 3 Correlation between stereoregularity andspherulithic growth rate at 110°C: (l) ZN grades (ex-cept HAPP); (h) MC grades.

Figure 4 Correlation between weight average molarmass and spherulithic growth rate at 110°C for stan-dard stereoregularity ZN-grades (REPP and CRPP).

Table III Results of WAXS Investigation onCompression-Molded Plates

MaterialMFRa

(g/10 min)Xc Totalb

(%) Betac Gammad

REPP 2 28 42.7 0.05 0CRPP 2 20 41.3 0 0MCPP 1 28 40.6 0 0.36HCPP 2 8 43.5 0 0

a 230°C/2, 16 kg.b Total crystallinity.c Turner–Jones index.d Gamma concentration index.


tive studies at Linz University have not beenpublished yet]. Therefore we conclude that a re-tarded growth of the a modification favors a de-velopment of the g modification.

Another point worth discussion is the compar-atively low melting point for MCPPs, which is,again, rather similar to the behavior of random-C2-copolymers. Obviously, the chain defects re-sulting from head–head or tail–tail linkage ofmonomer units result in a stronger melting pointdepression than the tacticity irregularities nor-mally present in products based on Ziegler–Nattacatalysts. At the same time, the melting ofMCPPs occurs in a more narrow temperaturerange as seen from DSC scans, resulting from themore homogeneous structure of the molecules andexplaining the better relation between meltingand heat deflection temperature.

Processing and Mechanics

Especially in the case of injection molding, theprocessability of metallocene-based iPPs wasfound to be only slightly different from conven-tional products.22–25 For rheological reasons, themold-filling behavior is closer to CR grades. In ourstudy, no negative effect of the slightly reducedgrowth rate could be observed, while problemswere encountered in case of the extremely slowcrystallization of HAPP, which led to problems indemolding (samples were not solid enough formechanical demolding after the standard cycletime and had to be taken out by hand). Investi-gations of the morphology of the injection moldedsamples did not yield any surprising facts: Thedifference in skin layer thickness at comparableMFR between REPP and CRPP samples was al-ready known from the literature31 and own inves-tigations.17 The same applies for the core mor-phology (spherulite size), where CRPP and alsoHCPP exhibit a more coarse structure; in thelatter case, this can be explained from the highergrowth rates at comparable nucleation density.MCPP samples structurally resemble the REPPsamples more with the skin layer being in thick-ness between REPP and CRPP as a consequenceof structure determination by rheological andcrystallization effects.

Regarding the mechanical properties, it ap-pears that the stiffness is roughly correlated tostereoregularity (see Fig. 5), but only for the caseof ZN-based products (in contrast to the correla-tion between stereoregularity and overall crystal-linity, where MCPPs just sow a somewhat higher

scatter). With respect to the regularity parame-ter, the metallocene-based iPPs have a signifi-cantly higher modulus than the rest, giving asecond possible explanation for the different rela-tion between melting point (or, in case of process-ing, sealing initiation temperature) and stiffnessor heat deflection temperature frequently statedin the literature.8 As mentioned before, this isalso affected by the difference in the distributionof irregularities along the chain for productsbased on different catalysts.

No such deviation was found in case of thetoughness, where the average molar mass as pri-mary defining factor for the impact strength ap-pears to be generally valid (see Fig. 6). This cor-relation was frequently observed before for thecase of PP-homopolymers.17,31

CONCLUSIONS

● For PP-homopolymers based on Ziegler–Natta-type catalysts, in principle the crystal-lization behavior can be explained by the nu-cleation density (governed by the MMD) andthe growth rate (governed by the chain reg-ularity).

● At MMDs that are narrow as for CR grades ofZN-based iPP, metallocene-based productsshow a significantly higher nucleation den-sity. The growth rate of these products is, asto be expected from their lower tacticity, re-

Figure 5 Correlation between stereoregularity andstiffness (flexural modulus): (l) ZN grades (exceptHAPP); (h) MC grades.


duced and can be included into a generalcorrelation between stereoregularity andgrowth rate.

● A combination of literature data with ourown results leads to the conclusion that aretarded growth of the a modification favorsdevelopment of the g modification. This isobserved in the case of metallocene-iPPs aswell as normal ZN products with comono-mers or reduced tacticity.

● The morphology observed in injection-molded samples is in accordance with thesteady-state crystallization parameters andearlier results. While the MMD only deter-mines the skin layer formation, the core mor-phology is a complex function of MMD andstereostructure.

● Metallocene-based iPPs do not follow thenormal correlation between tacticity andmodulus for ZN products. This can explainthe different relation between melting pointand stiffness, which is a significant advan-tage of these grades.

● No general correlation can be drawn betweencrystallinity data from DSC or WAXS inves-tigations and mechanical properties, as themorphology of the samples plays an impor-tant role in determining the actual behaviorunder load.

● For all investigated products, the averagemolar mass as primary defining factor for theimpact strength appears to be generallyvalid.

Part of the work presented here was financially sup-ported by the Austrian Forschungsforderungsfonds furdie gewerbliche Wirtschaft under project no. 5/570/809.The authors are grateful to Prof. H. Janeschitz-Kriegland Dr. G. Eder for coordinating the work carried outat J. Kepler University Linz, Prof. P. Zipper for carry-ing out the X-ray investigations, as well as Dr. D.Leistner of Borealis AG for preparing several of thesamples in the pilot plant. Thanks also to Dr. K. Ham-merschmid and Dr. C. Paulik at Borealis AG for helpfuldiscussions.

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effects of the catalyst system on the crystallization of polypropylene*

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