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The Effect of Co-monomer Type on the Mechanical and Thermal Properties of Metallocene and Conventional LLDPEs.

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Dev. Chem. Eng. Mineral Process. 12(1/2), pp. 67-76,2004.
The Effect of Co-monomer Type on the
Mechanical and Thermal Properties of
Metallocene and Conventional LLDPEs
S. Walker, G.M. McNally and P.J. Martin
Polymer Processing Research Centre, The Queen 's University Belfast,
Stranmillis Road, Belfast BT9 SAH, Northern Ireland, UK
A range of metallocene and conventional linear low-density polyethylenes (LLDPEs).
with different material properties, were prepared by injection moulding. An
assessment of the effect of cooling rate and polymer properties on the mechanical
performance of the specimens was conducted to establish any signifcant correlations.
Tensile results showed that hexane-based metallocene (m)LLPDEs exhibited higher
elongation to break while the Young's modulus of the materials was found to be more
influenced by density. Impact results demonstrated that mLLDPEs have superior
impact strength at room temperature over conventional LLDPEs. Rheological studies
of the materials under high shear rates experienced in injection moulding, were
performed to determine flow characteristics of the materials. Dzferential scanning
calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) were used to
study the influence of the co-monomer type and degree of branching on the properties
of the materials.
Introduction
Linear low-density polyethylene (LLDPE) is an important polymer commodity
representing 25% of the total polyethylene (PE) market and continues to be the fastest
growing material in the PE family [l]. This has been accelerated by recent
developments in metallocene and other constrained geometry catalysts, and has
renewed the growth and demand for PE resins. Metallocenes display controlled
molecular weight (MW), narrow molecular weight distributions (MWDs) and may
contain sparse long chain branching. The ability to tailor PEs with specific molecular
architectures can be very useful in fulfilling physical and mechanical properties [2,3];
however compared to those produced using conventional catalysts with the same
MW, they often have poor processabilities due to high viscosities. In practice this is
67
S, Walker, G.M. McNally and P.J. Martin
seen during extrusion by high melt pressures and motor loads. However, there is
limited information on how their processability is affected during injection moulding.
It is well known that the microstructure of polymers plays a critical role in
determining their physical and mechanical properties. The co-monomer plays an
important role in these properties. As the co-monomer content is increased there is a
decrease in the crystallinity and melting point of the copolymers [4]. In addition the
differences in molecular structure in co-monomer length, i.e. 1-butene, 1-hexenel and
1-octene can also affect the final properties of the item [ 5 ] . An important focus in the
development of these commercially available mLLDPEs is the mechanical properties,
which are critical in determining the suitability of a material for a given application.
Parameters such as MW and MWD, co-monomer type and concentration all play a
significant role in the final properties. In addition, processing parameters, and in
particular the nature of cooling, can significantly affect the properties of a polymer.
Slow cooling promotes crystal perfection and an increased level of crystallinity,
whereas fast cooling limits the developments of crystallites and therefore favours a
reduction in crystallinity [a]. This in turn influences the mechanical properties of the
ethylene co-polymers. Therefore it is important to gain an understanding of how these
variables interact and contribute to the physical properties of the materials.
In this investigation the thermal and mechanical properties of ethylenela-olefin
copolymers synthesized with metallocene and conventional Z-N catalysts were
compared, with particular focus on the effect of catalyst type, co-monomer type,
density, and MW and its distribution.
Experimental Details
(a) Materials. The polymers used in this study were commercial grades, the
conventional LLDPE, Lupolen supplied by Elenac; the ethylene 1-octenes, Elites
from Dow; and the ethylene 1-hexene, metallocenes, Luflexen from Elenac. The
material specifications are shown in Table I.
Table 1. Materials properties of conventional and metallocene LLDPEs.
68
Efect of Co-monomer Type on Properties of Metallocene and LLDPEs
(6) Injection Moulding. Tensile, flexural and impact specimens for all the materials
were manufactured using an Arburg 320s Allrounder 500-350 injection moulding
machine, with a general purpose screw (d = 45 mm and LA3 = 18). It was fitted with a
test mould with tensile, flexural and impact impressions. The barrel temperature
profile from the feed section to the nozzle was maintained at 170, 190, 200, 210 and
210°C during the manufacture of the various samples. After injection mouldmg and
prior to testing, specimens were allowed to condition under ambient conditions for a
minimum of 48 hours.
(c) Rheological Studies. A Rosand dual capillary rheometer, model no. FW7, was
used in the rheological studies to profile each material and compare the viscosity
profiles of metallocene and conventional LLDPEs. Approximately 50 g of material
was used for each run and samples were examined at shear rates ranging from
1000 s-' to 12000 s-' at temperatures of 160, 180, 200 and 220°C. The calculated
rheological data results were Bagley corrected.
(d) Mechanical Analysis. The tensile properties of the samples were tested according
to ASTM 638. These tests were performed using an Instron 441 1 universal tester with
a load cell of 5 kN and a constant crosshead speed of 200 d m i n . The impact
properties of the samples were determined using a CEAST automatic fractovis free
falling dart impact tester, fitted with a DAS4000 WIN data acquisition system. The
peak force (N), peak energy (J) and total energy (J) were recorded and the impact
strength was calculated by dividing the peak energy by cross-sectional thickness of
the samples.
(e) Crystallinify. In order to investigate the effect of molecular variables and cooling
rate on the degree of crystallinity of each of the polymers, DSC analysis was
performed under controlled cooling rates of 5"C/min, 1O"C/min and 20"C/min. using a
P e r k Elmer DSC 6 modular thermal analyser. A 5-10 mg representative sample of
each specimen was heated over a temperature range of 20°C to 200°C at lO"C/min.
The crystallinity was determined from the enthalpy (AH) values of the melt
endotherms, using the AH value 290 J/g for 100% crystalline polyethylene [7].
Q) Dynamic Mechanical Thermal Analysis. DMTA analysis of the samples was
performed using a Polymer Lab MARK I1 dynamic mechanical analyser. The samples
of dimensions 46 x 13 x 3 mm were mounted in the dual cantilever mode. The
samples were scanned over the temperature range -120°C to 100°C at a temperature
scan rate of 2°C per minute, and a frequency of 1 Hz.
Results
(a)Rheological Studies. Figure 1 shows the effect of shear rate on apparent viscosity
of the octane-based mLLDPEs at 200°C. The results possessed typical pseudoplastic
behaviour with the apparent viscosity decreasing with increasing shear rate for all the
materials. The mPE 4 and mPE 3 with MFIs of 0.85g/min showed very similar
behaviour, with mPE 2 and mPE 5 exhibiting the lowest initial viscosity of approx.
69
S. Walker, G.M. McNally and P.J. Martin
400 Pa.s at 500 s-'compared to viscosities greater than 600 Pa.s for the other materials
at this shear rate. This may be attributed to these materials having a lower MW and
higher MFI of 4.0 and 5.75 g/10 min compared to much lower values for the other
octane-based mLLDPEs. At the hgher shear rate of 10000 s-' all the materials
exhibited viscosities around 50 Pa.s. Also shown in Figure 1 are the rheograms for the
hexene mPEs (mPE 8-13) and the conventional materials (cPE 1-3). The rheograms
illustrate that at lower shear rates the differences in viscosity between materials was
greater than at shear rates more associated with injection moulding. The conventional
Z-N material, cPE 3, had a lower viscosity profile than mPE 9 which had similar MFI.
Ruksakulpiwat [8] also reported lower shear viscosities for mPE resins than
conventional Z-N polyethylenes. At higher shear rates all these resins possessed
viscosities of around 55-60 Pa.s. In general the octene resins showed hgher
viscosities than the hexene mLLDPEs for similar MFIs. For example, mPE 1 (MW
117500, MFI lg/lO min) had a viscosity of 600 Pa.s at 500 s-',compared to mPE 11
(MW 113500, MFI lg/10 min) with a viscosity of 661 P a s at the same shear rate.
These results suggest that the longer chain branching present in the octene
copolymers affected the viscosity at these lower shear rates. In addition the
polydipersity was found to have little affect on the rheological properties of the
materials at these high shear rates.
800
600
400
200
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Figure I . Rheograms of octene mPEs, hexene mPEs and conventional LLDPEs.
70
Eflect of Co-monomer Type on Properties of Metallocene and LLDPEs
(6) Tensile Analysis. Figures 2 and 3 show the tensile modulus of the octene and
hexane-based metallocene LLDPEs. There was a progressive increase in modulus
with increase in density for all materials. For the octene range of materials an increase
in density from 916 kg/m3 to 939 kg/m3 gave an increase in modulus from 72.5 MPa
to 295 MPa; whilst in the hexene range, an increase in density from 916 kg/m3 to
924 kg/m3 corresponded to an increase in modulus from 55.8 MPa to 109 MPa. For
the conventional materials, the tensile modulus values were comparable to the
metallocene catalysed materials of similar densities, so catalyst type did not have a
significant effect on modulus. Mould temperature also had a positive effect on the
tensile modulus, with each material showing a modulus increase with increase in
mould temperature from 20°C to 60°C.
T
Figures 2 and 3. Eflect of mould temperature on tensile modulus of octene mPEs,
hexene mPEs and conventional LLDPEs.
Figures 4 and 5 show elongations to break of the octene and hexene copolymers at
each mould temperature. There was a general decrease in elongation with increase in
density for both the octene mLLDPEs and hexene mLLDPEs. The lower density
hexene LLDPEs in fact exceeded the limit of the testing machine and did not break.
Hexene mLLDPEs had higher elongations than the octenes of similar densities. The
mPE 1 (octene, p = 916 kg/m3) exhibited an elongation of 732% compared to mPE 8
(hexane, p = 9 1 6 kg/m3) with an elongation of >1000%. For the conventional
materials, the butene LLDPE exhibited higher elongations than the hexene LLDPEs.
An overall decrease in elongation to break was also found amongst the materials as
the mould temperature increased, being most noticeable for the octene mPEs. These
results indicated that the mould temperature and co-monomer type affected the
elongation, with the shorter length co-monomer promoting higher elongations.
Figures 6 and 7 present the stress at yield for the various materials. The octanebased mLLDPEs (mPE 1-4) exhibited lower yield stresses than mPE 5-7, indicating
that yield stress was higher for the higher density materials. In addition, MW had an
71
S. Walker, G.M. McNally and P.J. Martin
~rIl€rrRrIl€nR:d€d€bE
9 X J l l l 2 1 3 1 2 3
Figures 4 and 5. Effect of mould temperature on elongation of octene mPEs, hexene
mPEs and conventional LLDPEs.
effect on the yield stress, with increased yield stress being recorded for higher MW
resins. For example mPE 2 (MW 83800) had the lowest recorded yield stress of
8.87 MPa while mPE 4 (MW 122500) had a yeld stress of 12.5 MPa. Similar trends
were recorded for the hexene mLLDPEs with both density and MW affecting the
results. The conventional LLDPEs exhibited slightly lower yield stresses than the
mLLDPEs. Increasing the mould temperature slightly increased the yield strength of
the materials. The cPE 3 (Z-N) resin with slightly higher MW and density than
mPE 9 (metallocene) which would favour higher yield stresses, actually exhibited a
lower yield stress. The octene LLDPEs had the highest stress at yield followed by
hexene and butene with comparable densities and MW. These results suggest that comonomer type, density and MW affect the yield stress of the LLDPEs.
25
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Figures 6 and 7. Effect of mould temperature on yield stress of octene mPEs, hexene
mPEs and conventional LLDPEs.
72
Effect of Co-monomer Type on Properties of Metallocene and LLDPEs
The impact properties at room temperature of each material investigated are
shown in Figures 8 and 9. The octane-based mLLDPEs exhibited the highest impact
strength. The material with the higher density, mPE 7, had the greatest overall impact
strength of 1 1.2 J/mm while mPE 10 had the lowest impact strength of 7.4 J/mm
amongst the metallocene materials. Melt flow index affected the impact properties,
with an increase in MFI producing a corresponding decrease in impact properties of
the metallocenes. In addition the conventional materials exhibited poorer impact
performance; cPE 3 (MFI 2.8; p = 918 kg/m3) had an impact strength of 6.8 J/mm
compared to mPE 9 (MFI 2.8; p = 919 kg/m3) with an impact strength of 7.8 J/mm.
Figures 8 and 9. Impact properties of octene and hexene mPEs and conventional
LLDPEs.
I,",
.
N ' m ' * ' v)' a 'Ic
w w w w w w w
T
a a a a a a a
E E E E E E E
Figure 10. Efect of cooling rate on
melting temperature of octene LLDPEs.
Figure 11. Eflect of cooling rate on
melting temperature of hexene LLDPEs.
73
S. Walker, G.M.McNally and P.J.Martin
(c) Crystalliniq. The results in Figures 10 and 11 show the effect of DSC cooling
rate on the peak melting temperatures of the polymers. There was only slightly higher
melting temperatures for the slower cooling rate, with a progressive increase in melt
temperature with increasing density, and the octene mLLDPEs exhibited higher
values than the hexene mLLDPEs. However the cooling rate had a more pronounced
effect on the hexene mLLDPEs, as illustrated by larger differences in the melting
temperature. The conventional materials showed higher melting temperatures than the
metallocenes as was also found by Xu et al. [8], who attributed this phenomenon to
the distribution of the short chain branches amongst the polymer chains.
Figure 12. Efect of cooling rate
on % crystallinity of ociene mPEs.
Figure 13. Effect of cooling rate
on % crystallinity of hexene mPEs.
The overall crystallinity of the various polyethylenes shown in Figures 12 and 13
decreased with increase in cooling rate and increase in density. The highest
crystallinity (62%) was recorded for mPE 7 at a cooling rate of 5"C/min, whilst cPE 3
recorded the lowest value (16.5%) at a cooling rate of 2OoC/min. These results
indicate that both thermal treatment and density affect the degree of crystallinity of
these polyethylenes
(d) DMTA Analysis. DMTA analysis was performed on all the samples to investigate
the effect of molecular characteristics on the phase transitions of these materials. The
Tg and storage modulus (E') at 25°C for each material were recorded from the DMTA
thermograms and are shown in Table 2. For each group of polymers there was a
general increase in the temperature of the Tan 6 peak with increase in density. The
results also show that an increase in MW resulted in a corresponding increase in the
74
Effect of Co-monomer Type on Properties of Metallocene and LLDPEs
transition temperature with increase in MW for the hexane-based mLLDPEs of the
same density. Co-monomer type did not appear to have a significant effect on this
transition temperature. The storage modulii (Log E’) at 25°C increased with an
increase in polymer density. This was most apparent for the octene mLLDPEs where
an increase in density from 916 kg/m3 to 939 kg/m3 gave a corresponding increase in
the storage modulus from 8.30 MPa to 8.82 MPa. These storage modulus values
correlate well with the tensile and flexural modulii recorded in Figures 2, 3, 7 and 8.
The mPE 5-7 exhibited both the highest tensile modulii and had storage modulus
values greater than 8.60 MPa at room temperature.
Table 2. Dynamic mechanical properties of polymers at 1 Hz.
mPE 12
mPE 13
cPE 1
cPE 2
cPE 3
Hexene
Butene
Hexene
Hexene
MITSUI SP 250
Phillips D350
-74
Exxon MMA 043
Exxon LL3002
Elenac 180FA
-76.5
-19
-17
-82
8.40
8.46
8.18
8.16
8.47
924
933
918
917
919
Conclusions
This investigation reports on the effect of a-olefin co-monomer type, mould cooling,
and polymer structure on the mechanical performance of injection moulded samples
produced from a range of metallocene-catalysed octene and hexene co-monomer
based PEs. Rheological studies showed that both material types exhibited viscosities
around 50-60 Pas at typical injection moulding shear rates, with the longer chain
branching present in the octene mLLDPEs, and having little effect on the flow
characteristics of these materials. In addition, the mLLDPEs presented no processing
problems at these shear rates
The increase in tensile modulus was shown to be dependent on mould
temperature, density and the overall crystallinity, with the higher tensile modulii
being recorded for the higher density octane-based mLLDPEs. In contrast, the
elongation results showed that hexene mLLDPEs had higher elongation to break than
75
S.Walker, G.M. Mchrally and P.J. Martin
octene mLLDPEs with similar densities, and the mould temperature having a less
significant effect OR this property. Impact strength depended not only on polymer
density but also on MFIs of the polymers, increasing with decrease in MFI.
Conventional LLDPEs showed poorer performance in both yield stress and impact
strength.
DSC anaIysis highlighted that the octane-based mLLDPEs had generaIly higher
crystallinities, especially at the higher mould temperatures, than the hexane-based
materials. The DMTA analysis of the resin showed that the various relaxations were
dependent on co-monomer type and density. DMTA analysis of the various polymers
highlighted that both the phase transitions and storage modulus were dependent on
co-monomer type and density. The results also tend to suggest that the hexene
a-olefin co-monomer mPE types exhibited a lower storage modulus than the octene
a-olefin co-monomer mPE type for materials of similar densities.
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Catalyst. Ann. Tech. Conf. SOC.Plastic Eng. 2 1578.
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Polymer 5957-5965.
7. Starck, P., Malmberg, B., and Lofgren. B. 2002. Thermal and Rheological Studies on the Molecular
Composition and Structure of Metallocene and Ziegler Natta Catalyzed Ethylene-a-Olefin
Copolymers. J. Appl. Poly. Sci. Q 1140-1156.
8. Ruksakulpiwat, Y. 2001. Comparative Study of Structure and Property of Ziegler-Natta and
Metallocene Based Linear Low Density Polyethylene in Injection Mouldings. Ann. Tech. Conf. SOC.
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9. Xu, X., Xu, J, Feng L., and Chen W. 2000. Effect of Short Chain-Branching Distribution on
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