Note: Descriptions are shown in the official language in which they were submitted.
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WO 98/04403 PCT/US97/12995
OPAQUE FILMS COMPR1STNG 1SOTACTTC POLYPROPYLENE
BACKGROUND OF THE INVENTION
The present invention relates to an opaque polypropylene-based multilayer film
structure which exhibits enhanced moisture barrier and mechanical properties.
Opaque polymeric films are used in many commercial applications. One
particularly
important application is the packaging of products such as snack foods and
candy bars. Films
employed in the food packaging industry are chosen and/or designed to provide
characteristics
necessary for proper food containment. Such characteristics include water
vapor barrier
properties, oxygen and gas barrier properties and flavor and aroma barrier
properties.
Polypropylene is a polymer commonly employed in the manufacture of opaque
films
used in the food packaging industry. In the case of multilayer films,
polypropylene is typically
used in the base or core layer and is cavitated with a cavitating agent such
as polybutylene
terephthalate (PBT). Typically, it has been necessary to use from 6% to 12% by
weight of the
cavitating agent in the prior art films to achieve the desired degree of
cavitation.
However, the use of the PBT cavitating agent within the aforementioned ranges
is not'
without its drawbacks. Particularly, the loading ranges typically employed in
the art may
negatively impact production uptime. Moreover, the PBT itself, at the loading
ranges typically
employed in the art, represents a significant percentage of the raw material
costs.
The mechanical properties of a polymeric film structure are another important
2 o characteristic, particularly with respect to such applications as food and
candy wrappings.
Films having enhanced mechanical properties facilitate handling and packaging
because such
films are more readily accommodated by typical industrial machinery. Attempts
have been
made to enhance the mechanical properties of polypropylene-based films (as
measured by the
stiffness and moduli of the film) through increased orientation and/or by the
addition of
additives. However, increased orientation often increases the likelihood
offilm splitting
during manufacturing, while the addition of additives typically provides
limited enhancement
of mechanical properties but can negatively impact other film characteristics
such as
dimensional stability.
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2
Thus, there is a need in the art for an opaque polypropylene-based film which
may be
readily manufactured (i.e., improved production uptime and reduced raw
material costs), and
which exhibits enhanced moisture barrier and mechanical properties.
The present invention, which addresses the needs of the prior art, relates to
an opaque
polymeric film having enhanced moisture barrier and mechanical properties. The
film has a
base layer which includes a blend of a first polypropylene polymer having an
isotactic
stereoregularity greater than 93% and a cavitating agent in an amount
effective to cavitate the
base layer.
In one preferred embodiment, the base layer includes less than 6% by weight of
1 o cavitating agent, and preferably from 2% to S% of cavitating agent. In
another preferred
embodiment, the base layer further includes a resin modifier in an amount up
to 9% by weight
of said base layer. The resin modifier is preferably a hydrogenated
hydrocarbon.
In still another preferred embodiment, at least one tie layer is adhered to
the cavitated
base layer. A skin is preferably adhered to the other side of the tie layer.
The tie layer, which
facilitates adherence of the skin layer to the cavitated base layer, is
preferably formed from
polypropylene. The skin layer is preferably formed from an ethylene-propylene
random
copolymer or an ethylene-propylene-butene-1 terpolymer.
As a result, the present invention provides an opaque polypropylene-based film
structure exhibiting enhanced moisture barrier and mechanical properties.
These improved
2 o properties are obtained without a negative impact on such film
characteristics as dimensional
stability. More to the point, these improved properties are obtained while
using a significantly
lower level of cavitating agent, thus providing improved production uptime and
reduced raw
material costs. The film structure further exhibits a high degree of
machinability and
processability.
2 5 The present invention is prepared by blending high crystallinity
polypropylene (HCPP)
with a cavitating agent in an amount effective to cavitate the HCPP. The HCPP
has an
isotactic stereoregularity of greater than 93%, preferably from 94% to 98%.
The base layer
preferably includes less than 6% by weight of the cavitating agent.
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Commercially suitable PPs include Fina 3371 (available from Fina Oil and
Chemical
Co., Dallas, Texas), Exxon 4612 and Exxon X052 (available from Exxon Chemical
Co.,
Houston, Texas) and Amoco*6361 (available from Amoco Chemical Co., Chicago,
Illinois).
Commercially suitable HCPPs include Amoco 9117, Amoco 9119 and
Amoco''~218(available
from Amoco Chemical Co., Chicago, Illinois), and Chisso"i~5010 and Chisso~2805
(available from Chisso Chemical Co., Ltd., Tokyo, Japan). Suitable HCPPs are
also available
from Solvay in Europe.
The HCPP has a high isotactic stereoregularity, resulting in higher
crystallinity than
conventional isotactic polypropylene, i.e, greater than 93%. (Conventional
isotactic
polypropylene is defined herein as having an isotactic stereoregularity of
from 90% to 93%.)
The HCPP thus exhibits higher stiffness, surface hardness, lower deflection at
higher
temperatures and better creep properties than conventional isotactic
polypropylene. Further
information relating to HCFP, including methods for preparation thereof, is
disclosed in U.S.
Patent No. 5,063,264.
For purposes of the present invention, stereoregularity can be determined by
IR
spectroscopy according to the procedure set out in "Integrated Infrared Band
Intensity
Measurement of Stereoregularity in Polypropylene," L L. Koenig and
A.VanRoggen, Journal
of Applied Polymer Science, Vol. 9, pp. 359-367 (1965) and in "Chemical
Microstructure of
Polymer Chains," J. L. Koenig, Wiley-Inerscience Publication, John Wiley and
Sons, New
2 o York, Chichester, Brisbane, Toronto. Stereoregularity can also be
determined by
decahydronaphthalene (decalin) solubility and nuclear magnetic resonance
spectroscopy
The use of HCPP in the base layer unexpectedly lowers the amount of cavitating
agent
required to cavitate the film and produce the desired opacity. More
particularly, the use of
2 5 HCPP in the base layer lowers the amount of cavitating agent necessary to
cavitate the film
from the prior art levels of approximately 6% to 12% by weight, or greater, to
a level in the
present invention of less than 6% by weight, and preferably from 2% to 5% by
weight. This
reduction in loading level of the cavitating agent is significant in that it
results in greater
production uptime. In addition, the reduction in the amount of cavitating
agent necessary to
3 o cavitate the base layer translates into a significant cost savings in raw
materials.
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More to the point, the blend of HCPP and cavitating agent in the base layer
provides a
significant improvement in the moisture barrier of the resultant film. As
demonstrated in the
example set forth hereinbelow, improvements in moisture barrier of up to 20%
to 30% may be
achieved with the film structures of the present invention. It will be
recognized by those
skilled in the art that this 20% to 30% improvement in moisture barrier is
significant in the
food packaging industry.
Preferred cavitating agents include polybutylene terephthalate (PBT), calcium
carbonate (CaCO,),.polyester (FET), polyamides including nylon and other
polymers and/or
inorganic materials which are incompatible.with HCPP.
In one preferred embodiment, the HCPP in the base layer is blended with a
conventional isotactic polypropylene (PP). The base layer preferably includes
an amount of
PP effective to plasticize the base layer and thus increase the processability
of the resultant
extruded film structure. The blended base layer increases manufacturing
efFrciency by
increasing line operability and line percentage uptime. For example, the
blended base layer
reduces the force necessary to stretch the film and in addition facilitates
edge trimming of the
extruded film. Thus, the blended base layer results in a reduction in cost
associated with
manufacture of the film structure.
In another preferred embodiment, a resin modifier is blended with the base
layer
precursor. The resin modifier is present in an amount of up to 9% by weight,
and preferably
2 o from 3% to 6% by weight. The blending may be accomplished by the direct
feeding of HCPP
and resin modifier into the film extruder or by use of a masterbatch. One
preferred method of
blending the components utilizes a masierbatch formed of HCPP and resin
modifier, e.g., 80'%
HCPP and 20% resin modifier. The masterbatch is then blended with additional
HCPP, which
reduces the concentration of the resin modifier to the final desired level.
Applicant has discovered that polymeric film
structures formed from HCPP exhibit unexpectedly large decreases in water
vapor
transntission with small additions of resin modifier. The moisture barrier
increases as the
loading level of resin modifier is increased until a point is reached (less
than 10% resin
3 o modifier) at which further increases in the loading level of the resin
modifier do not provide
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WO 98/04403 PCT/US97/12995
further substantial increases in moisture barrier. Moreover, the HCPP-based
film structures
also provide enhanced mechanical properties without suffering from losses in
dimensional
stability, as is common in prior art film structures which may require upwards
of 20% resin
modifier to achieve maximum moisture barrier.
The resin modifier is preferably a low molecular weight hydrogenated
hydrocarbon
which is compatible with the HCPP polymer and which provides the desired
enhancement of
film properties. The preferred resin modifier has a number average molecular
weight less than
5000, preferably less than 2000, and more preferably from 500 to 1000. The
resin modifier
can be natural or synthetic and preferably has a softening point of from
60° to 180°C.
1o Particularly suitable hydrocarbons which can be subsequently hydrogenated
are the
hydrocarbon resins. Preferred hydrocarbon resins include among others
petroleum resins,
terpene resins, styrene resins and cyclopentadiene resins.
Examples of commercially available hydrogenated hydrocarbon resins suitable
for use
in the present invention are those sold under the trademarks PICCOLYTE,
REGALREZ and
REGALITE by Hercules Corporation of Delaware and under the trademark ESCOREZ
by
Exxon Chemical Company of Houston, Texas. The ESCOREZ resins are particularly
preferred hydrogenated hydrocarbon resins.
One particularly preferred resin modifier is referred to herein as a saturated
alicyclic
resin. The saturated alicyclic resins are obtained by the hydrogenation of
aromatic
2 o hydrocarbon resins. The aromatic resins are themselves obtained by
polymerizing reactive
unsaturated hydrocarbons containing, as the principal component, aromatic
hydrocarbons in
which the reactive double bonds are generally in side-chains. More
particularly, the alicyclic
resins are obtained from the aromatic resins by hydrogenating the latter until
all, or almost all,
of the unsaturation has disappeared, including the double bonds in the
aromatic rings.
The saturated alicyclic resins used in the present invention have a softening
point from
85 ° to 140 ° C, and preferably from 100 ° to 140
° C, as measured by the ball and ring method.
Examples of commercially available saturated alicyclic resins suitable for use
in the present
invention are those sold under the trademark ARKON-P by Arakawa Forest
Chemical
Industries, Ltd. of Japan.
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6
In one preferred embodiment, a tie layer of an olefinic polymer is adhered to
the base
layer. Inasmuch as the skin layers discussed below may not adequately adhere
to the cavitated
base layer, a tie layer compatible with the cavitated base layer is first
adhered to the base layer.
A skin layer of an olefinic polymer is, in turn, adhered to the tie layer.
Both the tie and skin
layers are preferably coextruded with the base layer. The tie layers may
include a whitening
agent such as Ti02.
Suitable olefinic polymers utilized for the skin layers) include i) ethylene
homopolymers, ii) copolymers of ethylene and propylene, iii) copolymers of
ethylene or
propylene and butylene or another alpha olefin having 5 to 10 carbon atoms,
iv) terpolymers
of ethylene, propylene and butylene or another alpha-olefin having 5 to 10
carbon atoms, and
v) mixtures thereof.
Olefinic polymers which are particularly preferred for the skin layers)
include
ethylene-propylene copolymers with propylene as the main constituent and an
ethylene content
of 2% to I 0% by weight (relative to the weight of the copolymer), propylene-
butylene
copolymers with propylene as the main constituent and a butylene content of
0.5% to 25% by
weight (relative to the weight of the copolymer), and ethylene-propylene-
butene-1 terpolymers
with propylene as the main constituent, 0.5% to 7% by weight of ethylene and
5% to 30% by
weight of butene-1 (each time relative to the weight of the terpolymer), and
mixtures of these
polymers. The co- and terpolymers are preferably random polymers.
2 0 In a still further preferred embodiment, a coating is applied to the outer
surface of the
skin layer(s). An acrylic coating, which provides improved printability,
machinability and
aroma barrier characteristics, may be applied to one of the skin layers. A
heat seal coating
such as ethylene methyl acrylate (EMA) or ethylene acrylic acid (EAA) may be
applied to the
other skin layer. Other suitable coatings include polyvinylidene chloride
(PVDC), polyvinyl
alcohol (PVOH) and low temperature heat seal coatings, as disclosed in
commonly-owned
U. S. Patent No. 5,419,960.
In order to further improve certain properties of the resultant film,
effective amounts of
additives such as antiblocking agents, antistatic agents and/or slip agents
may be contained in
the base layer, tie layers) and/or skin layer(s).
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Preferred antiblocking agents include silica, talc, clay, sodium aluminum
silicate, and
conventional inorganic anti-blocks. Other suitable antiblocking agents include
inorganic
additives, such as silicon dioxide, calcium carbonate, magnesium silicate,
aluminum silicate,
calcium phosphate, and the like, and/or incompatible organic polymers, such as
polyamides,
polyesters, polycarbonates and the like.
Preferred antistatic agents include alkali alkane sulfonates and essentially
straight-
chain, saturated aliphatic tertiary amines possessing aliphatic radicals with
10 to 20 carbon
atoms and being substituted by 2-hydroxyalkyl-(C, to C4) groups. Preferred
amines are N,N-
bis-(2-hydroxyethyl)-alkylamines having 10 to 20, preferably 12 to 18, carbon
atoms in their
1o alkyl groups. The effective amount of antistatic agent varies in the range
from 0.05% to 3%
by weight, relative to the weight of the layer.
Preferred slip agents include higher aliphatic acid amides, higher aliphatic
acid esters,
waxes, metallic soaps and silicone oils such as polydimethylsiloxane. The
effective added
amount of lubricant varies from 0.1% to 2% by weight.
The multilayer films of the present invention may be prepared employing
commercially
available systems for coextruding resins. As mentioned, the blended HCPP and
cavitating
agent precursor is preferably coextruded with at least one second and at least
one third
polymeric material, which form the tie and skin layers, respectively. The
polymers can be
brought to the molten state and coextruded from a conventional extruder
through a flat sheet
2 o die, the melt streams being combined in an adapter prior to being extruded
from the die. After
leaving the die orifice, the multilayer film structure is quenched.
The film structure of the present invention is preferably biaxially oriented.
In orie
preferred embodiment, the film structure is stretched from 4.5 to 6 times in
the machine
direction (MD) and from 6 to 13 times in the transverse direction (TD). The
overall
2 5 orientation (MD x TD) preferably ranges from 25 to 80. After orientation,
the edges of the
film can be trimmed and the film wound onto a core.
The film structures of the present invention also exhibit unexpectedly
increased
stiffness and moduli (MD and TD) over film structures having conventional
isotactic
polypropylene-based core layers. The increased stiffness and moduli provide
the film structure
3 0 with enhanced mechanical properties which, in turn, facilitate subsequent
handling and
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CA 02262081 2004-12-17
g
packaging. Moreover, the increases in MD and TD moduli are accomplished at
relatively low
orientation, thus reducing manufacturing costs and the associated likelihood
of splitting.
The film structures of the present invention are formed having a thickness
ranging from
microns to 60 microns, preferably from 15 microns to 50 microns. Each tie
layer
5 represents from 5% to 15% of the structure, while each skin layer represents
from 2% to 6%
of the structure.
EXAMPLES
Water Vapor Transmission Rate (WVTR) in each of the following examples was
measured at 37.8°C (100°F) and 90% Relative Humidity (ASTM F
3?2) and is expressed in
10 g/100 in2/day/mil.
EXAMPLE 1
Samples 1 a to 1 c were produced to compare the moisture barrier of an opaque
PP-
based film structure with an opaque HCPP-based film structure and an opaque
HCPP-based
film structure modified with a hydrogenated hydrocarbon.
Sample 1 a was produced with a core layer of 94% standard PP (Exxon*4252) and
6%
cavitating agent (Ceianese*1300a) having a thickness of 28.25 microns, and was
coextruded
with skin.layers of standard PP (Exxon*4252) each having a thickness of 6.0
microns to
produce an ABA extrudate. The ABA extrudate was stretched 5.3 times in the
machine
direction and 9.0 times in the transverse direction.
Sample lb was produced with a core layer of 96% HCPP (Amoco~911?) and 4%
cavitating agent (Celanese*1300a) having a thickness of 26.75 microns, and was
coextruded
with skin layers of HCPP (Amocd'9117) each having a thickness of 5.75 microns
to produce
an ABA extrudate. The ABA extrudate was stretched 5.3 times in the machine
direction and
9.0 times in the transverse direction.
2 5 Sample 1 c was produced with a core layer of 93% HCPP (Amoco*9117) and 4%
cavitating agent (Celanese*1300a) and 3% hydrogenated hydrocarbon (Arkon*P-
115) having a
thickness of 24.5 microns, and was coextruded with skin layers of HCPP (Amoco
9117) each
having a thickness of 5.25 microns to produce an ABA extrudate. The ABA
extrudate was
stretched 5.3 times in the machine direction and 9.0 times in the transverse
direction.
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9
Sample Core % Cav. % Resin WVTR
Layer Agent Modifier (Aged)
1 a PP 6.0 0.0 .301
1 b HCPP 4.0 0.0 .242
1 c HCPP 4.0 3 .0 .216
EXAMPLE 2
Samples 2a to 2d were produced by samples 2a to 2d were produced to compare
the
moisture barrier of an opaque PP-based film structure with opaque HCPP-based
film
structures.
Sample 2a was produced with a core layer of 92% standard PP (Amoco*6317) and
8%
cavitating agent (Celanese 1300x) having a thickness of 25.0 microns, and was
coextruded
with skin layers of 94% standard PP (Amoco*6317), 4% Ti02 (Titanium Oxide),
and 2% Talc
each having a thickness of 5.35 microns to produce a ABA extrudate. The ABA
extmdate
was stretched 5.5 times in the machine direction and 8.5 times in the
transverse direction.
Sample 2b was produced with a core layer of 30% standard PP (Amoco'1i317),
66.8%
HCPP (Chisso''~A4141) and 3.2% cavitating agent (Celanese*1300a) having a
thickness of
22.75 microns, and was coextruded with skin layers of 94% standard PP (Amoco
6317), 4%
Ti02 (Titanium Oxide), and 2% Talc each having a thickness of 4.85 microns to
produce a
ABA extrudate. The ABA extrudate was stretched 5.5 times in the machine
direction and 8.5_
time in the transverse direction.
Sample Zc was produced with a core layer of 45% standard PP (Amoco*6317), S
1.8%
HCPP (Chisso XA4141) and 3.2% cavitating agent (Celanese*1300a) having a
thickness of
22.75 microns, and was coextruded with skin layers of 94% standard PP
(Amoco*6317), 4%
Ti02 (Titanium Oxide), and 2% Talc each having a thickness of 4.85 microns to
produce a
ABA extrudate. The ABA extrudate was stretched 5.5 times in the machine
direction and 8.5
times in the transverse direction.
Sample 2d was produced with a core layer of 30% standard PP (Amoco*6317),
66.8%
HCPP (Amoco*4141X) and 3.2% cavitating agent (Celanese*1300a) having a
thickness of
22.75 microns, and was coextruded with skin layers of 94% standard PP
(Amoco'6317), 4%
Ti02 (Titanium Oxide), and 2% Talc each having a thickness of 4.85 microns to
produce a
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CA 02262081 2004-12-17
ABA extrudate. The ABA extrudate was stretched S.5 times in the machine
direction and 8.5
times in the transverse direction.
Sample Core % Cav. % PP' % HCPP WVTR
5 Layer Agent in Core in Core (Unaged)
2a PP 8.0 92.0 0.0 0.47
2b HCPP 3.2 30.0 66.8 0.39
2c HCPP 3.2 45.0 51.8 0.39
10 2d HCPP ~ 3.2 30.0 ~ 66.8 0.40
' Total % PP includes 30.0% of reprocessed polypropylene
EXAMPLE 3
Sample 3a was produced to demonstrate the moisture barrier of an opaque HCPP-
based film structure containing a low level of cavitating agent.
Sample 3a was produced with a core layer of 97% HCPP (Amoco 9218) and 3%
cavitating agent (Celenese*1300a) having a optical thickness of 28 microns.
This core was
coextruded with skin layers of 92% HCPP (Amoco''~218) and 8% whitening agent
(Schulman*
CTW5050) each having a thickness of 3.5 microns to produce an ABA extrudate.
The ABA
extrudate was stretched 5.3 times in the machine direction and 8.5 times in
the transverse
direction.
Sample Core % Cav. % Resin WVTR
Layer Agent Modifier (Aged)
3 a HCPP 3.0 0.0 0.24
It is thus readily apparent from the data set forth above that the present
invention
provides an opaque film structure exhibiting significantly improved moisture
barrier properties,
as compared to the prior art control samples (samples la and 2a). The films of
the present
invention exhibited substantially the same degree ofopacity as the prior art
control samples
(la and 2a), while utilizing significantly lower levels of cavitating agent.
Moreover, these film
structures exhibit enhanced mechanical properties, e.g., improved stiffness
and moduli and
provide ease of manufacturing, e.g., increased line operability and line
percentage uptime (as
observed during manufacture of the present film structures).
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