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The Manufacture and Performance of Polyethylene-Polyisobutylene Films for Cling Applications.

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Dev. Chem. Eng. Mineral Process., 11(1/2),pp. 169-183, 2003.
The Manufacture and Performance of
Polyethylene-Polyisobutylene Films for
Cling Applications
C.M. Small, G.M. McNally" and W.R. Murphy
Polymer Processing Research Centre, The Queen 's University Bevast,
Stranmillis Road, Bevast BT9 5AH Northern Ireland, UK
and A.Marks
Steve Orr Ltd, Dromore, Co. Down, Northern Ireland, UK
A range of Linear Low Density Polyethylene (LLDPE) films with polyisobutylene
(PlB) content from 2%-8% was manufactured using a Killion blown film extrusion
system and a cast film extrusion system. Thefilms were aged at 25°C 35°C and 45°C
for up 28 days, to enable tack (cling) development. The tack, in both blown and cast
jlms, improved significantly with ageing, at increased storage temperatures and at
higher film blow up ratios. FTIWATR analysis was used to investigate the surface
migration of PIB in the cast films. The results confirmed those determined by
mechanical tack (cling) analysis. DSC analysis showed only a slight decrease infilm
crystallinity with increasing PIB concentration. The film tensile modulus, elongation
and tear properties in both MD and TD were not signijicantly affected by increase in
PIB concentration.
A second series offilms with 8% PIB content was manufacturedfrom a range of
LLDPES. FTIWATR, DSC and mechanical tack analysis were used to investigate the
relationship between polymer properties and migration rates. The study established
that crystallinity was the most influential factor governing PIB migration and this
could be related to polymer density. Co-monomer type was found not to be a factor
influencing migration of PIB.
* Authorfor correspondence.
C.M. Small, G.M. McNally, W.R. Murphy andA. Marks
Extruded polymer films are being continually developed for new industrial and
packaging applications. Stretch or cling film packaging is a major application for
polyethylene and the surface characteristics are often critical to the overall
performance of these films, with a desire for enhanced printing, sealing, cling, etc.
The most common methods of manufacturing these films are by blown and cast
film extrusion techniques. Both these extrusion techniques have their advantages and
disadvantages, in respect of film properties, ease of processability, economy,
efficiency, etc. and studies have been conducted comparing and optimising film
manufactured by both these techniques [l-31. Recent improvements in blown film
technology have greatly improved cooling efficiency. However the rapid quenching
achievable on cast film lines enables greater control of film properties and these may
be more easily optimised to develop desirable film morphology. In the past decade,
linear low density polyethylene (LLDPE) and metallocene polyethylene (mPE) begun
to take over from low density polyethylene (LDPE) as the major commodity polymers
being used in the manufacture of packaging films. LLDPE films exhibit higher tensile
strength, improved draw down and enhanced tear and puncture resistance; these
properties being ideal for stretch wrap applications.
Post-processing surface modification is an extremely cost effective means of
adapting conventional polymers for specialist applications, without having a
detrimental impact on the key bulk properties of the film. Specialist additives may be
blended with the base polymer and then extruded by either blown or cast film
processes. The additive migrates to the film surfaces over time until equilibrium is
reached. This procedure offers numerous commercial advantages since it requires no
further processing of the film and requires no chemical modifications.
The creation of an adhesive or tack surface on stretch wrap films is often
desirable for applications where the film is required to remain in-situ over time,
providing a load retention capability or a sealed environment, e.g. palletising goods.
This can be achieved by incorporation of adhesive or tackifier additives in the
polymer. The diffusion of these additives can be compared to the diffusion or
migration of other small particles in polymer structures [4-61.From the study of slip
agents and stabilisers in film, it has been reported [4] that the migration obeys Fickian
diffusion characteristics. However, there are a number of factors influencing
migration including additive concentration and concentration gradient through the
film cross-section, film structure, compatibility of the polymer/additive mixture,
polymer bulk properties, extrusion processing variables (including co-extrusion
systems) and film conditioning.
This present work investigates the modification of LLDPE film, to produce tacky
or self-adhesive surfaces, by using the migration of polyisobutylene (PIB) from the
bulk of the material to the surfaces. This migration process occurs in the films after
extrusion by blown or cast film techniques. The processes associated with the
development of a cling or tack surface on polymer films has not been widely reported
in the published literature, although there are many patents held by individual
companies regarding the manufacture of such films. A number of different additives
can be used to impart tack on film surfaces [7] and the choice of additive will be
dependent on the particular application.
Manufacture and Performance of PE-PIB Films for Cling Applications
The main aim of this present work was to determine the effect of extrusion
conditions, PIB concentration, storage temperature and ageing on the mechanical
performance, cystallinity and migration characteristics of LLDPE-PIB based films.
Since the structure of the bulk polymer may have considerable influence over
migration, this work also examines the influence of polymer properties on diffusion
rates and specifically changes in polyethylene co-monomer type, polymer density and
melt flow index.
The conventional means of assessing migration in polymers is by mass-sorption
studies, i.e. monitoring the weight up take in polymer film exposed to a high
concentration additive source. However, more recent diffusion studies have utilized
Fourier-transform infrared spectroscopy (FTIR) in the attenuated total reflectance
mode (ATR) [8-121, for monitoring small particle diffusion in polymer film. This
technique can identify the constituents present on the polymer surface if these
additives are infrared active.
The principles of FTIWATR
When an infrared light beam is directed through an ATR crystal, the internal
reflectance produces an evanescent wave. When the polymer film sample is mounted
adjacent to the crystal surface the emitting wave penetrates several microns into the
film. The infrared active components of the surface absorb energy from the wave and
the decrease in intensity of the reflected wave can be measured using Fouriertransform infrared spectroscopy. The intensity of the diffisant peaks will increase
with time and analysis of peak height development, as a function of time can provide
a calculation of the diffusion coefficient. Figure 13 shows a typical FTIWATR scan
of a film surface containing PIB and highlighted in Figure 14, is a double eak
representing PIB components. The peaks occurring at 1365cm-' and 1385 cm- are
assigned to the C-H bond deformation associated with the gem-dimethyl groups of
PIB [ 131.
Experimental Details
(a) Materials
The properties of the materials used in this investigation are detailed in Table 1.
During the blown film extrusion trials using LLDPE, gross melt fracture was
encountered with random bubble distortions and overall bubble instability. This
problem is common during blown film extrusion of LLDPE, and is caused by the
lower elongational viscosities of LLDPE, associated with short chain branching. This
problem has been the focus of many reports [ 14, 151 and the processing problem has
been overcome by blending LLDPE with LDPE, to give enhanced bubble stability
during blown film extrusion. Therefore, in the present work, blends were produced
containing 0 , 2 , 4 , 6 and 8% PIB Masterbatch in an LLDPE-LDPE blend (70/30 w/w).
The components were mixed thoroughly prior to being fed directly into the feed
hopper of the extruder. The properties of the materials used in the second study are
presented in Table 2. Blends of these materials were produced containing 8% PIB.
C.M Small, G.M. McNali), W.R. Murphy and A. Marks
(b) Preparation offilms
Blown films were manufactured on a Killion- KN150 38mm Extruder, with a general
purpose screw (LA3 30; compression ratio 3: l), using a 75 mm annular die with a die
gap of 800 pm.The temperature profile was ramped from 195°C at the feed section to
210°C at the die. The melt was cooled on exit ftom the die using a single-orifice
cooling ring. The screw speed was set to 20 rpm and haul off rate adjusted, to produce
films with constant thickness of 25 pm, at blow-up ratios (BURS) of 1.52.0 and 2.5.
Masterbatch 52 f 2% PIB in LLDPE
Table 1. Material properties for extrusion trials ,
Po&mer type
g / I0 min
Table 2. Material properties for diffusion analysis.
Manufacture and Performance of PE-PIB Films for Cling Applications
Cast films were manufactured using a Killion cast film extrusion system. The
extruder was fitted with a 600 mm flexible lip sheet die, set to a die gap of 250
micron. The temperature profile was ramped from 200°C at the feed section to 220°C
at the die. The screw speed was held constant at 30 rpm. The chill roll temperature
was maintained at approx 15"C, with an air gap of 110 mm. A rubber-coated roller
was used to press the hot film extrudate onto the chill roll. A haul off ratio of 1.04 and
nip roll speed of 7.0 d s was maintained throughout to achieve a 25 pm film.
(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 mdmin. Samples were tested in both the transverse
and machine axis of the film. Tensile strength at break and Young's Modulus were
(d) Tear strength analysis
All tear samples were tested according to ASTM D 1938-94, single-tear method
(trouser tear). 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 250 mm/min. Samples
were tested in both the transverse and machine axis of the film and tear propagation
strength was recorded.
(e) Differential scanning calorimetry
Differential scanning calorimetry was used to investigate the effect of PIB
concentration and extrusion processing conditions on the crystalline development of
the film. Tests were performed on all samples using a Perkin Elmer DSCQ. Samples
were heated from 40°C to 140°C at a rate of 10Wmin. The crystalline melting
temperature was determined and the degree of crystallinity calculated for each
sample, using a reference AH value of 289 kJkg for a 100% crystalline polyethylene.
Film tack (cling) analysis
Film samples 100 mm 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 was
investigated by measuring the force required to peel these apart at a 90" angle, as
shown in Figure 1. The tests were performed using an Instron 44 1 1 Universal Tensile
Tester at a crosshead speed of 250 m d m i n and grip length of 50 mm, with a load cell
of 0.1 kN. In order to investigate the effect of storage temperature and time on the
tack properties of the various films, samples were conditioned at 25"C, 35°C and
45°C for up to 28 days prior to analysis.
I 73
C.M. Small, G.M. McNalk, W.R. Murphy and A. Marks
Figure 1. Tack strength analysis.
(g) FTIRA TR analysis
Spectra were obtained using a Perkin-Elmer FTIR spectrometer (Spectrum 1000)
fitted with an ATR diffusion cell. The equipment was positioned in a laboratory
maintained at 25 rt 1°C. A ZincSelenide (ZnSe) ATR crystal, having dimensions
50 mm x 20 mm x 2 mm,an angle of incidence of 45" and a reltactive index of 2.4,
was fitted to the spectrometer. The instrument operated with a resolution of 4 cm-'
and 20 scans were collected for each sample. The IR absorbance scans were analysed
between 1350 cm-' and 1400 cm-', for changes in the intensity of PIB peaks.
Results and Discussion
(0 Tensile properties
The effect of extrusion processing conditions and PIB concentration, on the break
strength in both the machine direction (MD) and transverse direction (TD) are shown
in Figures 2 and 3. The results indicate that the tensile strength of cast film decreases
initially with the addition of PIB, however increasing concentration has no further
effect. There was also very little difference in the MD tensile strength and TD tensile
strength for these films. The results also show that increasing PIB concentration
causes a progressive decrease in break strength in blown film at all BURs. Figures 2
and 3 have an overall decrease in MD and TD break strength especially at the higher
Manufacture and Perjbrmance of PE-PZB Filmsfor Cling Appiications
Figure 2. The effect of extrusion
processing conditions and PIB
concentration on MD break strength.
Figure 3. The effect of extrusion
processing conditions and PIB
concentration on TD break strength,
The effect of PIB concentration and film processing conditions on the Young’s
modulli for all films is given in Figures 4 and 5 . The cast films have the lowest
modulus in both the MD and TD compared to blown films. Although there was little
difference between MD and TD, there was a gradual increase in modulus with
increased PIB concentration. In the blown films, increase in BUR led to a reduction in
both the MD and TD modulli and there was a significant reduction in modulus with
progressive increase in PIB concentration.
Figure 4. The eflect of extrusion
Figure 5. The effect of extrusion
processing conditions and PIB
processing conditions and PIB
concentration on MD Young’s Modulus. concentration on TD Young’s Modulus.
(ii) Tearproperties
The effect of extrusion conditions and PIB concentration on the tear propagation
resistance of all films is shown in Figures 6 and 7. In all films tear strength was
considerably higher in the TD compared to the MD. The cast films showed the lowest
overall MD and TD tear strength of all the films investigated. Generally there was a
slight increase in MD tear with progressive increase in PIB concentration especially at
the lower BURs. Also there was a slight decrease in TD tear with progressive increase
in PIB concentration especially at lower BURs and for the cast films.
C.M. Small, G.M. McNally. W.R. Murphy and A. Marks
Figure 6. The effect of extrusion
processing conditions and PIB
Concentration on MD tear propagation
Figure 7. The effect of extrusion
processing conditions and PIB
concentration on TD tear propagation
(iii) Crystallinity
The effect of extrusion conditions and PIB concentration on film crystallinity is given
in Figure 8. The cast films had the lowest percentage crystallinity of all films and this
may be attributed to the much more efficient cooling, due to excellent contact of the
melt with the chill rolls. Films manufactured with higher BURs possessed a
marginally lower crystallinity and this may be due to the larger surface area exposed
to the cooling medium, but because the film was relatively thin, this effect was
minimised. Incorporation of PIB gave a slight increase in the crystallinity of cast
films (1 7% to 20%) and a slight reduction (23% to 20%) in blown films.
Figure 8. The effect of extrusion processing conditions
and PIB concentration on % crystallinity ofpims.
(iv) Tack strength
The migration of PIB from the bulk to the surface of LLDPE films was characterised
by measuring the tack strength as a function of time and these results are shown in
Figures 9-12. In general, tack strength was shown to be higher in blown films
especially at higher BURs, compared to cast films. Increase in tack strength was
recorded for all films with progressive increase in PIB concentration.
Manufacture and Performance of PE-PIB Films for Cling Applications
Figure 9. The effect ofextrusion
processing conditions and PIB
concentration on tack strength.
(Films conditioned at 25°C at 28 days)
Figure 10. The effect of conditioning
temperature and ageing on tack strength.
(Samplefilm BUR 2.0 6% PIB)
The tack strength of films improved significantly with ageing and films conditioned at
25°C continue to increase in tack strength throughout the time scale (28 days) of this
present investigation. An increase in conditioning temperature from 25OC to 35OC and
45OC gave a significant increase, by approximately 100% (45°C) in tack strength for
all the films. However, for films containing the higher PIB concentrations (6% and
8%), the maximum tack strength develops much earlier than films containing lower
PIB concentrations. Figures 11 and 12 tend to indicate that there are optimum
conditions of ageing and storage temperature required to create films with a high
degree of surface tack properties.
Figure 11. The effect of conditioning
Figure 12. The effect Of extrusion
temperature and ageing on tack strength. Processing conditions and ageing on
tack strength. (6% PIB films conditioned
(Samplefilm cast 6% PIB)
at 25 "C.
C.M. Small, G.M. McNally, W R. Murphy and A. Marks
(v) FTIR-A TR Study
FTIR-ATR absorbance scans of LLDPE film, with and without PIB additive, are
given in Figure 13. The two spectra are very similar apart for a pronounced doublet
peak occurring between 1350 cm-' and 1400 cm-' in the film containing PIB. A
localised portion of the scan is presented in Figure 14, and from this the PIB doublet
is more easily identified. This scan also identifies significant differences between the
absorbance spectra of the different co-monomer polyethylenes. A peak occurs at 1375
cm-l, but this has the highest magnitude in butene co-monomer polymer, then hexene
and finally octenes. This compares directly to the short chain length of the various
LLDPEs. The shorter butene side chain is recognised as being the most mobile of the
short chain structures and this is reflected in the high absorbance peak. However,
movement or deformation of the C-H bonds in the polyethylenes with the larger
octene side chain elements is more restricted.
-8% PIB
_-- _ _ _--
Wive Nnmbcrcm-1
03 -
Wave Number cm
Figure 15. The eflect of storage
temperature on FTIR-ATR spectra of
LLDPE (2% PIB)film (at 28 days).
Figure 14. FTIR-ATR spectra showing
PIB doublet peaks and different comonomer polyethylene.
Figure 13. An FTIR-ATR spectra in
absorbance mode for LLDPEfilm.
(0% and 8% PIB)
Figure 16. The eflect of storage
temperature on FTIR-ATR spectra of
LLDPE (8% PIB)fllm (at 28 days)
Wave Number em
Manufacture and Performance of PE-PIB Filmsfor Cling Applications
The effect of PIB concentration and storage temperature on absorbance is
presented in Figures 15 and 16. There is a direct correlation between PIB
concentration and peak height of absorbance scans. As well as an increase in the
height of the principle peak (1365 cm-'), increasing PIB concentration causes an
increase in the prominence of the secondary peak (1400 cm"). In all samples an
increase in storage temperature produces a stepped increase in peak heights and this
influence is most noticeable in higher PIB concentration films, where the
concentration gradient between the bulk and the surface is greatest.
The effect of PIB concentration, storage temperature and ageing on the peak
height absorbance of sample films is given in Figures 17 and 18. The results
demonstrate a slight increase in peak height with increase in storage temperature and
progressive ageing, for low PIB concentrations. At higher concentrations there is a
more rapid increase in peak height within the first 7 days, with a 10°C increase in
temperature producing up to a 25% increase in peak height absorbance during this
period. Depending on the storage temperature, peak height absorbance reaches a
maximum between 7 and 21 days (for higher PIB concentrations). Beyond this stage
the peak height decreases and then stabilises.
Figure 17. The efect of storage
temperature and ageing on peak height
absorbance of LLDPEfilm. (2% PIB).
Figure 18. The eflect of storage
temperature and ageing on peak height
absorbance of LLDPEjilm. (8% PIB).
The trends observed in tack strength measurements are very similar to those for
increase in peak heights during the FTIR investigation, both being influenced by
temperature, ageing and PIB concentration. However, the migration behaviour
identified in the FTIR analysis precedes changes in tack strength by 7 to 10 days. It
may be suggested that this reflects tack strength being more dependent upon the
propenies of the PIB on the film surface, than FTIR-ATR analysis. Although the
molecular weight distribution of the PIB masterbatch is not known, it is believed that
the masterbatch contains a range of molecular weights, in order to produce both initial
and long-term tack in film. Lower molecular weight species could migrate to the
surface quickest and so the FTIR analysis would identify this, but surface tack may
improve with a blend of higher molecular weight PIB, which may take longer to reach
the surface. Therefore the quantity of PIB at the surface may not directly reflect
improved tack strength.
C.M. Small, G.M. McNaI(y, W.R. Murphy and A. Marks
(vi) Polymer properties
The FTIR spectrum in Figure 14 demonstrates the difference in IR absorbance of
different co-monomer polyethylene. The effect of material properties, including comonomer type, on peak height absorbance and tack strength of films containing 8%
PIB (at 28 days) is shown Figure 19. There is a direct relationship between peak
height absorbance and tack strength, and, this follows a similar pattern as described
above. With materials segregated into co-monomer and polymer type, there is no
evidence from these results to suggest that co-monomer type is a major factor with
regard to PIB migration in polyethylene. The results would also tend to suggest that
polymer MFI, density or polydispersity did not have a significant influence on
migration of PIB in these films.
Figure 19. Eflect of material properties
on peak height absorbance and peel
strength of thin polymerJilms (8% PIB).
Figure 20. E@ct ofmaterial properties
on crystallinity of thin polymer films
(8% PIB).
(vii) Polymer properties Crystallinity
The effect of co-monomer type and MFI on film crystallinity is shown in Figure 20.
Since all films were manufactured using the same extrusion processing conditions,
then the inherent properties of the bulk polymers must be responsible for the
crystalline structure that has developed in Figure 20. The lowest density materials
correspond to the lowest crystallinity films. Comparison of Figures 19 and 20 show
that film crystallinity is the most prominent factor effecting migration rate of PIB
through films of the same co-monomer. There is a direct relationship between
increase in crystallinity and decrease in peak absorbance and tack strength, in films
with the same co-monomer. This relationship is evident in hexene (samples A C)
and octene (samples D E) co-monomer LLDPE’s.
The DSC analysis shows conventional octene polyethylenes (A/B-LLDPE) do
not indicate the same relationships with regards difhsion of PIB, as metallocenes.
These Zeigler Natta catalysed LLDPE’s have a more random chain
branching/molecular architecture inhibiting the development of highly ordered
crystalline morphology, which may enable enhanced migration to occur.
Manufacture and Performance of PE-PIB Filmsfor Cling Applications
This work reports the effects of extrusion processing conditions and PIB
concentration over the range 2%-8% w/w PIB, on the mechanical and tack
performance of films manufactured from blends of LLDPE and LDPE. The results
show that these low concentrations of PIB in films have only a minimal effect upon
the mechanical performance of films manufactured by blown and cast extrusion
techniques. The tack strength of these films improves progressively with increase in
PIB concentration. The tack strength also improves considerably in all film types with
ageing, showing maximum tack development after approximately 20 days at room
temperature. At the elevated storage temperatures of 35°C and 45°C the films
developed significantly higher tack by up to 100% over a shorter time period (1 4 to
21 days).
Several investigators [ 16-181 have suggested that the transport of small molecular
species in semi-crystalline polyethylene films occurs almost exclusively through the
non-crystalline phase and that the crystalline regions only act as physical barriers
impeding migration. Therefore, in this work it may have been expected that the
highest tack strength should have been developed in cast films, since DSC analysis
has shown that the cast films are less crystalline than the blown films, thus enabling
more rapid migration of the small molecular weight polymer PIB. However, further
investigation of the DSC thermograms highlights a distinct difference between cast
and blown traces. Figures 21 and 22 show the effect of process conditions and PIB
concentration on the recorded DSC thermograms. In both blown film and cast films, a
single peak occurs, however the cast film endotherm exists over a wider temperature
range. This may suggest a slightly less ordered crystalline structure being developed
in these rapidly quenched films and a higher level of co-crystallisation of the LLDPE
and LDPE. This would tend to confirm the work by Drummond et al. [19] who
reported co-crystallisation occurring in blends of LLDPE and LDPE. This different
crystalline structure may therefore be impeding the PIB migration in these films.
22 7
20 18 -
Figure 21. The effect of extrusion
processing conditions on DSC
Temperature OC
Figure 22. The effect of extrusion
processing conditions and PIB
concentration on DSC thermograms
(6% PIB concentration).
C.M. Small, G.M. McNally, W.R. Murphy and A. Marks
In this work PIB migration was characterised using FTIR-ATR analysis as a
hnction of time, alongside mechanical tack analysis. The results indicate that a small
percentage increase in PIB concentration can have a significant effect on migration
rates; an increase in concentration from 2% to 8% was shown to produce an increase
in peak height or tack strength, of 100% to 200% over a 28-day period. The rate of
migration decayed with time as the concentration gradient decreased, and in most
films the migration rate was negligible by 2 1 days. There is some evidence to suggest
that beyond this point the concentration gradient is slightly reversed and this is
reflected in both migration studies.
FTIR-ATR analysis has proved very competent for measuring the diffusion of
PIB in thin polymer films and the changes in peak height absorbance from IR scans
compare very favourably with tack strength measurement.
Using FTIR-ATR and mechanical tack strength analysis, it has been possible to
quantify the migration through a range of LLDPEs with different co-monomer types
and film crystallinity. The results of this study show that the migration of PIB
molecules through these films was not dependent on co-monomer type, MFI or
polydispersity. However migration was dependent on overall film crystallinity.
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Continuedfrom back cover
Polymer Properties, Processing and Applications
127 Effect of Pigment Type and Concentration on the Rheological Properties of
A. F. Marks, G.M. McNally, W.R. Murphy and P. Orr
137 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
147 The Effect of Cooling Processing Conditions on the Crystallinity and
Mechanical Performance of Pigmented Polypropylene Extruded Film
A. F. Marks, M. Leathem, G.M.McNally and W.R. Murphy
159 The Effect o f Cure Conditions on the Performance of Styrene Butadiene
Rubber for Industrial Conveying Applications
G.M. McNaIly, J.L. Clarke, C.M. Small, W.J. Skelton andJ. McCall
169 The Manufacture and Performance of Polyethylene-Polyisobutylene Films for
Cling Applications
C,M. Small, G.M. McNally, W.R. Murphy and A. Marks
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