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The Effect of Extrusion Processing Conditions on the Properties of Blown and Cast Polyolefin Packaging Films.

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Dev. Chem. Eng. Mineral Process., 1I (1/2), pp. 137-146, 2003.
The Effect of Extrusion Processing
Conditions on the Properties of Blown
and Cast Polyolefin Packaging Films
M. Billham, A.H. Clarke, G. Garrett, G.M. McNally"
and W.R. Murphy
Polymer Processing Research Centre, The Queen 's University of
Belfast, Stranmillis Road, Belfast BTP 5AH, Northern Ireland, UK
Thin mono-layerfilms of metallocene catalysed polyethylene, linear low density and
conventional low density polyethylene, as well as polypropylene, were producedfrom
a 38mm extruder through a 75mm diameter blownplm die and a 6OOmm castfilm die.
By using the same die gap on each die to achieve equal draw-down ratios, the
influence of orientationj?om the two processes on the mechanical properties of each
firm was investigated. Tensile strength at break, Young's Modulus, percentage
elongation at break, tear propagation resistance, in both machine and transverse
direction all gave significant differences in properties with cast film when compared
to different blow-up ratios for blown film. Di@erentiaI Scanning Calorimetry was
used to measure the percentage of ctystallinity in eachfilm. Differences were found
to show that the cooling process as the melt exits the die has a signtficant effect on the
percentage of crystallinity.
Introduction
Over the past decade or more the use of linear low density polyethylene (LLDPE) and
metallocene-catalysed polyethylenes (mLLDPE) have been used to replace
conventional low density polyethylene (LDPE) in thin film packaging applications
due to their higher tensile strength and suitability for down-gauging due to better
draw-down. This has led to considerable reduction in the weight of polyethylene, up
to 30% in some packaging film applications. Blown film and cast film extrusion are
the two most common techniques used worldwide for producing flexible thin films.
* Authorfor correspondence.
137
M. Billham, A . H. Clarke, G. Garrett, G.M. McNally and W.R. Murphy
In the blown film process, polymer melt is extruded through an annular die and the
melt is drawn upwards and cooled by passing through an air cooling ring. During this
drawing and cooling process, the melt tube formed from the die is normally inflated
by air to four or five times the diameter of the annular die. The degree of bubble
inflation is referred to as the Blow-up Ratio and this inflation process enables
transverse orientation of the polymer melt during cooling.
In the cast film process, the polymer melt is extruded through a straight die and
on exiting the lip of the die the polymer melt is quenched by contacting a temperature
controlled chill roll system.
Although there are many reports on the effect of extrusion conditions for blown
film [1-3] and cast film (4, 51 extrusion, very little information is currently available
on the comparison of the two processes. Therefore there is an urgent need to quantify
the difference in the mechanical performance of the different polyolefins
(polyethylenes, polypropylenes) when processed into blown film and flat cast film.
Many improvements have been made in the cooling efficiency on blown film
extrusion lines however the rapid quench rate of cast film offers a more significant
and controllable advantage to the film properties affected by rapid quenching.
Although it is widely recognised in the industry that cast LDPE film, for example,
produces a slightly stiffer and clearer film when compared to blown film, the relative
difference for a particular polyolefin, and in comparison to other polyolefins is not
well quantified for the two film extrusion techniques.
The main aim of this investigation is therefore to quantify the effect of extrusion
processing systems and conditions on the mechanical performance, crystallinity and
optical properties [6] of a range of polyolefins including conventional low density
polyethylene (LDPE), linear low density polyethylene (LLDPE), metallocene
catalysed polyethylene (mPE) and polypropylene (PP).
Experimental Details
(a) Marerials
Samples of polymer systems were supplied by Exxon Chemicals, Dow Plastics and
Quarain. The material properties including density and melt flow indices are shown
below.
Table 1. List of materials and suppliers.
138
Dens@
Melt Flow Index
(g/cm7
(gll Omin)
Escorene
0.923
1.2
Dow Plastics
Elite
0.925
0.85
LLDPE
Exxon Chemicals
Escorene
0.919
1 .o
PP
Quarain
0.910
2.1
Material
Producer
Trakname
LDPE
Exxon Chemicals
mLLDPE
Efect of Extrusion Processing Conditions on Properties of Polyolefins
The polyethylene based resins were chosen for having similar densities and
similar Melt Flow Indices (MFI) with a polypropylene resin as a comparable
polyolefin.
(6) Thinfirm preparation
Cast films were produced from all resins using on a Killion KN150 38 mm extruder,
fitted with a general purpose PE screw (LID 30, 3: 1 compression ratio) attached to a
600 mm wide coat-hanger type flat sheet die. During the trials the die gap was pre-set
to 800 pm and the extruder was run at a constant screw speed of 30 rpm. On exiting
the die the melt was quenched on a temperature controlled polished chill roll
maintained at a temperature of 32°C and this was located at a distance of 110 mm
from the die lips. The extruded film was then passed through a pair of nip rolls onto a
winder. The speed of the chill roll and nip rolls were adjusted to ensure constant film
thicknesses of 50 pm. The barrel temperatures were profiled from 180°C at the feed
end up to 210°C at the die for the polyethylenes. Since the shear viscosity of PP is
higher than for the polyethylenes, the barrel temperatures were 200°C to 220°C
respectively for polypropylene.
Blown films were produced from all resins using the same Killion extruder
attached to a 75 mm diameter spiral mandrel type blown film die. An annular die gap
was 800 pm and the extruder was set to a constant screw speed of 20 rpm. The melt
was cooled on exiting the die using a single-orifice cooling ring. The haul-off rate
was adjusted to produce 50 pm films at four Blow-up Ratios of 1.1, 1.5, 2 and 2.5.
The extruder barrel and die temperatures were profiled once again from 185°C at the
feed end up to 210°C at the die for the polyethylenes and 205°C to 220°C for the
polypropylene.
Table 2 below shows variation in the recorded melt temperatures, melt pressures
and frost line height (FLH) during the extrusion trials for each film at all BUR’S.
Table 2. Variation in recorded melt temperatures, melt pressures
andfrost line height (FLH)for eachfilm at all BURS.
139
M. Billham, A.H. Clarke, G. Garrett, G.M. McNally and W.R. Murphy
(c) Film tensile analysis
All tensile tests were performed using an Instron 441 1 Universal Tensile Tester
according to BS2782, using a crosshead speed of 500 mm/min and an initial gauge
length of 50 mm. Ten samples were tested in both machine and transverse directions.
Tensile strength at break, Young's Modulus and percentage elongation at break were
recorded.
(a) Tear strength analysis
All tear tests were performed using an Instron 441 1 Universal Tensile Tester
according to the Tear Propagation Resistance standard ASTM D1938-94, using a
crosshead speed of 250 mm/min and the single-tear method (trouser tear) test piece.
Six samples were tested in each direction, the mean tear propagation strength was
recorded.
(e) Differential scanning calorimetric analysis
In order to investigate the effect of extrusion processing conditions, polymer density
and polymer type on the overall crystallinity of the various films, samples of the films
were prepared for differential scanning calorimetric analysis. Tests were performed
using a Perkin Elmer DSC-6thermal analyser, samples were heated from 40°C to
200°C at a temperature increase of 10°C mK'. The crystalline melting temperature
was determined and the degree of crystallinity was calculated using a AH value of
198 M g-' for 100% polyethylene and 86 kJ g-' for polypropylene.
(f) Shrinkage analysis
Many polyethylene films are used for shrink wrap applications and in order to
investigate the effect of process Shrinkage tests were performed on a Ray-Ran Hot
Plate Heat Shrinkage Apparatus according to the manufacturers test procedure and the
hot plate was set to a temperature of 130°C for the polyethylenes. Four samples of
each film were tested and the shrinkage of the 50 mm diameter discs in both the
machine and transverse direction was recorded.
(g) Haze and gloss analysis
Aesthetic properties such as clarity and gloss in packaging films are now regarded as
extremely important in the marketing of products. Gloss was measured at angles of
20" and 60" using a BYK-Gardner micro-TRI-gloss reflectometer in accordance with
ASTM D523. Observations were also recorded with regard to the clarity and haze of
each film.
140
Efect of Extrusion Processing Conditions on Properties of Polyolefins
Results and Discussion
(i)
Tensile Properties
The effects of extrusion processing conditions on the tensile stress at break, in both
machine and transverse direction, of cast film and blown films at different blow-up
ratios for the different polyolefins are shown in Figure 1. The increase in blow up
ratio in LDPE, LLDPE and PP showed a gradual decrease in the tensile strength at
break in the machine direction due to the reduction in molecular orientation. The
tensile strength of mPE was not influenced by B.U.R. in the MD, but showed a
gradual decrease in the TD tensile strength as B.U.R.was increased. With mPE the
higher frost line height at a BUR of 2.5 and the slower haul off speed, (see Table 1)
resulted in a greater relaxation time between the die and the frost line resulting in
chain disentanglement and relaxation to give less orientation and a lower tensile
strength. Overall the tensile strength of mPE and LLDPE was significantly higher
than LDPE due to the closer chain packing ability of the shorter branched polymers.
PP had comparable tensile properties to mPE and LLDPE. The cast films have the
highest tensile strength in the TD when compared to the blown films especially the
mPE and LLDPE, apart from the lowest BUR in LDPE.
Machine Direction
50
,
Transverse Direction
I
I
Figure 1. The effect of extrusion processing conditions on the tensile stress at break,
in both machine and transverse direction, of cast film and blown films at
dgerent blow-up ratiosfor the different po&olefins.
The effect of extrusion processing conditions on Young’s Modulus, in both
machine and transverse direction, of cast film and blown films at different blow-up
ratios for the different polyolefins are shown in Figure 2. The cast films have the
lowest Young’s Modulus in both MD and TD than the blown films on all polyolefins
tested. There was very little difference in modulus, i.e. stiffness in either direction.
LDPE has comparable modulus to mPE and slightly higher than LLDPE. Only the
modulus of LDPE is influenced by BUR as it is decreased in both directions with
increase in BUR. The modulus of PP is typically more than twice the modulus of the
polyethylenes.
I41
M. Billham, A. H. Clarke, G. Garrett, G.M. McNally and W.R. Murphy
Machine Direction
Transverse Direction
600
&OO
v
-2400
$300
P200
F
a100
>
0
LDF€
mpE
LLOPE
LDF€
FP
mFE
L L E
PP
Figure 2. The efsect of processing conditions on the Young's Modulus, in both
machine and transverse direction, of cast film and blown films at
dflerent blow-up ratiosfor the difserentpolyolefins.
The effect of extrusion processing conditions on elongation at break, in both
machine and transverse direction, of cast film and blown films at different blow-up
ratios for the different polyolefins is shown in Figure 3. The elongation of LLDPE is
approximately 20% higher than mPE and up to double that of LDPE, PP is very notch
sensitive during sample preparation and is difficult to stretch at room temperature.
Only the cast film of PP stretched suggesting a low orientation and crystallinity is
required to allow any stress thinning. Elongation was greater in all polyethylene films
in the TD with less orientation due to less draw-down from the die gap in this
direction. The cast films with no orientation in the TD gave the highest percentage
orientation compared to the blown film.
Machine Direction
Transverse Direction
1200
1200
rlOOO
ClOOO
.Q
.Q
c
(
Y
% 800
% 800
4 600
3 600
S 400
&?
400
200
0
200
0
LDFE
mpE
LLDF€
FP
LDPE
mpE
LLDPE
FP
Figure 3. The effect of extrusion processing conditions on the percentage elongation,
in borh machine and transverse direction, of cast film and blown fdms at
different blow-up ratiosfor the different polyolefins.
I42
Effect of Extrusion Processing Conditions on Properties of Polyolefins
(ii) Tearproperties
The effect of extrusion processing conditions on tear propagation resistance, in both
machine and transverse direction, of cast film and blown films at different blow-up
ratios for the different polyolefins are shown in Figure 4. The tear strength of LLDPE
was generally higher than that of mPE, which was higher than LDPE. The tear
strength in the TD was higher in all the films than that in the MD. Apart from PP the
tear strength of all the cast films was always lowest in both directions, with the blown
film at a BUR of 2.5 giving the highest tear strength in the MD for mPE and LLDPE.
The mPE and the LLDPE gave a tough tear with a tendency to stretch along the tear
in both directions whereas LDPE gave a straight ‘splitty’ tear in the TD and tended to
tear across the film in the MD. Cast PP gave a comparable tear strength to LDPE in
the MD but the PP blown films resulted in a very poor tear resistance. The tear
resistance of PP in the TD was good for the cast film and the BUR of 1.1 but dropped
significantly as the blow up ratio is increased.
Figure 4. The eflect of extrusion processing conditions on the fear strength, in both
machine and transverse direction, of cast film and blownfilms at diferent
blow-up ratiosfor the duerent polyolefins.
(iii) Shrinkage
The effect of extrusion processing conditions on film shrinkage at 130°C,in both MD
and TD, of cast film and blown films at different blow-up ratios for the different
polyolefins are shown in Figure 5 . The results show that as the BUR is increased the
film shrinkage decreases in the MD and increases in the TD. This is most evident
with LDPE due to a lower crystalline melting temperature (T,) than mPE and
LLDPE. Table 3 shows that LLDPE has a lower T, than the mPE, the heat shrinkage
results show no significant difference.
I43
M. Billham, A. H. Clarke, G. Garrett, G.M. McNally and W.R. Murphy
Figure 5. The efect of extrusionprocessing conditions on the heat shrinkage, in both
machine and transverse direction, of castfilm and blownfilms at diferent
blow-up ratiosfor the differentpolyolejns.
(d) Crystallinity
Figure 6 shows the effect of extrusion conditions on percentage crystallinity for each
film, as measured using DSC analysis. The percentage crystallinity of mPE, at
between 38% and 47%, was higher than that of LLDPE and LDPE at around 32% to
38%. The cast film compared to the blown films had the lowest level of crystallinity
due to the temperature of the chill roll, better contact with the film surface and a more
efficient cooling medium than air cooling. BUR of 2.5 for LDPE, LLDPE and PP
were shown to have the highest levels of crystallinity. The crystallinity of mPE was
not seen to be affected by BUR in blown film even with an increase in the frost line
height. There is a significant increase, from 32% to 38%, in the crystallinity of
LLDPE at a BUR of 2.5 from the cast and lower BUR respectively. PP shows a
general trend of increase in crystallinity with increase in BUR.This is due to keeping
a constant FLH and the haul off running slower to maintain 5 0 ~ mthere was more
time for crystalline development in the melt, between exiting the die and cooling to
below the crystallisation temperature.
(e) Gloss and clarity.
The effect of extrusion processing conditions on the gloss and clarity of cast film and
blown films at different blow-up ratios for the different polyolefins are shown in
Table 3. Both cast films of mPE and LLDPE exhibited excellent gloss and clarity.
The gloss and clarity of the mPe and LLDPE was greatly reduced even at the lowest
BUR and further reduced as the BUR was increased. This can be correlated with the
increase in crystallinity from the cast to the highest BUR and concluded that the
crystallinity of mPE and LLDPE is highly dependent upon the cooling rate on exiting
the die and is directly related to the optical properties. The gloss and clarity of LDPE
is superior in the cast film than blown film but is not significantly affected, unlike
mPE, LDPE and PP, by increasing BUR even though the crystallinity is highest in the
highest BUR. PP is clear in the cast film but even at the lowest BUR is very hazy and
does not appear to deteriorate at higher BUR. This is because even at the low BUR
I44
Esfect of Extrusion Processing Conditions on Properties of Polyolefins
minimum cooling was necessary to achieve a stable bubble, therefore allowing the
film to cool at a slow rate and crystallinity to develop.
Figure 6. The eflect of extrusion processing conditions on the degree of crystallinity
of castjilm and blown films at dgerent blow-up ratios for the diflerent
polyolefins as measure by DSC analysis.
Conclusions
This work investigates the effect of processing conditions during blown film extrusion
and cast film extrusion on the mechanical performance, related crystallinity and
optical properties for a range of polyolefins including LDPE, LLDPE, mPE and PP.
The results show that the Young’s Modulus of films manufactured using the cast
film extrusion process were much lower than for films manufactured using the blown
film process. DSC analysis showed that these films generally had lower crystallinity,
which would tend to c o n f m this recorded reduction in modulus.
The mPE films were shown to have the highest stress at break, highest Young’s
Modulus and relatively high elongation (800%) in comparison to the other
conventional polyethylenes. PP was shown to have the overall highest modulus in
both MD and TD, which would tend to indicate that this was a much stiffer film, and
this is reflected in the relatively high crystallinities observed for this polymer
especially at higher blow-up ratios.
The conventional LLDPE was shown to have the highest tear strength of all the
polymers studied especially in the MD. The tear strength of PP was shown to be
much lower than the polyethylenes in the MD, especially in blown films. The results
also show that the MD tear strength for polyethylenes is dependent on BUR,with tear
strengths increasing with higher BUR for the mPE and LLDPE, and a decrease in MD
tear strength with increasing BUR being recorded for LDPE.
The LDPE resin was shown to have the highest MD shrinkage values of all the
polymers investigated, with slight decrease in MD shrinkage being recorded for
increase in BUR.
M.Billham, A.H. Clarke, G. Garrett, G.M. McNally and W.R. Murphy
The effect of extrusion processing conditions showed that the films manufactured
using the cast film technique exhibited more clarity and gloss than films
manufactured using the blown film technique. The gloss and clarity of the mPE and
LLDPE was greatly reduced in blown film even at the lowest BUR. and reduced
further as the BUR was increased. These optical properties are seen to be closely
related to the changes in percentage crystallinity observed for all the films.
Table 3. The efSect of extrusion processing conditions on the gloss and clarity of cast
film and blownfilmsat diferent blow-up ratiosfor the different polyolefins.
References
1.
2.
3.
4.
5.
6.
146
Beagan, C.M., McNally, G.M.. and Murphy, W.R. 1998. Metallocene Catalysed Polyethylene in
Blown Film Applications. Antec, pp. 128-133.
Beagan, C.M., McNally, G.M,. and Murphy, W.R. 1998. The Blending and Co-extrusion of
Metallocene-catalysed Polyethylene in Blown Film Applications. Anfec, pp 90-94.
Snell, P. 1982. Comparison of Properties of LDPE, LLDPE, HDPE. Film Extrusion Conference
Proceedings, TAPPl Press, Atlanta, pp.5-10.
Canning, K., and Baigui, B. 1999. Film Casting Low Density Polyethylene Melt. Anrec, pp.386-390.
Degroot, A.T., Doughty, K., and Stewart, B. 1994. Effects of Cast film Fabrication Variables on
Structure Development and Key Stretch Film Properties. Journal of Applied Polymer Science,
Volume 52, pp.365-376.
Johnson, M.B., Wilkes, G.L., Sukhadia, A.M., and Rohlfing, D.C. 2000.Optical Properties of Blown
and Cast Polyethylene Films: Surface Versus Bulk Structural Considerations. Journal of Applied
Polymer Science. Volume 77, pp.2845-2864.
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