binary and ternary mlldpe blends: tailoring their properties using complex viscosity measurements

Binary and Ternary mLLDPE Blends: Tailoring Their

Properties Using Complex Viscosity Measurements

Miguel Angel Flores,1 Raquel Alicante,2 Ruth Perez,2 Elena Rojo,2 Mercedes Fernandez,2 Anton Santamarıa*2

1Centro de Tecnologıa, REPSOL-YPF, Ctra. N-V, km. 18, 28931 Mostoles, Madrid, Spain2Department of Polymer Science and Technology, POLYMAT, Faculty of Chemistry, University of the Basque Country,P. O. Box 1072, E-20080, San Sebastian, SpainFax: (þ34) 943-212-236/015270; E-mail: [email protected]

Received: January 24, 2005; Revised: April 19, 2005; Accepted: May 10, 2005; DOI: 10.1002/mame.200500043

Keywords: complex viscosity of blends; LLDPE blends; metallocene polyethylene;molecular weight distribution; shear; ternaryblends

Introduction

Metallocene or single-site catalysts are able to produce

polyethylenes and polyethylene copolymers with narrow

molecular distributions and uniform distribution of short

chain branches. This has repercussion on superior mechan-

ical properties, but at the same time provokes undesirable

high viscosities. The latter usually leads to flow instabil-

ities, in particular sharkskin surface instability, which are

observed at shear rates typically involved in processing

methods, like extrusion.[1] Unfortunately, the poor proces-

sability of the metallocene polyethylenes and polyethylene

copolymers tends to limit considerably their commercial

penetration. Four possible strategies are envisaged to avoid

or postpone sharkskin of these polymers improving their

industrial extrusion conditions: (a) rise die temperature to

decrease viscosity, (b) decrease die temperature to reach a

so-called ‘low temperature window of extrudability’,[2–6]

(c) use a polymer processing additive to provoke slip at the

wall of the die and (d) mix or blend the base polyethylene

with other polyethylenes to modify the rheological res-

ponse. The principal inconvenient for the use of the first two

ways to postpone sharkskin is that they imply amodification

of the temperature profile of the extruder, which is not

usuallywell acceptedbypolymer transformers.On theother

hand, polymer manufacturers try to avoid processing aid

additives [route (c)], because they have to purchase them to

other companies and increase the final price of the polymer.

Summary: Metallocene-catalysed linear low density poly-ethylenes (mLLDPEs) of different molecular weights aremixedwith a basemLLDPE to obtain a pre-determined gradeand a suitable extrusion processability. Complex viscosityresults are revealed as a very suitable tool to tailor the pro-perties of the prepared binary and ternary blends. Theinvestigated blends show symptoms of miscibility like time-temperature superposition and observance of Cox-Merz rule.This allows to evaluate the molecular weight distribution,supplying also practical data related to the polyethylenegrade and ‘sharkskin’ instability. The complex viscositycurve of a model sample that would have both, a suitablegrade and processability, is created; blends are prepared tomatch the model curve. The blend with a low molecularweight mLLDPE shows a parallel shift of the complexviscosity to lower values (molecularweight distribution is notbroadened), postponing sharkskin, but changing the polymergrade. Satisfactory results are neither obtained with blends ofa high molecular weight mLLDPE. Best results are obtainedwith ternary blends, which combine the modifications pro-

vided by low and high molecular weight mLLDPEs,approaching the complex viscosity and molecular weightdistribution of the model.

Complex viscosity results of PR-207 (– —), PR-209 (��) andPR-210 (– –) ternary blends, as compared with those of basesamples PR-201 (symbols) and the model (line).

Macromol. Mater. Eng. 2005, 290, 704–709 DOI: 10.1002/mame.200500043 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

704 Full Paper

In the field of polyethylene blends, where this paper is

placed, two routes are currently contemplated to avoid or

postpone the sharkskin of metallocene-catalysed linear low

density polyethylenes (mLLDPEs): (a) addition of a low

molecular weight second component to decrease the visco-

sity, which eventually produces a shift of sharkskin to high

shear rates, since the critical shear stress is practically

constant[7] and (b) addition of a high molecular component

to increase the internal cohesion of the polymer provoking

an enhancement of the melt strength, which would avoid

tearing or cracking at the exit of the capillary.[8] A new

parameter (the ‘reconfiguration rate’) has been recently

defined by Migler et al.[9,10] to investigate this mechanism

of sharkskin formation. However, avoiding or postponing

surface instability is not the only practical condition re-

quired to a polyethylene blend. Polyethylenemanufacturers

are impelled to offer blends with the same application grade

as the base polymer, due to commercial reasons. The melt

flow index (MFI) test, created in the laboratories of Imperial

Chemical Industries in the first stages of development of

polyethylene,[11] still constitutes the most useful parameter

for distinguishing thevarious grades of polymers in general.

In commercial terms, the grade labels the processing me-

thod and the final applications of the material. In the

particular case of polyolefins, manufacturers use MFI to

specify the suitable end use of a certain grade of poly-

ethylene or polypropylene; for instance, the eight different

grades ofHDPE range from0.05 to 0.15 g per 10min,which

is typically extruded to make profiles, to 13–15 g per

10 min, which is injected to obtain screw caps.

In this work, different mLLDPEs are mixed with a base

mLLDPE, tailoring its properties with the aid of dynamic

viscoelastic measurements to obtain a pre-determined

grade and a suitable processability.

Experimental Part

The molecular characteristics of the base polyethylene and theother mLLDPEs used to prepare the blends, are presented inTable 1. The investigated blend compositions are given inTable 2.

Rheological measurements were carried out in an extrusionrheometer (Gottfert Rheotester 1000) and in a rotationalrheometer (ARES Rheometrics Scientific rheometer).

Extrusion rheometry measurements were carried out with a1-mm diameter and L/D¼ 30 capillary die, at 190 8C.

Dynamic measurements were performed in the rotationalrheometer using a parallel plate geometry with d¼ 25 mm, attemperatures and frequencies detailed in the figures.

The method we have used to infer the molecular weightdistribution from complex viscosity data was proposed byWood-Adams and Dealy[12,13] and constitutes a modified ver-sion of the technique developed by Shaw and Tuminello.[14]

The equation used to calculate the logarithmic differentialmolecular weight distribution w(logm) is

wðlogmÞ

¼ � ln 10ð Þmn2

� �Z�

Z0

� �1=2ad2 ln Z�

d lno2þ n

d ln Z�

d lnoþ d ln Z�

d lno

� �2" #

ð1Þ

where n is the slope in the power law region, which varies from0.94 to 0.98 in our samples, a is the exponent in the relationshipbetween viscosity and molecular weight (3.6 in this work) andm is the reducedmolecular weight, given bym:M/Mw whichis assumed to be related to the frequency aso¼oc/m

a/n [oc isthe value at which the viscosity plateau (Z¼ Z0) intersects theline fitted to the power law portion of the viscosity curve]; oc

varies from 45 to 144 s�1 in our sample.The use of this equation implies the availability of experi-

mental complex viscosity data that include the Newtonianplateau and the power law region. This is achieved by fitting thedata to the model proposed by Cross:[15]

Z0 ¼ Z01þ ðol0Þa

ð2Þ

where Z0 is the linear or Newtonian viscosity, l0 is therelaxation time (whose inverse accounts for the onset of non-linear behaviour) and a the shear thinning index.

Themolecular weight distribution obtained curves, functionof the reduced molecular weight m, are normalised to unity forimproving the comparison purpose.

Results and Discussion

Our base sample mLLDPE PR-201 has an MFI value of

1.35 g �min�1, which indicates that extrusion must be, in

principle, a suitable processing method. But, the extrusion

rheometry data obtained at 190 8C reveal that sharkskin and

‘‘slip-stick’’ limit processing severely, for instance, shark-

Table 1. Molecular parameters of the base sample (mLLDPE1) and themLLDPEs used to prepare the blends. The highmolecularweightof mLLDPE is noticed by its low HLMI value. (HLMI: high load melt index).

Mw Mw=Mn d HLMI Hexene

g � cc�1 g per 10 min %

mLLDPE 1 89 000 2.38 0.918 – –mLLDPE 2 343 000 2.60 0.917 1 1.66mLLDPE 3 – – – 0.03 2.35mLLDPE 4 67 000 2.06 0.921 – 2.55

Binary and Ternary mLLDPE Blends: Tailoring Their Properties Using Complex Viscosity Measurements 705

Macromol. Mater. Eng. 2005, 290, 704–709 www.mme-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

skin appears at only 48 s�1 (Figure 1). This is a clear

example of the typical undesirable effect observed in

metallocene-catalysed polyethylenes, associated to the

narrow molecular weight distribution of this type of

polyethylenes. Needless to say that the goal of metallo-

cene-catalysed polyethylene manufacturers is to postpone

sharkskin to higher shear rates, but maintaining the same

grade (similar MFI). As it is mentioned in the Introduction,

mixing polyethylenes of different molecular characteristics

constitutes one of the routes to achieve this objective.

According to the results presented below, dynamic visco-

elastic measurements and, in particular, the plots of the

complex viscosity as a function of frequency are revealed as

a useful tool to obtain blends which procure the aforemen-

tioned properties.

As can be seen in Figure 2, the complex viscosity data

and the extrusion flow viscosity data coincide at corre-

sponding values of frequency and shear rate, following the

Cox-Merz empirical rule Z( _gg)¼ Z*(o).[16] Applicability ofthis rule for polyethylenes was proved almost 40 years ago

by Onogi et al.[17] All the blends considered in this work

fulfil this equivalence. Therefore, the complex viscosity

measurements, which have the advantage of requiring a

very small amount of sample, give information of the visco-

sity function Z( _gg) in a wide range of shear rates. The

viscosity data are very well adjusted to the aforementioned

Cross model [Equation (2)]. The values of the adjustable

parameters for the base sample and the investigated blends

are included in Table 3. Concerning these results, the

observance of the Cox-Merz rule must be considered as a

symptom ofmelt state homogeneity in polymer systems.[18]

Besides, the time-temperature superposition holds for all

the considered blends. The activation energy values, ob-

tained from horizontal shifts to superpose data, are included

in Table 3. In Figure 3, the plots of tan d vs G* reflect time-

temperature superposition. Mavridis and Shroff[19] used

such plots to investigate HDPE and LDPE samples, reach-

ing to the conclusion that long chain branched polyethy-

lenes require both horizontal and vertical shift activation

energy to superpose data. More recently, Van Gurp and

Palmen[20] have shown the tan d-G* plots irregularities as a

sign of polydispersity or phase separation. In the particular

case of LLDPEs, Trinkle and Friedrich[21] infer structural

properties, like the presence of very small amounts of LCB,

using the so-called Mavridis-Shroff or Van Gurp-Palmen,

tan d vs G* plots. Considering these previous findings, the

Table 2. Composition of the investigated binary and ternaryblends.

Material mLLDPE1

mLLDPE2

mLLDPE3

mLLDPE4

% % % %

PR-201(base sample)

100 – – –

PR-203 92 – 8 –PR-206 70 – – 30PR-207 62 8 – 30PR-209 52 8 – 40PR-210 42 13 – 45

Figure 1. Flow curve of base polyethylene PR-201, obtained bycapillary extrusion rheometry at 190 8C. The arrow indicates theonset of sharkskin.

Figure 2. The complex viscosity data and the extrusion flowviscosity data of base polyethylene PR-201 at correspondingvalues of frequency and shear rate, following the Cox-Merzempirical rule (see text). The line corresponds to the Cross model[Equation (1) and Table 3] taking Z* instead of Z0.

Table 3. Values of the parameters of the Cross model [Equation(1)], used to adjust dynamic and steady state flow viscosity data(see text and Figure 2), and activation energy values.

Material Z0 l0 a Ea

Pa � s�1 kcal �mol�1

PR-201 6.2� 103 0.022 0.84 6.4PR-203 1.1� 104 0.052 0.66 6.2PR-206 3.7� 103 0.014 0.85 6.4PR-207 6.9� 103 0.064 0.50 6.5PR-209 5.8� 103 0.061 0.48 6.6PR-210 7.9� 103 0.26 0.40 6.2mLLDPE 3 1.7� 106 158.4 0.56 4.8mLLDPE 4 6.7� 102 Not defineda) Not defineda) 7.1

a) Viscosity is independent of frequency.

706 M. A. Flores, R. Alicante, R. Perez, E. Rojo, M. Fernandez, A. Santamarıa


results of Figure 3 reinforce the hypothesis of having no

symptoms of immiscibility in the investigated blends. In

this sense, complete melt homogeneity for LLDPE blends,

investigated using small angle neutron scattering, has been

reported by Alamo et al.[22]

In view of more industrial purposes, the trend followed

by MFI can be deduced from the Z0 data of the samples,

considering the results observed for other thermoplastics.

This allows to establish an approximate estimation of the

grade of a polyethylene blend analysing the Z* data at low

frequencies. On the other hand, it is known that sharkskin

instability appears typically at shear stresses in the range

0.15–0.25 MPa,[7] so the lower the viscosity, the larger the

shear rate is for the onset of the instability. Having in mind

that the Cox-Merz rule is obeyed for our polymer series,

it can be settled that Z*(o) vs o results give the basic

information about the suitability of a blend sample, in terms

of its grade (MFI) and its processability (absence of

sharkskin at reasonable shear rates). This is illustrated in

Figure 4(a), which shows the complex viscosity of our base

sample, compared with the complex viscosity of a model

sample that would have the same grade but a better pro-

cessability. In other words, this model sample is a quanti-

tative illustration of a desired polymer. Both polyethylenes

would possess very close MFIs, as indicated by a very

similar linear viscosity Z0, but the sharkskin of the model

samplewould be considerably postponed, as a consequence

of its lower viscosity at high shear rates. On the molecular

basis, this rheologicalmodification requires an enlargement

of the molecular weight distribution, displayed in

Figure 4(b): this figure shows the relative molecular weight

distributions of the samples of Figure 4(a), determined

using the method developed byWood-Adams et al. (see the

Experimental Part).

The strategy followed to approach the model blend

presented in Figure 4 is based on the effect of the addition

of both, a low molecular weight and a high molecular

mLLDPEs. The complex viscosity results of blends PR-206

and PR-203 (which contain, respectively, a low molecular

weight and a highmolecularweightmLLDPEs, seeTable 2)

are presented in Figure 5(a). The addition of low molecular

weight mLLDPE (blend PR-206) gives rise to a parallel

shift of the viscosity function to lower values. It is well

known that a broadening of the molecular weight distribu-

tion provokes an increase of the relaxation time l0 of

Equation (2), changing the onset of the shear thinning

Figure 3. Time-temperature superposition: d vs G* taken atdifferent frequencies and temperatures for all the investigatedblends. No break of time-temperature superposition is observedfor any blend.

Figure 4. (a) The complex viscosity of the base polyethylenePR-201 (symbols), compared with the complex viscosity of amodel (line). (b) The molecular weight distribution of the samplesof Figure 4(a), determined using the method developed by Wood-Adams et al. (see theExperimental Part) PR-201 (symbols),model(line).



behaviour. Therefore, since the viscosity curves of base

sample and blend PR-206 are parallel, no significant broad-

ening of the molecular weight distribution is expected. In

agreement with the aforementioned considerations on the

correlation between Z*(o) curve and the basic performance

of mLLDPE blends, the grade is changed (MFI¼ 1.35 g per

10 min for base sample andMFI¼ 2.1 g per 10 min for PR-

206 blend) and the onset of sharkskin increases from

_ggc ¼ 48 s�1 for base sample to _ggc ¼ 92 s�1 (Table 4). Al-

though the latter result is encouraging, the goal marked in

Figure 4 is not achieved, since the grade of the base

mLLDPEPR-201 is notmaintained. Satisfactory results are

also not obtained with the addition of a high molecular

weight mLLDPE. The relaxation time increase (the onset of

shear thinning shifts to a lower _gg21) observed for the blend

PR-203 is associated to a slight broadening of themolecular

weight distribution [Figure 5(b)]. However, this does not

cause any improvement in order to attain the ideal blend

depicted in Figure 4: the MFI is undesirably decreased, due

to the enhancement of the linear viscosity Z0, and the onsetof sharkskin is not practically postponed (Table 4), in spite

of a presumable material cohesion enhancement (see the

Introduction).

The information obtained from these binary mixtures

suggests the possibility of preparing ternary blends, which

could combine the general viscosity decrease promoted by a

low molecular component, with the effect at low shear rate

of a high molecular weight component on Z0. Figure 6

shows the complex viscosity results [Figure 6(a)] and the

corresponding relative molecular weight distribution

[Figure 6(b)] of PR-207, PR-209 and PR-210 ternary

blends. No shoulder is detected at highmolecular weight, as

was expected considering the presence of mLLDPE 2

(Mw ¼ 343 000) in these blends. This is probably due to the

Figure 5. (a) The complex viscosity results of the basepolyethylene PR-201 (*), and binary blends PR-203 (– –)and PR-206 (� � �) (see Table 2). (b) Broadening the molecularweight distribution of blend PR-203 (– –), with respect to basepolyethylene (*). The relative molecular weight distributionsare determined as described in the Experimental Part.

Table 4. MFI values and critical shear rate values for the onset ofsharkskin of ternary blends (see text).

Material _��c MFI

s�1 g per 10 min

PR-201 48 1.35PR-203 50 –PR-206 92 2.09PR-207 64 1.39PR-209 80 1.67PR-210 72 1.40

Figure 6. (a) Complex viscosity results of PR-207 (– —), PR-209 (��) and PR-210 (– –) ternary blends, as compared with thoseof base samples PR-201 (*) and the model (line). (b) Thecorresponding relative molecular weight distribution of thesamples of Figure 6(a), as compared with base sample PR-201(*) and the model (line). The relative molecular weight distri-butions are determined as described in the Experimental Part.

708 M. A. Flores, R. Alicante, R. Perez, E. Rojo, M. Fernandez, A. Santamarıa


low amount of this polymer in the investigated blends.

In any case, an approach to the complex viscosity and

molecular weight distribution of the model polyethylene of

Figure 4 is observed. Although our ternary blends do not

reach the conditions established in Figure 4, they constitute

an actual alternative to base sample PR-201, since they offer

an improved extrudability (sharkskin is postponed), main-

taining a similar grade, as can be seen in Table 4.

Conclusion

Complex viscosity results are revealed as a very suitable

tool to tailor the properties of mLLDPE/mLLDPE blends,

in order to obtain a pre-determined grade and a suitable

processability. The investigated binary and ternary blends

show symptoms ofmiscibility in themolten state, like time-

temperature superposition and observance of Cox-Merz

rule. Under these conditions, complex viscosity results

allow to evaluate the molecular weight distribution (using

the Wood-Adams method) of the blends, supplying also

practical data concerning MFI (related to the polyethylene

grade) and sharkskin instability. In an attempt to obtain a

mLLDPE of MFI close to 1.35 g per 10 min with no flow

instability, the complex viscosity curve of amodel sample is

created. The hypothetical molecular weight distribution of

the model is shown to be broader than that of the base

mLLDPE. The base polyethylene is mixed with mLLDPEs

of various molecular weights. The addition of a low

molecular weight mLLDPE gives rise to a parallel shift of

the complex viscosity to lower values (themolecularweight

distribution is not broadened), postponing sharkskin, but

increasing excessively MFI. On the other hand, binary

blends prepared mixing the base mLLDPE with a high

molecular weight mLLDPE, increase the shear thinning

behaviour of the complex viscosity, but the obtained results

are not suitable. Best results are obtained with ternary

blends, which combine the modifications provided by low

and high molecular weight mLLDPEs, approaching the

envisaged complex viscosity and molecular weight dis-

tribution of the model.

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binary and ternary mlldpe blends: tailoring their properties using complex viscosity measurements

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