Ultra-thin Nano-layer Stretch-Shrink Films
Henry G. Schirmer, BBS Corporation, Spartanburg, SC
Tom Schell, Curwood, Inc., Oshkosh, WI
Abstract
Clear ultra-thin polyolefin films based on nano-layers
composed of PP/EVA have been made and characterized.
These films exhibit shrink properties as well as stretch
properties and are surprisingly tough in spite of thinness
as low as 0.1mil total. In order to avoid unmanageable
tack, a high degree of slip was used. Surprisingly the high
slip films made did cling well to food dishes and bowls
making it highly useful as a lidding film. The high degree
of shrink shown in the film makes it also useful for shrink
packaging soft materials. Overlap seals were made during
the shrinking process.
room temperature. Therefore, any stretch orientation of PP
should have to be done immediately.
The quickest way to orient is as the PP containing film is
being blown. Fortunately, the specific material
composition used was very amendable to long stalk
processing where a very high blow up ratio (BUR) of 5:1
was obtained. Picture 1 below shows the first attempt.
Introduction
Oriented shrink films are another film type that has shown
unique improved properties through the use of nano-layers
within the film structure. Tough thin shrink films as low
as 0.3mil thick have been seen performing as well as
thicker films for packaging applications. The work
reported here is still another example of an even thinner
shrink film showing stretch properties as well. Ultra-thin
films as low as 0.1mil have been made because of the
high melt strength imparted by nano-layers.
The tool for creating nano-layers within the films
described in this work is the Layer Sequence Repeater
(LSR). As film research continues so does our knowledge
of what the LSR can do. Nano-layer films create many
surprises that we had not seen before and some of these
have been reported earlier at PO 2009, 2010, 2011 and
listed in the references. Some of the knowledge gained
has been used to develop the LSR further but cannot be
reported at this time because of patent reasons.
Nylon 6/EVOH nano-layer films reported in the above
papers showed some surprises that eventually led to an
understanding that blown films containing crystalline
EVOH in nano-layer form had a lower density, lower
M.P. crystal structure that was consistent with the film
being easily deformed. We also suspected that similar
things were happening to N6 as well. To test this,
N6/EVOH nano-layer films were stretch oriented above
the Tg of both but below 100C. Stretching the film in this
low temperature range showed clearly how deformable
this new crystal structure actually was.
Polypropylene is another crystalline polymer that also
may experience a new crystal structure when in nanolayer form. But it has a Tg well below room temperature
and that means that any crystalline changes may only be
temporary. Both N6 and EVOH both have Tg above 60C
so the new crystal forms remained rather permanent at
PICTURE 1
LONG STALK BUBBLE PROCESS
High Density Polyethylene shopping bags gain much of
their strength and toughness through this method of
orientation. Generally, a high molecular weight polymer is
the key to successful processing and the EVA component
in the 11-60-70 series structure was of 0.3MI. However,
the nano-layer structure of this new stretch-shrink film
gave it far different properties from an opaque stiff HDPE
counterpart. These properties were indeed due to the
nano-layers within the microstructure of the film series
described here. Picture 2 shows the micro-structure of a
similar but thicker precursor film.
PICTURE 2
25 NANO-LAYER FILM
1
Definition of Terms
The Patented Modular Disk Die has produced films
containing 25 and more micro-layers independently of the
LSR. While it is generally true that as the number of
micro-layers increases, the individual layer thickness
decreases for a given total film thickness. But the
thickness of each of the structural micro-layers is
generally all in the same order of magnitude. This doesn’t
preclude that some thin micro-layers may contain the
same material to make what would appear to be fewer and
thicker micro-layers. The reason for doing this might be to
simply gain increased output or to gain some other
processing attribute.
The Layer Sequence Repeater (LSR) operates as an
independent unit within a Modular Disk Die and inserts
nano-layer bundles within the matrix of micro-layers
wherever desired. These are truly an order of magnitude
thinner than the surrounding micro-layers. So the term
nano-layers as depicted here will be used to differentiate
nano-layer thickness relative to micro-layer thickness and
the difference was very apparent in picture 2 above and
other microphotographs shown in this paper.
For the purposes of this paper, the number of nano-layers
issued by the LSR was set at 25 in alternating layers of
EVA/PP. This nano-layer bundle appeared to be well over
one third of the total thickness of the film in all microphoto’s taken. In fact, the measured proportional thickness
ratio was 2125/4625 or 0.46T where T=total thickness.
Since the bundle contained 25 nano-layers, then each
nano-layer averaged .46/25T or .018T. At a total thickness
T=0.1mil, each nano-layer averaged 0.0018mil thick.
That’s 45.7 nanometers and will be the average of each
PP/EVA nano-layer in this paper.
Oriented films can be biaxial or monaxial. The ultra-thin
films within this report are biaxial. Oriented films can be
made by either a reheat process or by a cooling process.
The ultra-thin films here are from a cool down process.
Generally oriented films from cool down processes are
tougher, stretchier and in a less stressed state (low shrink
force) than films derived from the heat up process (high
shrink force). Some processes share a little of both as is
seen in US Patent 5,456,979.
were made at 4mil because we first thought to orient them
using a reheat process. However, there were problems
with hot tack and open ability of the tubing. So that
approach was temporarily abandoned until we could
provide a means to prevent hot tack. Picture 3 shows the
nano-layer structure of the 4mil film. Other thinner films
in this series all have the same structure but become
increasingly harder to microphotograph.
PICTURE 3
SAMPLE 11-54
The above 4mil film blown film structures also had a
tendency to blow with a stalk. So to enhance the stalk
stability, the EMA was switched to an EVA with a 0.3
MI. This higher MW resin indeed made long stalk
processing more stable and processed well in the LSR
with PP.
Samples 11-57 through 11-60 were made at a high BUR
and increasing draw speeds. Sample 11-61 coupled this
with a drastically reduced delivery rate. The highest draw
speed for this lab equipment was 56 fpm. Picture 4 shows
the overall setup.
Ext. d
Ext. b, A, c
2” Die &
Air ring
Experimental Procedure
TEST FILM STRUCTURES
Samples 11/54 & 11-55=
LDPE/EMA/{EMA/PP.. 25nano..PP/EMA}/EMA/LDPE
The above structures were the start of the PP nano-layer
series. EMA having about a 2 MI was first chosen to
sequence with PP because it adheres well. These films
PICTURE 4
LABORATORY FILM LINE
Extruders d, b, c = 0.75”
Extruder A = 1.25”
Extruders b, A, c were all commonly driven at a fixed
output ratio of 20/60/20 referred as a triplex extruder.
2
Samples 11-62 to 69=
LDPE/EVA/{EVA/PP.. 25nano..PP/EVA}/EVA/LDPE
With the stability of the process well defined by the 11-54
to 11-61 samples, the parameters were set so that thinnest
and widest film could be obtained. All films made were
15-16” wide with the rate of the triplex extruder b, A, c at
29rpm and extruder d at 30.
Because the glass transition temperature (Tg) of PP is
below room temperature at about –10C, any deformable
crystalline state produced within the nano-layers during
cooling would only be temporary at room temperature and
would quickly revert to the more stable higher melting
more rigid state. As reported earlier this is similar to the
two crystalline forms of polybutene –1.
The DSC curves displayed in Fig.1 show the first heat to
be lower than the second annealed heat.
Samples 11-70 to 11-73
LDPE/EVA/{EVA/PP.. 25nano..PP/EVA}/EVA/LDPE
The triplex extruder b, A, c was then lowered in rate to 20
rpm while extruder d was also lowered to 20 rpm. All
sample rolls were 15-16” wide and in 4 increments the
draw speed was increased from sample to sample roll until
the fastest setting at Sample 11-73. Many rolls of this
were collected until the EVA ran out. This gave us enough
of this thinnest sample to test in several areas of
application.
162.55 C
111.92 C
CONTROL FILM STRUCTURES
In order to see the true effect of the nano-layer core in the
above test films, the die module was switched to a similar
module containing a three-layer core. Everything else
remained the same.
163.57 C
111.59C
Samples 11-74 & 75
LDPE/EVA/(EVA/PP/EVA)/EVA/LDPE
These films were also run with a high stalk high BUR
bubble into films of 14-16” widths. Attempts to draw 1175 to thinner film failed and the only stable processing
condition was close to 11-74. Both films were 1 mil thick.
Samples 11-76 & 77
LDPE/EVA/(EVA/PP/EVA)/EVA/LDPE
Extruder screw speeds were changed to lower rpm similar
to 11-73. Sample 11-76 was drawn to 0.5mil thickness at
16” width. Sample 11-77 was very unstable and could not
be blown much past 12” wide. This was 0.2mil compared
to 0.1mil for the 11-73 16” wide nano-test films. Clearly,
25 alternating nano-layers of EVA/PP/EVA imparted high
melt strength and pinhole resistance caused by an
occasional gel in the bubble as it was drawn very thin
biaxially. The 3-layer EVA/PP/EVA control films showed
no bubble stability as they were drawn thinner than
0.5mil.
Discussion of Results
DSC TESTS
ΔT=1.02C
FIG. 1 DSC CURVES FOR SAMPLE 11-73
This is consistent with having a less dense form of crystal
structure. The ΔT at 1.02C is surprising and may be a
result of the orientation of the film during processing. The
stretching of the film during the cooling from the melt
immediately oriented the PP layers that were being cooled
probably even before the LDPE layers solidified.
Orientation during the cooling from a melt usually takes
place at a lower temperature than heating from a solid as
is usually done. A lower density form of crystal structure
may very well be the signature of this difference.
Since at least the nano-structure of this film was oriented
as the melt was cooled, the stress involved in stretching
the film to high BUR was much lower than it would have
been for orientation by the reheat methods. This is seen in
the low shrink tension and force during the shrink process
that sample 11-73 displayed during shrink packaging tests
described below. It therefore seems plausible that the
lesser stressed oriented PP crystal may also display a less
densely packed but still stable crystal structure to account
for the ΔT between first and second heat.
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Also notable on the DSC curves is the wide temperature
range that exists while there are still solid nano-layers of
PP remaining. So from room temperature all the way to
160C the film would have some degree of strength due to
the solid PP. This wide range of hot strength is in many
ways similar to x-linked PE shrink films.
STRETCH - SHRINK TESTS
Sample 11-73 was selected for shrink performance tests as
well as stretch lidding film because it was the thinnest that
was produced at 0.1mil. This ultra-thin film was
surprisingly tough and handled well in lidding trials where
it was drawn tight over the rim of a bowl. Although the
film was not tacky due to the slip added to the outer PE
layer, it had a tendency to cling to the bowl. This was
enhanced further if any moisture was present. There is no
question that the thinness and ease in draping of the film
also added to this good performance. Picture 5 shows the
lidded bowl along with a shrunk package showing overlap
seals made during shrinking.
Lap seals during shrink
PICTURE 6 - TRACED 10mm CIRCLE ON FILM
Tight lid cover
PICTURE 5 SHRINK-STRETCH OF 11-73
During the shrinking of 11-73, the heat strength became
noticeable when the lap seals were made during shrinking.
Obviously, the film temperature was above 111C in order
to make the seal but there were no burn through holes
observed as the film sealed and shrunk at the same time.
The presence of the PP nano-layers was no doubt
responsible for holding the film together over this wide
shrink temperature range especially in the range between
111C and 162C, the melting points of LDPE and PP.
Since the shrink took place in this tacky temperature
region, testing the film for ultimate free shrink and shrink
tension was complicated by the extreme thinness and film
tack. With the LDPE surfaces melted, the shrinking film
stuck immediately to itself on contact and the thinness
produced extremely low shrink force. Pictures 6 and 7
show how the ultimate free shrink was obtained by
draping the film with a 10x10mm traced circle from a
table. With a helper to keep the film from folding onto
itself, hot air was applied by means of a heat gun set at
low air flow. The result was free shrink in all directions as
shown in picture 7.
PICTURE 7 – FILM SHRUNK BY HOT AIR GUN
The better samples showed approximately 50% free
shrink in all directions and selected articles shown in
picture 8 were shrink-wrapped. These included an
answering machine and 2 rolls of tissue.
PICTURE 8 – 50% SHRINK, FILM & PACKAGES
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Measurement of shrink force was also complicated by the
same things as free shrink. The film became tacky at the
measurement temperature and the shrink force was low.
By trial and error, we found that 2 paper clips and a strip
of wood weighing 24 grams provided enough weight to
counterbalance the longitudinal shrink so that no shrink
occurred in that direction. Picture 9 shows the results.
The free shrink film sample was held with paper clips on
all 4 corners. This was not truly free shrink because there
was some pendulum like resisting force to do this as the
device was heated within an oven.
Both sides of the device held suspended 3-1” strips of film
cut from the longitudinal and transverse directions. Each
strip was weighted with paper clips at 1.2 grams each and
progressed from one to three clips for increasing weight.
Each strip was marked with 10 cm measurement lines that
increased in length as the samples became heated and
stretched beyond the shrink force limit. The shrink tension
then was derived at the weight where the film shrink was
in balance with the weight suspended in a manner similar
to the above earlier crude measurements.
The stretch-temperature-weight tabulation below Graph 1
in the next column shows that this occurred in the 210220F-temperature range where the shrink was below 10%.
There was virtually no shrink force beyond this
temperature.
PICTURE 9 – SHRINK TENSION MEASURMENT
Since the film was 13” wide and averaged 0.1mil thick,
the shrink tension was calculated as follows:
24gms. /13”x 0.1mil x .001”/1mil =
24gms. /13” x 0.0001”= (1lb/454gms)24gms. /0.0013in2
Shrink Tension = 40.7 psi
All these were crude measurements and more control was
desired to obtain a better characterization. So a sample
holding device was assembled that overcame the film tack
problems. This is shown in Picture 10.
Free shrink was easier to measure and the results are
shown in Graph 1. Please note the difference between the
L & T free shrink was partially due to the slight pull of the
suspension clamps on the film corners during shrink and
is more of a reflection of the differences in the extremely
low shrink tensions i.e. the differences between the lower
transverse value and the higher longitudinal value.
Please note picture 8 again where the shrink was 50% in
both directions. This was a situation where there was a lot
less shrink tension influence on restraining free shrink in
the transverse direction. The less restraint on the film
means a truer value of free shrink, so we averaged both to
arrive at the 50% average free shrink in both directions.
% FREE SHRINK OF SAMPLE 11-73
3 shrink force strips with progressive wts.
Longitudinal
Average
Free shrink film
held with 4 clamps
Transverse
GRAPH 1 FREE SHRINK VS. TEMPERATURE
PICTURE 10 SAMPLE HOLDING DEVICE
Graph 1 shows that shrink began at about 200 F close to
the melting point of the LDPE outer surfaces. It then
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progressed as the temperature rose to 260F where total
melting of the sample took place. Somewhere within this
region the shrink forces came into play but all test strips
measuring shrink force failed at 230F and higher.
Significant stretch occurred between 210 and 220F so
these 2 temperature readings were averaged and shown
tabulated and underlined below in blue.
Shrink force @ 210F
Longitudinal stretch
Transverse stretch
1.2gms. 2.4gms. 3.6gms
10.1
10.2
10.8
10.2
11.0
20.5
Shrink force @ 220F
Longitudinal stretch
Transverse stretch
Average stretch (long.)
Average stretch (trans.)
1.2gms.
10.0
10.1
10.05
10.15
decreased. Generally, as the stretching and cooling speed
increased the elongation decreased. This is a clear
indication of orientation taking place. Oriented films
simply have less elongation than blown films.
Table 1 TENSILE –ELONGATION (Instron, psi)
2” Span @ 20” per minute crosshead
Sample
Machine direction
Cross Machine
TEST
mils
T
E
T
E
11-63
1.06
4599
480%
4325
425%
11-64
0.41
4399
268%
3848
287%
11-67
0.26
4475
177%
4284
246%
Clearly the lowest shrink force occurred in the transverse
direction because there was more stretch occurring at the
2.4gm value. The transverse shrink force at 1.2 grams
resisted any significant elongation and the longitudinal
shrink force of 2.4 grams also resisted any significant
elongation. So in choosing these 2 values, the shrink
tension was calculated as follows:
11-73
0.15
4956
124%
1676
402%
1.2gms. /1”x 0.1mil x .001”/1mil =
1.2gms. /1” x 0.0001”= (1lb/454gms)1.2gms. /0.0001in2
2.4gms.
10.5
12.0
10.35
11.50
3.6gms.
11.4
11.4
11.10
15.95
Therefore between 210 & 220F:
Shrink Tension Transverse = 26.4 psi
Shrink tension longitudinal = 2(26.4) = 54.8 psi
As shown here, the more precise measurements of both
shrink and shrink tension were essentially a confirmation
of the less controlled measurements taken earlier.
OTHER FILM CHARACTERIZATION TESTS
Tensile – Elongation (Test Film)
We have seen from the shrink and shrink tension tests that
the more highly stressed orientation appeared to be in the
Machine Direction (MD) than in the Cross Machine
Direction (CDM) using the thinnest sample 11-73. Tensile
& Elongation tests, however, should help to confirm this
and show how the orientation behavior is influenced by
sample thickness. In other words the thinnest sample
should show the most orientation because of higher
stretch speeds and faster cooling.
The main point to note in looking at the test data is that
blown films usually have lower tensile strength and higher
elongation in most cases than oriented films. Table 1
above shows a summary of the T & E data as the films
were drawn faster and the thickness was accordingly
CONTROL
11-74
1.27
4554
494%
4451
479%
11-75
1.13
4850
516%
4642
479%
11-76
0.40
3612
315%
3095
306%
11-77
0.20
2993
105%
2768
326%
There was one exception in the cross machine direction of
sample 11-73 elongation. This may have been due to a
less oriented streak in the film. Cross machine direction
properties often can be less consistent than machine
direction properties because of film differences due to
streaks, die lines etc.
While the elongation goes down, the tensile strength goes
up as orientation increases. Again this is seen in the
machine direction with the same exception in the cross
machine direction particularly with the thinnest sample.
The least oriented sample appeared to be in the thickest
sample 11-63 with the highest elongation but its tensile
strength was also surprisingly high in both directions.
This probably was due to the differences in crystalline
size and coarseness of the aged polypropylene in the less
oriented state. Oriented polypropylene has a very fine
crystalline structure while unoriented polypropylene has a
course sphereulitic structure.
Tensile – Elongation (Control Film)
The control films containing EVA/PP/EVA instead of 25
nano-layers showed a marked decrease in tensile as the
films were drawn thinner showing that they were weaker.
Note the blue values in table 1 above. This is consistent
with the poor bubble stability shown during manufacture.
6
The machine direction measurements show this vividly
while the cross machine measurements were less
pronounced especially with the elongation. Sample 11-77,
the thinnest control sample, could not be blown as wide as
its test film counterpart 11-67. So, table 1 shows at a
glance the higher tensile and lower elongation of the more
oriented test films. While the control films became more
oriented during stretching, there was much less strength
developed.
Graph 2 details the test film data more vividly. Films
made below .4 mils have decreased elongation and
generally higher tensile strength. This implies higher
orientation is obtained with thinner test films.
Oriented Films
GRAPH 3 - TEAR STRENGTH (Gms)
Permeability
Blown Film
Permeability of the film to gases and vapors tends to
either get a bit lower or remain about the same with
respect to orientation. What truly counts is thickness. As
the films become thinner the permeability increases.
GRAPH 2 - TENSILE & ELONGATION DATA
Tear Strength
Graph 4 shows that both Oxygen and water vapor follow
the same path of increasing permeability as the films
became thinner.
Just as the ultimate elongation of the more oriented films
was lower than the less unoriented films in the above film
series, tear strength also was lower. Since the thinner
films had increasingly lower elongation (a sign of
increasing orientation) , we now can also follow the same
decrease in tear strength over the same film thickness
path. Graph 3 on the next column shows this relationship.
Please note the steep drop in tear strength as the film was
reduced in thickness from 1 mil to.41 mil. This .41mil
thickness value appears to be the threshold to the film
becoming truly oriented. This is also the thickness where
the tensile strength began to climb and the elongation
became lower.
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although they could be made into 1-mil
films with a high stalk, high BUR process.
Summary and Conclusions
1.
Test film containing a core of 25 alternating
nano-layers consisting of Polypropylene and
EVA had a propensity to be blown into very
thin films at a blow up ratio of about 5:1with
a long stalk process. Low MI EVA stabilized
the process as would be expected.
1.
2.
The thinnest test films made with this
process were about .1mil thick.
3.
3.
The thinnest test films showed all the signs
of being oriented with free shrink at up to
50% and shrink tension about 25-50 psi.
4.
5.
DSC measurements showed the PE/EVA
melt peaks at 96-111C and the PP peak at
162.55C. This extra 50C temperature spread
gave the film more hot strength during
shrinking.
DSC measurements also showed a
difference in the PP crystallinity on 1 st heat
at 162.55C to 2nd heat at 163.57C or a
∆T=1.02C. This difference is usually
indicative of a lower density crystalline
structure due perhaps to orientation.
6.
The test films sealed during heat shrinking
using simple overlap seals as would be used
in film over wrapped trays or rolls of paper.
7.
The relatively high elongation of even the
thinnest test film was helpful in making it
very useful as a stretch lidding film for food
trays and bowls.
8.
Graphs of the tensile, elongation and tear
strength all showed a decided tendency to
higher and higher orientation as the test film
was drawn thinner and thinner.
9.
Permeability to Moisture Vapor and Oxygen
increased as the test film became thinner.
10.
A special shrink property device was needed
and assembled for use to avoid film-to-film
contact. The high tack of the test film due to
the melting of the polyethylene surfaces
made measurement difficult with standard
devices in the shrink temperature region.
11.
Control films containing 3 alternating
EVA/PP/EVA layers as a core could not be
blown with stability into ultra-thin film
REFERENCES
2.
4.
5.
6.
7.
8.
9.
10.
11.
12.
The Modular Disk Coextrusion Die –
Schirmer Polyolefins 2000
New Compositions of Matter from The
Modular Disk Coextrusion Die - Schirmer,
Love, Schelling, Loschialpo - ANTEC 2000
Micro-layer Coextrusion Technology Baer,
Jarus, Hiltner - ANTEC 1999
Modular Disk Coextrusion: Production Rate
Tests with the 9” flex-Lip Die Schirmer Future-Pak 1999
25 Micro-layer Blown Film Coextrusion Die
– Schirmer - Polyolefins 2008
Exploratory Experiments on Solid-State
Foaming of PLA films and COC/LDPE
Multi-layered Films - Lu, Kumar, Schirmer ANTEC 2009
Improved
Flexible
Packaging
Film
Performance via Layer Multiplication- Sam
Iuliano – Polyolefins 2009
Nano-layers in Blown film – Schirmer,
Jester, Medlock – Polyolefins 2009
Nano-layers in Blown Barrier Films –
Schirmer, Jester, Medlock, Schell – PO 2010
Oriented Blends of Polybutene –1 and
Polypropylene –Schirmer–US Pat. 3,808,304
A.M. Chatterjee, “Butene Polymers”,
Encyclopedia of Polymer Science and
Engineering, Vol 2, 2nd. Ed, 590 (1985)
Nano-layers in Blown Barrier Films (part 2)
– Schirmer, Schell, Pucci & Chatterjee
AUTHOR CONTACTS
Henry G. Schirmer
BBS Corporation
2066 Pecan Drive
Spartanburg, SC 29307
Tel: (864) 579-3058
E-Mail: hschirmer@att.net
Tom Schell
Curwood, Inc.
2200 Badger Avenue
Oshkosh, WI 54904
E-Mail: TASchell@bemis.com
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