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The Effect of Orientation on Extrusion Cast Metallocene Polyethylenes and the Role of Processing Conditions in the Die Swell of Metallocene and Conventional Polyethylenes.

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Dev. Chem. Eng. Mineral Process. 12(1/2), pp. 21-35, 2004.
The Effect of Orientation on Extrusion Cast
Metallocene Polyethylenes and the Role of
Processing Conditions in the Die Swell of
Metallocene and Conventional Polyethylenes
B.G. Millar, P. Laughlin, W.R. Murphy and G.M. McNally
Polymer Processing Research Centre, The Queen 's University Belfast,
Stranmillis Road, Belfast BT9 5AH, Northern Ireland, UK
Castfilms were preparedfiom a range of metallocene polyethylenes (mPEs) of varied
co-monomer types Fexene, octene) using a Killion single-screw extruder, using
diferent haul of speeds (8-4 m/min) and die gaps (700-250 p).Samples with
greater orientation in one direction had increased tensile strength and shrinkage in
that direction. DSC analysis showed crystallinity to decrease with decreasing haul of
speed. Extrusion of mPEs and conventional linear low density polyethylenes
(LLDPEs) by single capillary rheology showed that die swell increased with
increasing extrusion rate and decreasing melt temperature. Increased die swell was
found for the broader molecular weight distribution (M.W.D.) LLDPEs and in the
higher molecular weight resins. Furthermore, long chain branching was found to
increase die swell.
Introduction
The use of linear low density polyethylenes (LLDPEs) has increased in recent times,
especially in packaging film applications where its use has enabled considerable
down-gauging of film thickness. Packaging film is the biggest application of LLDPEs
21
B.G. Millar, P. Laughlin, W.R. Murphy and G.M. McNally
and must meet certain specifications. To contain a product successfully, a packaging
film must have a specific tensile modulus and tensile strength range and attention
must also be given to the tear resistance and shrink properties. The co-monomers
used in most commercial LLDPEs are 1-butene, I-hexene, and 1-octene. However,
conventional LLDPEs polymerised using Ziegler-Natta catalysts do not have as
uniform a comonomer distribution as those using the new generation metallocene
catalysts [ 13. MPEs have attracted considerable attention from film manufacturers.
These single site metallocene catalysts permit the design of polymer chain structures
at a molecular level and enable the tailoring of properties [2]. T h s allows polyolefins
with comparatively narrow molecular weight distributions to be produced [3].
However, the processability of mPE is more challenging than conventional LLDPE.
In order to overcome some of these difficulties, blends of mPE with conventional PE
have received much attention recently [l]. The effect of blown film extrusion
processing conditions and molecular variables such as MFI, M.W.D. and comonomer
type on the mechanical performance of mPE blended with conventional LLDPEs has
been reported [4]. The effect of quench temperature on the mechanical performance
and crystalline development of mPE cast films has also been studied [5] while the
effect of processing conditions on the orientation developed in LLDPE blown films
has received attention [6]. In blown film and cast film production, films are hauled off
or blown up at various rates in order to maintain film gauge, compensate for die swell
and in the case of blown films, achieve isotropic mechanical performance. Orientation
is known to affect the tear resistance of films [6] and in cast film extrusion t h s
orientation is developed in the film in the short air gap between the die and the chill
roll before solidification.
The extrudate swelling (Barus effect) of polymers during extrusion is always of
concern to processors. This die swell phenomenon is due to the elastic recovery of the
melt and will be dependent on the rheological properties of the polymer, which, in
turn will be affected by the molecular chain architecture, temperature, and shear
stress. The influence of molecular structure on the rheology and processing behaviour
of conventional polyethylenes has been recently reported [ 101, and the extrudate swell
of conventional LLDPEs in capillary rheometry was reported to be affected by
molecular weight distribution, with higher swell being recorded for broader M.W.D.
polymers [l 11. The effect of shear stress, temperature and capillary W D ratio on the
extrudate swelling of polyethylene has been discussed and die swell has been shown
to increase with long chain branching [12]. Other reported works on the die swell of
polyolefins have been on HDPEs and polyolefm blends. However, very little has been
published on the effect of shear stress, shear rate, temperature and molecular structure
on the rheological properties and die swell of the new generation mPEs.
Therefore the main aims of this current work are to investigate the effect of haul
off speed, die gap and molecular characteristics on the crystalline development,
tensile properties and tear resistance of a range of commercially available hexene and
octene based mPEs of different MFIs and molecular weight distributions.
Additionally, to investigate the effect of shear stress and temperature on the
rheological response and die swell of a range of commercially available conventional
LLDPEs and mPEs that are currently used for cast film extrusion.
22
Efect of Orientation on Extrusion Cast Metallocene Polyethylenes
Experimental Details
(a) Materials
The polymers used to manufacture films were commercially available cast film
extrusion grades of M E with similar densities of 0.917 and 0.918 g/cc. Four
ethylene-hexene co-polymers and one ethylene-octene co-polymer were selected for
study. The material specifications are shown in Table 1. The die swell of these
polymers was also investigated along with two conventional LLDPEs.
1
Density
gkc
MFI
UlOmin
Hexene
0.918
4.50
73850
2.1
Hexene
0.918
3.40
79650
2.2
Resin
Comonomer
mPE-1
mPE-2
MW
M.W.D.
mPE-3
Hexene
0.918
2.50
85900
2.2
mPE-4
Hexene
0.918
3.40
77850
3.1
mPE-5
Octene
0.917
4.00
78100
3.O
LLDPE-1
Butene
0.929
4.10
72000
3.5
LLDPE-2
Butene
0.920
2.40
81000
3.8
Table I . Material specificationsfor experiments.
(Bold print relates to informationreceived from the resin suppliers)
(b) Cast Film Extrusion
A series of films were manufactured from the various resins using a Killion cast film
extrusion system, comprising a 38 mm extruder fitted with a 600 mm wide coathanger
type flat sheet die. The extruder speed was set at 28 rpm and the temperature profile
of was ramped from 225 to 240°C for the hexene resins and 160 to 175°C for the
octene resin, in accordance with the product data sheets. In order to investigate the
effect of melt draw down, the die gap was set at different settings of 700, 550, 400
and 250 pm. The films were quenched using a highly polished water-cooled chill
roller at 40°C.A range of films were produced at line speeds of 4 and 8 d m i n to give
film thicknesses of 50 and 25 pm respectively throughout the trials.
(c) Molecular Weight Characterisation
In order to determine the effect of M.W.D. on the mechanical performance of the
various polymers it was necessary to determine the weight average molecular weight
(M.W.) and number average molecular weight (M.N.). This analysis was performed
by Rapra Technology Ltd. using a Waters 150CV gel permeation chromatography
unit. These values are shown in Table 1.
(d) Tensile Analysis
The tensile properties of the various film samples were tested according to ASTM
D882-95using an Instron 441 1 universal tester with a load cell of 100 N at a constant
23
B.G. Millar, P. Laughlin, W.R. Murphy and G.M. McNally
crosshead speed of 500 d m i n . Samples were tested in both the machine direction
(MD) and transverse direction (TD) using an average of 10 samples. The resultant
break strengths for the films are shown in Figures 1 to 5.
(e) Differential Scanning Calorimetry
The crystallinity of the various film samples was determined using a Perkin-Elmer
DSC6 modular thermal analyser. An accurately weighed 7 to 9 mg representative
sample of each specimen was scanned over a temperature range of 30 to 180°C at
10"C/min. The crystallinity was then determined from the enthalpy (AJ3)values of the
melt endotherms, using the AH value 289.9 J/g for 100% crystalline polyethylene [4].
These values are shown in Table 2.
( f ) Tear Resistance Analysis
The tear strength of the various film samples was tested using a trouser tear method
according to ASTM D1938-94 using an Instron 441 1 universal tester with a load cell
of 100 N at a constant crosshead speed of 250 mm/min. Samples were tested in both
MD and TD using an average of 8 samples. The values are shown in Figures 6 to 10.
(g) Shrinkage Analysis
Samples for shrinkage analysis were cut from the films using a circular template with
a diameter of 50 mm. These samples were placed between aluminium discs on a hot
plate at 120°C for 75 seconds. All surfaces were coated in silicon oil and were free of
surface defects and air bubbles to ensure even contact with the heat source. At least 5
samples were tested for each film. The mean percentage MD and TD shrinkage of
each film was recorded.
(h) Melt Flow Indexer
The rheological properties and extrudate diameter for each resin in Table 1 were
determined using a Kayeness Galaxy Model No. 7053 melt flow indexer at three
different test temperatures: 180, 190, and 200°C. The mass flow rate of resin was
measured using a range of weights; 1.20, 2.16, 3.26, 6.20, 7.16, and 8.26 kg. These
results were then used to calculate the shear stress (7), shear rate (y) data. The
extrudate diameters were used to calculate the percentage die swell for each resin
under the same range of weights and under the three test temperatures.
Results and Discussion
(a) Tensile Analysis
The effect of molecular characteristics and processing conditions on the break
strength of the cast films is shown in Figures 1 to 5. Generally, there was an increase
in break strength for all the films with increase in haul off speed from 4 d m i n to
8 d m i n . This indicates increased molecular orientation of the films was achieved in
both MD and TD by increasing the line speed.
24
Effect of Orientation on Extrusion Cast Metallocene Polyethylenes
.M
700 550
)400
250
700
500
400
250
Me Gap (pm)
.M
700
)
550
mm
400
250
700
700
550
400
500
400
250
700
500
400
250
me Gap (rm)
STD
Figure 1. Efect of die gap and haul of
speed on the break strength of mPE-I.
I
WM)
Eflect of die gap and haul off
on the break strength of mPE-2.
AFigure 2.
dpeed
250
me GP (rm)
(sm
.M
700
)
550
400
250
700
500
400
250
me Gap (rm)
Figure 3. Eflect of die gap and haul ofl AFigure 4. Effect of die gap and haul o f f
Jpeed on the break strength of mPE-4.
speed on the break strength of mPE-3.
.M
700 550
)400
QTD
250
700
500
400
250
Me Gap (rm)
Figure 5. Effect of die gap and haul of
speed on the break strength mPE-5.
There was a progressive increase in break strength with increasing die gap over the
range 250 pm to 700 pm.Again suggesting that increase in molecular orientation in
the film was achieved by using wider die gap settings. The results also show that there
was a more pronounced decrease in both MD and TD break strength with decreasing
die gap for the lower MFI hexene based mPEs. However, the lower overall MD and
TD break strengths recorded for mPE-4 would indicate that films manufactured from
wider M.W.D. hexenes exhibited much lower values. Films manufactured from the
octene based LLDPE, mPE-5 had the lowest MD and TD break strengths of all the
films with break strengths up to 40% lower than the hexene mPE-3 that possessed a
similar MFI (4.5 g/10 min). For almost all films the TD break strength was lower than
the MD break strength.
25
B. G. Millar, P . Laughlin, W R. Murphy and G.M. McNah'y
(b) Differential Scanning Calorimetry
The effect of molecular characteristics and processing conditions on the crystallinity
of the cast films is shown in Table 2.
Haul off Speed - 4 d m i n
Haul off Speed - 8 d m i n
Resin
700pm
SSOpm
400pm
250pm
700pm
SSOpm
400pm
250pm
mPE-1
32.9
28.1
24.7
23.1
27.2
26.9
21.8
20.1
mPE-2
27.9
25.9
23.4
22.6
27.7
26.7
20.0
19.2
mPE-3
27.2
25.6
23.0
20.1
25.8
22.2
19.1
17.8
mPE-4
30.2
28.5
26.7
19.2
28.8
22.0
20.8
18.2
mPE-5
25.9
24.6
21.5
19.6
24.4
23.9
21.3
20.4
Table 2. Efect of die gap and haul ofspeed on the percentage clystallinity
for allfilms.
Generally, there was a significant increase in crystallinity with increase in haul off
speed from 4 d m i n to 8 d m i n for all films. This increase in crystallinity may be due
to the decrease in heat transfer from the film to the chill roll at the higher line speeds.
The results also show a considerable increase in crystallinity of the films with
increasing die gap from 250 pm to 700 prn This maybe due to the increase in draw
down ratio experienced by the melt in the region between the die exit and the surface
of the chill roll with increasing die gap and can be attributed to the effect of strain
induced crystallization and to the lower in line stresses with these lower viscosity
polymers. A progressive decrease in crystallinity for the hexane-based mPEs with
increasing MFI was also recorded. However, the crystallinity of mPE-4 (MFI 3.4 g/10
min) was shown to be higher than mPE-2 (3.4 g/10 min) caused by the wider M.W.D.
of mPE-4 (3.1) compared to mPE2 (2.2). The octene based mPE-5 (4.0 g/10 min)
films were shown to have lower crystallinities than mPE-3, which had a similar MFI
(4.5 g/10 min). The octene based mPE-5 also had a wider M.W.D. than mPE-3 and it
would have been expected that the crystallinity may have been higher as a result of
this. Therefore, the lower crystallinity recorded for the octene films were attributable
to the molecular structure of this longer chain branched polymer inhibiting crystal
growth [7].
(c) Tear Analysis
It has been proposed that the tear propagation behaviour may be related to the
lamellar arrangement and orientation with respect to MD and TD, which is closely
related to processing conditions [8]. The more the lamellae are orientated towards TD,
the higher the stress that will be borne by them thus leading to higher resistance to
tear propagation along the TD. The effect of molecular characteristics and processing
conditions on the tear strength of the cast films is shown in Figures 6 to 10.
26
Effect of Orientation on Extrusion Cast Metallocene Polyethylenes
c
IM)
700
550
400
IT0
250 700 500
01. &P (rm)
1
700
250
400
550
400
250
700
500
400
250
Ue Gap (um)
mTD
Figure 6. Effect of die gap and haul off Figure 7. Effect of die gap and haul off
speed on the tear strength of mPE-I.
speed on the tear strength of mPE-2.
5 3
P
m 2
I
c
,m
PTD
1
700
550
400
250
700
500
400
250
me G.PIrm)
IM)
700
550
400
250
700
500
400
250
me G.PIrm)
DTD
Figure 8. Effect of die gap and haul off Figure 9. Effect of die gap and haul off
speed on the tear strength of mPE-3.
speed on the tear strength of mPE-4.
g4
2 3
8f ,
c
1
.m
la TD
700
550
400
250 ‘700 500
me G.P(rml
400
250
Figure 10. Effect of die gap and haul ofl
speed on the tear strength of mPE-5.
Overall, the results show that there is a general decrease in both MD and TD tear
strength for all the films with increase in line speed from 4 d m i n to 8 d m i n . The
decrease in tear strength is more noticeable with decrease in MFI, with the lowest
values being recorded for mPE-1 with an MFI of 2.5 g/10 min. There was greater
differences in MD and TD tear strength recorded for a progressive increase in die gap.
This confirms earlier work, which indicated that higher draw down ratios favour TD
tear resistance [8]. The results in Figures 7 and 9 show enhanced TD tear strength
with wider M.W.D. The highest tear strengths were recorded for the octene based
resin d E - 5 .
27
B. G. Millar, P. Laughlin, W.R. Murphy and G.M. McNally
(d) Shrinkage Analysis
The effect of molecular characteristics and processing conditions on the MD and TD
shnnkage of the film samples upon heating at 120°C is shown in Table 3.
-
-
Haul off Speed 4 d m i n
Haul off Speed 8dmin
e
700pm
550pm
400pm
250pm
700pm
550p.m
400pm
250pm
mPE-1
14.3
12.3
11.8
11.3
5.3
5.0
4.7
4.3
mPE-2
20.1
19.3
17.8
16.3
14.3
11.2
9.6
6.2
mPE-3
23.2
22.0
21.9
21.6
16.2
14.3
12.1
11.7
mPE-4
21.6
19.5
17.9
15.2
13.2
11.8
9.8
7.2
mPE-5
24.2
21.9
19.8
18.6
15.8
13.9
11.5
10.8
mPE-1
5.8
6.3
7.0
7.8
0.0
0.3
1.1
1.8
mPE-2
6.8
7.0
7.1
7.7
1.3
1.8
2.1
2.5
mPE-3
8.I
8.2
8.0
8.1
2
2.1
2.5
2.8
mPE-4
6.2
6.6
7.0
7.8
2 .o
2.5
2.6
2.7
mPE-5
-6.3
-5.6
-4.2
-3.7
-1.8
-1.2
-0.2
0.0
Shrinkage of films at elevated temperatures is due to the recovery of amorphous
and crystalline phase orientation developed during processing and so is an excellent
indicator of the disparate degrees of orientation in the MD and TD of an extruded
film. Increased orientation in one direction is demonstrated by increased shrinkage
along the same axis. The results in Table 3 show an overall increase in MD and TD
shrinkage with increase in line speed from 4 d m i n to 8 d m i n , as a result of increase
in orientation at the higher line speed. There was a considerable increase in shrinkage
with increase in die gap from 250 pm to 700 pm. For the hexene based resins there
was an increase in MD shrinkage with increase in MFI. The highest MD shrinkage
was recorded for the octene based mPE-5 at the higher line speeds. TD shrinkage was
much smaller than the MD shrinkages with the TD shrinkage increasing with
decreasing die gap and increasing MFI for all hexene based films. The lowest TD
shrinkage was recorded for the octene based mPE material and in fact these film
samples actually expanded in the TD direction as a result of heating to 120°C on the
shrinkage tester hotplate. Overall, the main factors affecting the shrinkage of these
films were the line speeds, MFI and co-monomer type. M.W.D. did not appear to
have a significant effect on either MD or TD shrinkage confirming other work on the
effect of molecular characteristics on the shrinkage of mPEs [9].The increase in MD
shnnkage with increasing die gap correlated well with the increase in MD break
strength with increase in die gap for the hexene based polymers.
28
Effect of Orientation on Extrusion Cast Metallocene Polyethylenes
:!
f
1
#::
4
1.-
1
10
1w
+
% @ -
B
+l 8 0 T
::1.-
19PC
+200%
1
1.505
I
E
4
10
1oc
E
m_
w
i....--.
+1gooc
180~C
1
10
10
100
Shear Rate ( 0 . ' )
7igure 14. Effect of shear rate and
temperature on viscosity for mPE-4.
Figure 13. Effect of shear rate and
temperature on viscosityfor mPE-3.
Shear Fete (8.')
I
---t 200oc
Shear Rate (d)
1
100
Shear lbte (8.')
v---.---A
1.w5
10
1
Shear M e (d)
100
1
10
100
Shear Rate (8.')
29
B.G. Millar, P . Laughlin, W.R. Murphy and G.M. McNally
I
c
1
1
10
100
Shear Rate (8.’)
Figure 17. Eflect of shear rate and
temperature on viscosity of LLDPE-2.
(e) Calculation of Rheological and Die Swell Data
The rheological characteristics of all the resins at 180, 190, and 200°C were
determined by measuring the mass flow rate of the melts under different loading
conditions i.e. 1.20, 2.16, 3.26, 6.20, 7.16, and 8.26 kg in the melt flow indexer. The
shear rate and shear stress were then calculated using the following equations.
shear rate
y= 4Q
shear stress
7=
shear viscosity
21
-z
Pa= y
nr
’
Pr
-
where Q is volumetric flow rate, r, orifice radius (1.0475 mm), P, pressure exerted at
the die under the different loading conditions, and 1 is length of die (8 mm).
Rheograms for all the resins were constructed using these data and are shown in
Figures 11 to 17.
Most melts exhibit pseudoplastic non-Newtonian characteristics, i.e. the viscosity
decreases with increasing shear rate. The extent of this shear thinning is described by
the non-Newtonian index (n) of the melt. The non-Newtonian behaviour of melts can
be described using a power law equation.
where 7 is shear stress, k, the general consistency factor, y, shear rate and, n is the
non-Newtonian index. Determination of these values are described in detail elsewhere
[ 101. In this work n was calculated for all the resins using this power law relationship
and these results are shown in Table 4.
30
Effect of Orientation on Extrusion Cast Metallocene Polyethylenes
Activation Energy (kJ/mol)
n
Resin
1
10s"
20s"
40i'
200°C
mPE-1
36.23
29.77
nJa
0.952
mPE-2
34.83
33.49
da
0.93 1
mPE-3
34.23
33.15
32.29
0.932
mPE-4
35.21
34.45
n/a
0.923
mPE-5
36.34
34.41
24.36
0.830
LLDPE-I
28.91
26.32
25.36
0.800
LLDPE-2
25.63
24.82
23.90
0.824
Table 4. Activation energies and nowNewtonian indices.
It is well recognized that the viscosity of some melts are more sensitive to
temperature change than others and this dependency is referred to as the activation
energy of flow (Ea).The viscosity and temperature for most melts may be related by
the Arrhenius equation.
p o = Ae "%T
(5)
where A is a constant of the resin, E,, activation energy of the flow, R, gas constant
(8.3 J K-'mor') and, T is the temperature (K). In order to investigate the effect of
temperature on the viscosity of these resins, E, was calculated from the data at shear
rates of 10,20, and 40 s-' and the results are shown in Table 4.
The percentage die swell of the extrudates was determined by measuring the final
diameter of the rods 30 minutes after extrusion to ensure complete solidification.
Comparison of these measurements was made with the diameter of the orifice
(2.095 mm) and the results were expressed as a percentage change in diameter and are
shown in Figures 18 to 22.
~~
35
-
30 .
3
25-
u)
8
2015-
f
10-
a
5-
0
500
1000
1500
200(
Shear Stress (lpMa)
Figure 18. Effect of shear stress on die
swell at 180°C.
0
500
1000
Shear Stress (10'
1500
2001
Ma)
Figure 19. Effect of shear stress on die
swell at 190°C.
31
B.G. Millar, P. Laughlin, W.R. Murphy and G.M. McNally
30
f
u)
25
20
04
0
I
500
1000
1500
Shear Stress (10' M a )
0
ZOO(
Figure 20. Effect of shear stress on die
swell at 200°C.
1000
1500
Shear Stress (10' ha)
500
200(
Figure 21. Eflect of temperature on die
swell for mPE-3.
30
28
26
24
22
20
18
16
14
12
10
-
( f ) Effect of Shear Rate on Shear Viscosity Dependence on Molecular Features
The shear viscosities exlubited by the resins at different temperatures and shear rates
are shown in Figures 11 to 17. Initially as the shear rate increased, the shear viscosity
decreased for all resins. This behaviour is characterised by the non-Newtonian index,
which was determined for each of the rheograms and recorded in Table 4. The results
show that as the MWD of the resins increases, n decreases. The resins with the
narrower MWD, mPE-1, 2, and 3 (2.1-2.2) had least shear thinning of all the resins.
Conversely, the broadest MWD resins, mPE-5, LLDPE-1, and 2 all show distinctly
greater shear thinning over the shear rate range studied. Dependence of viscosity on
molecular weight was clear with resins mPE-1, 2, and 3. As molecular weight
increased (MFI decreased), a distinct increase occurred in the viscosity profiles of the
resins. Resin mPE-5 had a very different viscosity profile to the other mPE resins.
The increased shear thinning typical of the conventional resins LLDPE-1 and 2, was
32
Eflect of Orientation on Extrusion Cast Metallocene Polyethylenes
due to the inclusion of long chain branching in this particular mPE. Disruption of the
homogenous-branched structure led to improved processability whilst retaining the
typical properties of the metallocene-catalysedresins [7].
(g) Effect of Temperature on Viscosity
The effect of temperature on the viscosity profiles of all the resins is shown in Figures
11 to 17 and Table 4. There was a decrease in viscosity with increase in temperature
over the shear rate range studied. As the shear rate increased the activation energy
was shown to decrease. This is probably due to increase in molecular alignment in the
direction of flow at these higher shear rates. The mPE resins were more shear
sensitive to changes in temperature over the shear rate range studied and this can be
illustrated by the wider spacing of the profiles at each temperature in Figures 11 to 15
in comparison with Figures 16 and 17.
(h) Effect of Load on Die Swell
The effect of increasing the piston load on die swell for all polymers is shown in
Figures 18 to 20. As the load was increased, the die swell also increased for all resins.
Increasing the load on the melt increased the throughput in the melt flow indexer and
consequently decreased the time spent in the die, so the time available for melt
relaxation was reduced, leading to increased viscoelastic recovery of melt on exiting
the die.
(i) Effect of Temperature on Die Swell
The results in Figures 2 1 to 22 illustrate a clear dependence of die swell on extrusion
temperature. As the temperature increased, the die swell decreased, and thls affect
was more pronounced for the conventional LLDPE-1. It has been reported that at
higher temperatures molecular chain motion was quickened and the viscoelastic
relaxation process was shortened and die swell decreased [ 111.
(j)Effect of Molecular Structure on Die Swell
Apart from the affect load and temperature had on the die swell exhibited by the
resins, the results also show that molecular characteristics also mfluenced the degree
of die swell each resin experienced. LLDPE-I and 2 exhibited greater die swell in
comparison to the mPE resins. These broader MWD resins (3.5-3.8) had a less regular
structure than the mPE resins and therefore had more complex relaxation times. This
increased the internal stresses in the LLDPE resins. As the melt was forced through
the die, the induced stresses did not relax in the available time and caused swelling on
exit. As the mPE resins had shorter relaxation times and were less viscoelastic, they
experienced lower levels of die swell. The influence of MWD can also be seen with
mPE-4 (3.1) and mPE2 (2.2), which had identical MFIs. The broader MWD resin,
mPE-4 experienced greater die swell compared to the narrower MWD mPE-2. Resin
mPE-5, although a metallocene-catalysed resin, had long chain branching which
resulted in increased chain entanglement during shear flow thus increasing relaxation
times causing the resin to behave more like LLDPE in terms of die swell and viscosity
profile. Higher molecular weight resins had much greater chain entanglement than
lower molecular weight resins. The results show that as molecular weight increased
die swell also increased for the hgher molecular weight (M,) resins mPE-1, 2 and 3.
33
B. G.Millar, P. Laughlin, W.R.Murphy and G.M. McNally
Conclusions
The effect of die gap and haul off speed on the mechanical and thermal properties of a
range of cast films manufactured from commercially available mPEs have been
discussed. Processing conditions, molecular weight, M.W.D. and co-monomer type
were shown to affect the mechanical properties, crystalline development and
shnkage of the films. DSC analysis highlighted that as die gap and line speed were
increased crystallinity increased. Break strength increased with increasing die gap and
haul off speed. Higher molecular weight polymers with narrow M.W.D. were also
shown to have hgher break strengths. Film tear strength decreased with increasing
haul off speeds and decrease in MFI. Higher draw down ratios favoured TD tear
strength. Higher MD shrink films were produced at the higher line speeds and wider
die gaps, The calculation of the non-Newtonian flow index showed that as the MWD
of the resins increased, the extent of shear thinning exhibited by the resins increased.
The conventional LLDPEs and mPE-5, whch had some degree of long chain
branching content, all showed considerably more shear thinning than the other
metallocene-catalysed resins. Molecular weight influenced the rheological
characteristics of these polymers. As molecular weight increased in mPE-1, 2, and 3,
the viscosity of the resins was shown to increase. This is indicative of the greater
chain entanglements present in these higher molecular weight resins. Temperature
was also shown to have a strong effect on the viscosity of a resin. An increase in
temperature resulted in lower viscosities particularly at the lower shear rates. The
viscosity of the LLDPEs was found to be less temperature sensitive than that of the
mPEs. Die swell analysis showed that increasing the shear stress gave an increase in
die swell. Increase in melt temperature resulted in only slight decreases in die swell.
These factors are due to the viscoelastic nature of the polymer melt. As the flow rate
increased, the residence time in the die reduced which in turn reduces the time
available for the internal stresses created by flow in the die to relax causing the elastic
response of die swell. The increased temperature allowed melt relaxation within the
die, thus reducing the potential for die swell. The results also highlight that die swell
increased with increasing molecular weight. The extent of die swell was affected by
the molecular characteristics of the resin. The broader MWD resins were found to
exhibit increased die swell in comparison with the narrower MWD resins. The
mPE-5, with the long chain branching exhibited the greatest die swell for all the
polymers studied in this investigation. The benefit to processabilty achieved by
incorporation of the branching caused this resin to swell like a conventional LLDPE
rather than a metallocene-catalysed resin due to its increased chain entanglement.
References
1. Majumdar, A., and Kale, D.D. 2001. Properties of films made from ternary blends of mctallocene and
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Efect of Orientation on Extrusion Cast Metallocene Polyethylenes
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35
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