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The Characteristics of Polyethylene Film for Stretch and Cling Film Applications.

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Dev. Chem. Eng. Mineral Process. 12(1/2), pp. 5-20, 2004.
The Characteristics of Polyethylene Film for
Stretch and Cling Film Applications
C.M. Small, G.M. McNally, G. Garrett, and W.R. Murphy
Polymer Processing Research Centre, Zhe Queen 's University Belfast,
Stranmillis Road, Belfast BT9 5AH, Northern Ireland, UK
Part I. A range of polyethylene films were prepared from metallocene linear low
density polyethylene (m-LLDPE). linear low density polyethylene (LLDPE) and ultra
low density polyethylene (ULDPE) resins, containing 0 and 8% polyisobutylene
(PIB). FTIR, DSC and mechanical analysis techniques were used to investigate the
effect of co-monomer type, density and melt flow index (MFI) on the mechanical
performance, orientation and crystallinity of these films.The study established that
co-monomer type and MFI were the greatest factors influencing mechanical
performance and crystallinity. Crystallinity was found to be the most influential factor
governing PIB migration in these films and this in turn was related to polymer type,
density and MFI. High MFI, octene co-monomer films exhibited the highest
orientation, tear resistance and tack strength and would therefore be suitable f o r
stretch film applications. Ultra low-density polymers gave relatively low tack strength
and poor overall mechanical performance.
Part II. A range of ethyl vinyl acetate (EVA)/m-LLDPEIEVA co-extrudedfilms was
manufactured, with vinyl acetate (VA) co-monomer content of 6, 12 and 18% and PIB
contentfiom 0-20%. The films were aged at 45°C for up to 28 days, to enable tack
(cling) development. The results show that film tack strength improved significantly
with ageing. Increased VA concentration in the surface layer also showed signifcant
improvement in film tack strength. The film tensile strength, elongation and tear
properties in both machine direction (MD) and transverxe direction (TO) were not
significantly affected by increase in PIB concentration. However, increased VA
content showed slight improvement in MD mechanical perJ4ormance of the films, TD
properties were relatively unaffected. Films with 12 to 18% VA in the surface layers
produced high surface tack film and the mechanical performance of these films were
comparable to mono-layer polyethylenes. These films are suitable for stretch wrap
applications and have reduced the overall concentrations of tack additives, though
high VAfilms were more di&ult to process.
C.M. Small, G.M. McNally, G. Garrett and W.R. Murphy
Introduction
The principle of stretch wrapping is that the film is stretched around the article to be
wrapped and the residual tension gives a tight contour wrap. This is made possible by
stretching the film within its elastic region and in this loaded state the polymer
molecules attempt to regain their original conformation and thus exert force on the
article. The common problems associated with these stretch wrap films is mechanical
failure during stretching and optimisation of the surface tack or cling properties, so
that the film adheres to itself instantly and prevents film recovery or loss of the
containment forces. Polyethylenes are the most common resins used in this
application, The mechanical behaviour of LLDPE and m-LLDPE films make them
ideal for cling and stretch wrap film applications; exhibiting high tensile strength,
draw down and enhanced tear and puncture resistance.
The two most common methods of manufacturing thin polymer films are by
blown and cast film extrusion techniques. In cast film manufacture the polymer is
extruded through a slit die and stretched in air, before being cooled rapidly on
polished chrome chill rolls. Most, if not all, draw down and molecular orientation
occurs in the air gap, the distance travelled between exiting the die and contacting the
chill roll. In most commercial processes the air gap is rninimised and haul-off rates
are sufficiently high, such that the polymer spends very little time in this meltstretching zone. The orientation effects, which are frozen-in during melt stretching,
have been shown to have a major influence on mechanical behaviour, crystallinity and
other key film characteristics. The creation of an adhesive or tack surface on stretch
wrap films is also often desirable in packaging applications and this tack can be
achieved by incorporation of polyisobutylene (PIB) as a tackifier agent. The PIB
migrates through the bulk polymer to the surface creating the surface tack. This
migration will be affected by the morphology developed in the film during
manufacture. The effect of PIB concentration and extrusion processing conditions on
the diffusion behaviour of PIB in a range of LLDPE films has been reported [ 1, 21.
Gulmine [3] characterised polyethylene using FTIR spectroscopy, identifying specific
molecular characteristics. Zhang [4] conducted detailed investigation of orientation on
blown films using polarised FTIR techniques. This technique can identify both
crystalline and amorphous orientation in film and is an effective means of collecting
orientation data rapidly.
Unlike the investigations conducted by previous workers, one of the aims of this
work (Part I) was to determine the effects of molecular variables such as, comonomer type, MFI and density on the mechanical performance, orientation and
crystallinity of thin (25 pm) cast polyethylene films. The inclusion of a fixed
concentration of PIB (8 wt%) in the films also permitted investigations of the effects
of polymer characteristics on diffusion in LLDPE.
The principles ofpolarised FTIR orientation analysis
Infrared active components in polymer film absorb energy from the IR wave and the
transmittance through the film can be measured and analysed using Fourier-transform
infrared spectroscopy. The accuracy of the spectra measured will depend greatly on
the choice of scanning parameters, as well as, film thickness and axis alignment
[3,4]. Absorbance peaks at wave number 729 cm" and 718 cm" in polyethylene have
6
Characteristics of Polyethylene Film for Stretch and Cling Film Applications
been shown to represent elements within the chain structure whose transition moment
have been found to be parallel to the crystalline a-axis and b-axis respectively. Figure
l a shows an FTIR spectra for LDPE and the spectra in Figure l b shows the different
contributions in terms of amorphous, a-axis and b-axis crystalline phases in LDPE
[4]. MD and TD orientation can also be quantified using plane-polarisation of the IR
beam, so that the electric vector is aligned parallel or perpendicular to the machine
axis. Orientation analysis, using FTIR techniques [5-71, on blown film and bi-axially
orientated films, showed good correlation of tear resistance with blow-up ratio and
haul-off rates.
- 8% PIB
3500
2500
3000
2000
1500
1000
Wave Number cm”
Figure la A typical FTIR absorption spectra for LDPEfilm (0% and 8% PIB).
150
740
730
720
710
700
Wavenumber
Figure Ib. Decomposition ofpolyethylene curve at 718 and 729 cm”
wavenumber into a-axis, b-axis and amorphous phase contributions.
In the past stretch wrap and cling films have been manufactured from LDPE,
EVA and polyvinyl chloride (PVC). Recently however, Lipsitt [8] has compared the
mechanical performance of m-LLDPE with both EVA and flexible PVC. This work
has shown that m-LLDPEs exhlbit higher tensile strength, elongation and tear
resistance than EVA or PVC films, and can be processed at a substantially lower
7
C.M. Small, G.M. McNally, G. Garrett and W.R. Murphy
thickness. Polyethylenes however, require adhesive additives to develop significant
tack strength for use in cling film applications. In contrast EVA films are inherently
tacky and the tack strength is dependent on the ratio of ethylene to VA co-monomer,
the higher the VA content the stronger the tack performance.
To compensate for the physical and mechanical inadequacies of these materials
in stretch or cling film applications, polymer blends and co-extrusion techniques are
being used to improve the performance of these products. In co-extrusion the
properties of each polymer is not sacrificed by the inclusion of the other, as each is a
stand-alone entity in a composite sandwich type structure. This technique has been
successfully developed to produce film with specialist properties for specific
applications, by combining polymers with desirable properties, for example PE and
EVA for stretch and cling film. Theoretically, EVA formulations can vary from
1-99% VA co-monomer, but the majority of commercially available EVA contains
less than 50% VA. EVAs used in film manufacture generally contain about 5% VA,
with film toughness, as well as, tack strength increasing with increased VA
concentration (up to 20% VA). However, there is a limit to the acceptable levels of
VA in the films. High VA concentration can lead to film blockmg during
manufacture, and in cast film extrusion, adhesion to the chill roll increases line
tension and can cause differential cooling characteristics leading to changes in
mechanical performance.
This present work (Part 11) investigates the modification of m-LLDPE stretch
film, by using cast film co-extrusion techniques, to produce stretch film exhibiting
tacky or self-adhesive surfaces. The performance of 25 micron EVNm-LLDPE/EVA
co-extruded films and the effect of VA concentration (6-18% VA) on the mechanical,
morphological and tack behaviour of the films are reported. The thin EVA surface
layer (2.5 pm),with low VA concentration, provides inherent film tack strength. The
incorporation of increasing concentrations of polyisobutylene (PIB) (0-20%
masterbatch) into the EVA layers permits greater tack strength to develop by PIB
surface diffusion after extrusion. This eliminates processing problems associated with
extruding and collecting auto-adhesive films. Reports [ 1,2] identified the morphology
developed in the film and the post extrusion storage conditions as the main factors
influencing PIB tack development. Other studies [9, 101 have shown that the
amorphous/crystallineratio in EVA films increases significantly as the VA content is
increased. However, it was also shown that surface polarity was modified and t h s
could alter the diffusion characteristics of small molecules through EVA. This work
also reports on the diffusion characteristics of PIB as the VA content is increased.
Experimental Details
(a) Materials
The properties of the various LLDPEs used in Part I of this investigation are detailed
in Table 1. All blends were produced containing 8% PIB masterbatch (Polytechs SA).
The components were mixed thoroughly prior to being fed directly into the feed
hopper of the extruder.
8
Characteristics of Polyethylene Film for Stretch and Cling Film Applications
I
I
A
B
C
D
E
I
I
G
H
I
Metallocene
Metallocene
Metallocene
Metallocene
Metallocene
ULDPE
Metallocene
Metallocene
I
I
I
I
Hexene
Hexene
Hexene
Hexene
Octene
Octene
Octene
Butene
I
I
1
I
0.918
0.918
0.9 18
0.918
0.917
I
I
0.903
0.920
0.905
4.5
2.5
2.5
1.1
4.0
1.5
0.85
1.o
Material
Type
m-LLDPE
Producer
Vinyl
Acetate
Density
g/cm3
MFI
g/ 10 min
Exxon
nla
0.918
2.5
EVA-6
EVA- 12
Exxon
Exxon
Exxon
Polytechs
6.5% (wlw)
12 (WIW)
18 (wlw)
da
0.926
0.934
2.5
2.5
0.939
0.912
1.7
200-300
EVA- 18
I
I
9
C.M. Small, G.M. McNally, G. Garrett and W.R. Murphy
Part II. Co-extruded films were manufactured on a Killion co-extrusion system,
using a KN150 38 mm Extruder for the mLLDPE (with a general purpose screw L/D
30, 3: 1 compression ratio) and a KNlOO 25 mm Extruder for the EVA (with a general
purpose screw WD 30, 3: 1 compression ratio). The extruders were fitted to a 600 mm
flexible lip sheet die. A feedblock system attached to the die gave a film configuration
of ABA (EVA/mLLDPE/EVA). The temperature profile of the mLLDPE extruder
was ramped fiom 225°C at the feed section to 230°C at the die and the screw speed
was held constant at 15 rpm. The EVA extruder was ramped from 180°C at the feed
section to 200°C at the die and the screw speed was held constant at 12.5 rpm. The
chill roll temperature was maintained at approx 10°C, with an air gap of 100 mm. A
rubber-coated roller was used to press the hot film extrudate onto the chill roll to
ensure uniform cooling. Haul-off ratio and nip roll speed was adjusted to maintain
25 pm film throughout the trials.
(c) Film Tensile Analysis
All tensile samples were tested according to ASTM D882-95. Tests were performed
using an Instron 441 1 Universal Tensile Tester, with a load cell of 0.1 kN and a
constant cross head speed of 500 d m i n . Samples were tested in both the transverse
(TD) and machine (MD) axis of the film. Tensile strength at break and Young's
Modulus were recorded.
(d) Differential Scanning Calorimetry
Differential scanning calorimetry was used to investigate the effect of polymer type
on the crystalline development of the film. Tests were performed on all samples using
a Perkin Elmer DSC-6. Samples were heated from 4OoC to 140°C at a rate of
10"C/min. The latent heat of fusion (AH J/g) was calculated for each sample.
(e) FTIR Orientation Analysis
Spectra were obtained using a Perkin-Elmer FTIR spectrometer (Spectrum 1000)
fitted with a zinc selenide Graseby Specac 25 mm, ring mounted, wire grid polariser.
The equipment was located in a laboratory maintained at 25 f 1°C. The instrument
was operated with a resolution of 2 cm-' and an accumulation of 128 scans. The IR
absorbance scans were analysed between 710 cm-' and 740 cm-', for changes in the
intensity of absorbance peaks with the polariser gridlines aligned at 0 and 90" to the
MD direction of the film, i.e. parallel and perpendicular to the MD.
(f) Tear Strength Analysis
All tear samples were tested according to ASTM D1938-94, single-tear method
(trouser tear). Tests were performed using an Instron 44 11 Universal Tensile Tester,
with a load cell of 0.1 kN and a constant cross head speed of 250 d m i n . Samples
were tested in both the transverse (TD) and machine (MD) axis of the film and tear
propagation strength was recorded.
10
Characteristics of Polyethylene Film for Stretch and Cling Film Applications
(g) Film Tack Analysis
Film samples 100 x 250 mm, were laid in a double film ply, free from air pockets and
film imperfections. Samples were placed between thin metal sheets and a 1 kg weight
applied to the samples. The tack properties of the various films were recorded by
measuring the force required to peel these apart at a 90" angle, as shown in Figure 2.
The tests were performed using an Instron 4411 Universal Tensile Tester at a
crosshead speed of 2 5 0 d m i n and grip length of 50with a load cell of 0.1kN.
Samples were conditioned in an air-circulating oven at 45°C for up to 28 days prior to
analysis in order to allow development of the tack.
Figure 2. Tack strength analysis.
Results and Discussion
Part I,Investigating the Effects of Molecular Characteristics on Polyethylene Films
(a) Tensile Properties
The effect of molecular characteristics on the tensile strength and strain at break of
cast films, in both the MD and TD are shown in Figures 3 and 4. The results show
that the MD break strength of the hexene m-LLDPE films (density = 0.918 g/cm3)
decreases progressively with decrease in MFI. This same effect was shown in the
octene m-LLDPE films, but the decrease was not as significant. There was a similar
trend for TD break strength, although the TD strength for all films was significantly
lower than MD strength. The results show that decreasing MFI caused a progressive
decrease in TD break strength in hexene m-LLDPEs, with only a slight reduction in
TD strength for octene co-monomer LLDPEs.
C.M. Small, G.M. McNally, G. Garrett and W.R. Murphy
15
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15
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om
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B
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Figure 4. The effect of material
Vgure 3. The effect of material
properties on i D and TD break strength. properties on MD and TD strain at break.
Figure 4 gives the strain at break for the range of films. The pattern of strain
versus MFI and co-monomer type is similar to that found for break strength. Results
show a stepped decrease in strain for films with decreasing MFI. In general the
values of strain were quite similar for films with the same MFI, regardless of comonomer properties, however, the TD elongations recorded were much higher for the
octene m-LLDPEs
The effect of molecular characteristics on the Young's moduli for all films is
given in Figure 5 . The hexene co-monomer films were shown to have significantly
lower moduli in both the MD and TD compared to the octene films. In hexene films
the MD modulus decreased gradually with decreasing MFI and the TD modulus
decreased more significantly as the MFI was changed. The highest modulus values
were recorded for the octene films and a similar trend between modulus and MFI
occurred. The Young's moduli of the butene films were considerably lower than all
other films in both MD and TD.
Figure 5. The efect of material
properties on MD and TD tensile
modulus.
:igure 6. The efect of material
properties on film crystallinity.
(b) Crystallinity
The effect of molecular characteristics on the film crystallinity as determined by AH
(J/g) is shown in Figure 6 . The hexene mLLDPE films had considerably higher
crystallinities than the other films. There was a gradual reduction in crystallinity with
Characteristics of Polyethylene Film for Stretch and Cling Film Applications
decreasing MFI for hexene mLLDPE films. A combination of MFI and density
affected the film crystallinity for the octene mLLDPE films and in general, density
had a greater affect on crystallinity than MFI. A very small endothermic peak was
recorded for the butene mLLDPE.
I2
I I
I
p 09
e
4
-MD
08
--I
TD
07
06
05
0.4
I
4
750
740
730
720
710
Wivenumber
Figure 7. Inpared absorbance scans of MD and TD
orientation (polarisation 0 and 90').
(c) FTIR Orientation Analysis
Films prepared using the described extrusion conditions showed a change in FTIR
spectra, similar to that in Figure 7, when the polarisation is changed from 0" to 90".
The effect of molecular characteristics on film orientation as measured by FTIR
analysis is given in Figures 8 and 9. These graphs highlight the peak absorbance
pattern of the FTIR scans for the two identified peaks in polyethylene. In all films the
b-axis orientation (718 cm-') was higher than the a-axis orientation (729 cm-'). This
was probably due to the relatively low haul-off speeds (6.5 dmin), used during
manufacture of these cast films and the resultant low MD orientation.
I .l
Tgure 8. The effect of material
properties on MD and TD orientation
at 718 cm-l @-axis orientation).
I
I
Figure 9. The effect of material
properties on MD and TD orientation
at 729 cm-' (a-axis orientation).
For the hexene co-monomer films there was only a slight difference between MD
and TD, a-axis orientation and b-axis orientation. However there was a significant
reduction in all absorption values for polymers with lower MFI. In the octene-based
films the absorbance at 718 cm-' was notably higher in the TD than the MD,
13
C.M. Small, G.M. McNally, G. Garrett and KR. Murphy
indicating a higher degree of crystalline b-axis orientation in the TD direction. These
same films also showed much higher a-axis orientation in the MD than the TD,
indicating preferential a-axis orientation in the MD. The IR absorbance of the butene
film was similar to a hexene of equivalent MFI.
(d) Tear Properties
The effect of molecular characteristics on the tear propagation resistance of all films
is presented in Figures 10 and 11. In general, film tear strength was higher in the TD
compared to the MD. The hexene films had the lowest overall MD and TD tear
strength of all the films investigated, and in general the tear strength and tear energy
decreased with progressive reductions in MFI for these films. This confirms earlier
work [ 5 ] on LDPE film manufactured by the blown film extrusion process, which
reported that tear propagation behaviour of these films was closely related to the
lamellar arrangement of LDPE film with respect to the MD and TD directions. The
TD tear resistance was proportional to both MD-crystalline a-axis and TD-crystalline
b-axis orientation.
The octene co-monomer films had the highest overall tear strengths and in
general the tear strength and tear energy was considerably higher in the TD compared
to MD. Both MD and TD tear performance of the conventional LLDPE was
significantly higher than any of the mLLDPE films. This again corresponds well to
other work [ 5 ] that showed higher a-axis orientation along the MD and higher b-axis
orientation along the TD, lead to higher TD tear resistance.
Figure 10. The efect of material
properties on MD and TD tear stress.
Figure 11. The effect of material
properties on MD and TD tear energy.
(e) Tack Strength
The effect of molecular characteristics on the tack strength of cast films is given in
Figure 12. The migration of PIB from the bulk to the surface of the films was
characterised by measuring the tack strength as a function of time and the results are a
summary of film tack strength developed after 28 days. The main factors affecting
diffusion in polymers are (a) molecular structure of the polymer, (b) crystallinity and
(c) orientation. The hexene mLLDPE films showed a progressive increase in tack
strength as the crystallinity decreased, permitting easier diffusion of the PIB through
these more amorphous films. Similar tack strengths were recorded for films D, E, F.
However the crystallinity for octene based film E was lower than for D and F. If
difbion were affected solely by crystallinity it would be expected that film E would
14
Characteristics of Polyethylene Film for Stretch and Cling Film Applications
have much higher tack strength. However, FTIR analysis of these films shows film E
has a significantly hgher a-axis and b-axis crystalline orientation than films D and F.
Similar trends were recorded for films G and I, where crystallinity was significantly
low and low tack strengths were developed. However these films also had h g h a-axis
and b-axis crystalline orientation. This would tend to suggest that the PIB diffusion
process was controlled more by orientation factors than overall crystallinity.
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Figure 12. The effect of materialproperties on
film tack behaviour @lms containing 8% PIB).
Part I . Investigating the Performance of EVMm-LLDPIXVA Films
(a) Tensile Properties
The effect of VA content in the EVA layers and PIB content on the tensile strength of
the co-extruded cast films, in both the MD and TD is presented in Figures 13 and 14.
The MD break strength increased progressively with increase in co-monomer content
in the EVA layers. However the PIB concentration showed only a slight increase in
break strength, with increasing PIB content. The TD strength for all films was
marginally higher than MD and the results show increasing VA and PIB content
caused only a slight increase in TD tensile strength.
I
40,
6% VA
11% VA
7igure 13. The effect of vinyl acetate
concentration and PIB content on MD
break strength.
I
40,
6% VA
t
11% VA
I
Figure 14. The effect of vinyl acetate
concentration and PIB content on TD
break strength.
IS
C.M. Small, G.M. McNally, G. Garrett and W.R. Murphy
The MD and TD elongation to break for the films is given in Figures 15 and 16.
There was a slight increase in MD elongation for films with increasing VA. However,
the PIB had a significant positive effect on MD elongation. In general the values of
TD elongation were similar for all films, with the VA and PIB concentrations having
very little effect on TD elongation.
I
6% VA
IIXVA
6% VA
I~%vA
Figure 15. The effect of vinyl acetate
concentration and PIB content on MD
elongation to break.
12% VA
Figure 16. The effect of vinyl acetate
concentration and PIB content on TD
elongation to break.
The effect of VA and PIB content on the Young's moduli for all films is shown
in Figures 18 and 19. The MD modulus decreased significantly with increasing VA
and PIB concentrations. The TD modulus decreased more significantly with increased
VA and PIB concentrations.
These results indicate that the PIB additive was exhibiting a plasticising effect on
the film microstructure, as shown by the progressive decrease in modulus and higher
elongations recorded for films with higher PIB concentrations. The increase in VA in
the outer layers caused a significant decrease in tensile modulus and higher
elongations and this indicated that film crystallinity decreased with increasing VA.
110
,
1
6Y. VA
12% VA
t a n VA
6% VA
12% VA
18% VA
(b) Crystallinity
In-order to investigate the effect of VA on the crystallinity of the EVA, as determined
by AH (J/g), DSC analysis was carried out on all the films. The thennograms in
Figure 19 give the individual melting endotherms of the EVAs, m-LLDPE and PIB
masterbatch. The area under the EVA melt endotherm decreased significantly, as did
the melting temperature, with increasing VA. The same trend occurred for the
I6
Characteristics of Polyethylene Film for Stretch and Cling Film Applications
endotherm of EVNm-LLDPEEVA (2.5pd20pd2.5pm) co-extruded films in
Figure 20. These results suggest the formation of smaller crystalline structures, with
lower overall crystallinity, as VA is increased. Marais et al. [9,101 has reported that
increase in VA content created disorder within the EVA structure, limiting the ability
of PE to crystallise, effectively reducing the crystallinty of the EVA.
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The overall crystallinity (AH J/g) of the range of co-extruded films (c.f. Figure
21) decreased significantly with increasing VA. There was a gradual reduction in
crystallinity with increasing PIB content for films with 6% and 12% VA. The
crystallinity of the 18% VA films increased slightly as PIB content was increased
from 5% to 20%. The LLDPE carrier in the PIB masterbatch may be responsible for
this increase in crystallinity. The crystallinity of the LLDPE reduces the amorphous/
crystalline ratio in the EVA layers and thus overall crystallinity increases. This was
confirmed by DSC analysis, which showed an increase in the area of the higher
temperature peak as the PIB concentration was increased in 18% VA films.
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17
C.M. Small, G.M. McNally, G. Garrett and W.R.Murphy
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Figure 23. The effect of vinyl acetate
concentration and PIB content on TD
tear stress.
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Figure 24,~The effect of PIB content
on peel strength of 6% EVA films
(conditioned at 45°C).
(d) Tack Strength
The migration Gf PIB from the EVA layer to the surface of the films was
characterised by measuring the tack strength as a function of time as shown in Figures
24 to 26, as the effect of VA and PIB cantent on the surface tack strength of films
conditioned at 45°C. In general, an increase in tack strength was recorded for all films
with progressive increase in PIB concentration. In the 12% and 18% VA films, an
increase in PIB content from 5% to 15% caused an increase in tack strength of
approximately 300%. Increasing the PIB content to 20% had a lesser effect with only
a 7-8% increase in tack strength.
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Figure 25. The effect of PI. content
on peel strength of 12% EVA films
(conditioned at 45OC).
Figure 26. The effect of P I . content
on peel strength of 18% EVA films
(conditioned at 45 "C).
Tack strength also increased significantly with increasing VA content. Increasing
VA from 12% to 18% caused a progressive increase in the tack strength of 0% PIB
films. However as PIB concentration was increased the effect of VA was very small,
producing only a slight increase in tack strength. Tack strength improved significantly
with ageing. Films containing high PIB concentrations (15% and 20%) and stored at
45OC, developed maximum tack strength between 14 and 21 days, however beyond
this time the tack strength decreased significantly. These same trends were reported
earlier for PIB diffision in LLDPE film [ 11.
The diffision rate of PIB in 6% VA films was significantly lower than for higher
VA content and no tack was measurable for films with PIB content less than 15%.
However the crystallinity of these films was relatively high which would reduce
diffision of the PIB to the surface, so reducing tack.
18
Characteristics of Polyethylene Film for Stretch and Cling Film Applications
(e) Comparison of Mono-layer and Co-extruded mLLDPE films
Comparing Figures 13-26 with the data in Table 3 shows that reducing the thickness
of the mLLDPE layer in co-extruded films with EVA surface layers caused only a
slight decrease in the tensile strength of these films. Increasing the VA content in the
EVA layer to 18% produced a film with quite similar strength and elongation to a
mono-layer mLLDPE. The co-extruded film exhibited much higher tack strength,
however the tensile modulus and overall crystallinity was significantly reduced, as a
consequence of the VA molecules hindering crystalline development in the EVA
layer. Due to the lower crystallinity of the EVA layer and the high concentration of
PIB, the diffusion and tack development in the co-extruded film was much greater,
with the overall mechanical performance of the film being maintained by the LLDPE
in the core layer.
Table 3. Typical properties of mono-layer mLLDPEfilm. (PIB additive dispersed
throughout entirefilm thickness ( 2 5 ~ ) )
Conclusions
Thls investigation set out two major aims. Part I was to determine the effect of
molecular characteristics on the mechanical performance and the morphological and
orientation characteristics of a range of LLDPE films manufactured by the cast film
extrusion process. Part I1 reports on the effect of VA concentration and PIB content
on the mechanical performance, morphology and tack characteristics of co-extruded
EVA/mLLDPE/EVA films manufactured by the cast film extrusion process.
Thls work has shown that the co-monomer type, density and MFI of polyethylene
had a significant effect on the orientation characteristics of the films, which in turn
could be related to the tensile modulus, break strength and tear resistance of the films.
In general it has been found that crystalline a-axis and b-axis orientation decreased
with progressive reduction in MFI. As the orientation decreased the Young’s
Modulus, percentage crystallinity and tear propagation resistance also decreased.
The effect of co-monomer type on film properties was most evident in the tear
analysis where the octenes had much higher tear energy than hexene films. This may
relate to the longer chain branch structures in octenes, which would incur higher tear
resistance than the shorter chain branching in hexene and butene co-monomers.
The studies of PIB tack development have shown that the diffusion process was
dependent on both crystallinity and crystalline orientation.
These results would suggest that the properties of stretch films could be
optimised using high MFI, octene co-monomer, metallocene or conventional LLDPE,
19
C.M. Small, G.M. McNally, G. Garrett and W.R. Murphy
which developed preferential molecular orientation in the direction of stretch,
maximising the tear resistance and modulus of these films. Films made from these
polymers also exhibited good tack development over the 28-day test and were
relatively straightforward to process. Results indicated that ULDPEs should be
avoided for stretch and cling film applications, as their cling development and tear
performance was relatively poor.
The investigation of co-extruded films has shown tack strength and mechanical
properties for 12% and 18% VA films with 15% and 20% PIB were relatively similar,
although 12% VA had fewer processing problems. EVA content had little effect on
the overall mechanical performance of the film compared to mono-layer mLLDPE
film. EVNmLLDPE co-extrusion produced very good instantaneous tack strength
and with PIB only in the surface layer, it reduced the overall material costs.
The co-extrusion studies indicate that high tack strength can be achieved by PIB
loading in thin EVA surface layers. Results have shown that the 12 and 18% VA coextruded films have similar mechanical properties to the mono-layer polyethylenes.
Therefore EVA co-extrusion with a suitable octene polyethylene, as described in Part
I, would optimise the performance of stretch films.
References
I.
Small, C.M., McNally, G.M., Marks, A. and Murphy, W.R. 2002. The effect of extrusion processing
conditions and polyisobutylene concentration on the properties of polyethylene for stretch and cling
film applications. Antec, 230-234.
2. Small, C.M., McNally, G.M., Marks, A. and Murphy, W.R. 2002. The use of FTIWATR to
investigate the migration of polyisobutylene in polyethylene for cling film applications. Antec, 28822886.
3. Gulmine, J.V., Janissek, P.R., Heise, H.M. and Akcelrud, L. 2002. Polyethylene characterization by
FTIR. Poly. Test. 21,557 563,
4. Zhang, X., Ajji, A. and Jean-Marie, V. 2001. Processing-structure-propeTtiesrelationship of
multilayer films. I. Structure characterization. Poly. J. 42,8179 - 8195.
5. Ajji, A. and Zhang, X.2002. Correlations between orientation and some properties of polymer films
and sheets. Antec, 1651-1655.
6. Ajji, A., Auger, J., Huang, J. and Kale, L. 2002. Biaxial stretching and struchxe of various lldpe
resins. Antec, 1561-1565.
7. Krishnaswamy, R.K. and Sukhadia, A.M. 2000. Orientation characteristics of LLDPE blown films
and their implications on Elmendorf tear performance. Poly. J. 41,9205-9217.
8.
Lipsitt, B. 1998. Performance properties of metallocene polyethylene, EVA and flexible PVC films.
Med. Plast. Biomat. Mag. Sept/Oct.
9. Marais, S., Saiter, J.M., Devallencourt, C., Nguyen, Q.T. and Metayer, M. 2002. Study of transport of
small molecules through ethylene-co-vinyl acetate copolymers films. Part B: COI and O2gases. Poly.
Test. 21,425-431.
10. Devallencourt, C., Marais, S., Saiter, J.M., Labbe, M. and Metayer, M. 2002. Study of transport of
small molecules through ethylene-co-vinyl acetate copolymers films. Part A: Water molecules. Poly.
Test. 21,253-262.
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