Note: Descriptions are shown in the official language in which they were submitted.
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PERMEABLE POLYPROPYLENE FILM
BACKTROUND OF THE INVENTION
The invention relates to a permeable, propylene-containing film structure.
In particular, the invention relates to a film structure that is highly
permeable to
both gas and water vapor, at least the core layer of the film structure having
been
cavitated via beta-nucleated (beta-crystalline) orientation in the presence of
a
filler. The film structure also has uniform opacity, low density, and enhanced
stiffness. The invention takes advantage of a previously unknown synergy
between a beta-nucleating agent and a filler. The permeable film structure is
especially suited for labeling applications.
The market for polymer films continues to expand. One area of growth is
in the food and beverage industry. Polyiner films are increasingly being used
as
labels in the food and beverage industry, in part due to their printability
and their
ability to conform and adhere to the surface of a food package or beverage
container. The preferred label, however, is opaque and/or colored, e.g., a
white
opaque label. Polymer films, on the other hand, especially polyolefin films,
are
inherently clear and colorless. Therefore, polymer films to be used as labels
are
generally modified to render them opaque and/or colored.
A variety of techniques are known to modify a polymer film and render it
opaque and/or colored.
For example, it is well known in the art to include certain organic or
inorganic cavitating agents in one or more layers of a polymer film. The
organic
cavitating agent may be a polyester, such as polyethylene terephthalate (PET)
or
polybutylene terephthalate (PBT). The inorganic cavitating agent may be
calcium
carbonate (CaCO3). The presence of the cavitating agent in a layer of the film
during orientation of the film induces voids in the polymeric material
comprising
the layer of the film. The voids scatter light thereby causing the film to be
3 0 opaque.
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U.S. Patent 4,632,869 to Park, et al., discloses an opaque, biaxially
oriented film structure containing a voided polymer matrix layer, in which the
voids contain spherical void-initiating particles of polybutylene
terephthalate
(PBT). The structure may also include thermoplastic skin layers, and the
individual layers may include pigments, such as TiO2 or colored oxides.
However, the use of CaCO3- or PBT-type cavitating agents to induce voids
in a polymer film, as proposed by US '869 and others like it, is an example of
a
single component cavitation method. Single component cavitation of this type
tends to yield relatively large average pore sizes. As a result, the
mechanical
properties of the film suffer, leading to inferior resistance to permanent
deformation, e.g., label wrinkling, label buckling, or label shrinkage, when
the
film is subjected to bending and creasing stresses.
In addition, single component cavitation of this type tends to yield a non-
uniform void distribution due to dispersion problems with the filler.
Furthermore,
the cavitated films produced from single component cavitation of this type
tend to
have a density falling within the range of from greater than
0.45 g/cm3 to 0.90 g/cm3.
It is also known in the art to induce voids in a film layer containing
polypropylene by including therein a beta-crystalline nucleating agent. The
voids
formed by this type of single component cavitation method tend to have a
decreased average pore size.
There are three types of crystalline forms for polypropylene - alpha, beta,
and gamma. The alpha-crystalline form of polypropylene has a monoclinic
crystal
structure. The beta-crystalline form of polypropylene has a hexagonal crystal
structure. The gamma-crystalline form of polypropylene has a triclinic crystal
structure. The gamma-crystalline form of polypropylene has the highest
density,
while the beta-crystalline form has the lowest density.
However, the gamma-crystalline form of polypropylene only grows under
high pressure. In typical film processing conditions, the gamma-crystalline
form
is not observed. And between the alpha-crystalline and beta-crystalline forms,
the
alpha-crystalline form is the more stable crystalline form. Under typical film
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processing conditions, the majority of polypropylene will be the alpha-
crystalline
form. Therefore, a beta-crystalline nucleating agent is required in order to
produce a significant amount of the beta-crystalline form of polypropylene
during
melt crystallization.
EP 0 865 909 of Davidson et al. discloses biaxially oriented, heat-
shrinkable polyolefin films for use as labels, having a layer of a
polypropylene-
based resin with microvoids therein. The microvoids are formed by stretching a
web containing the beta-crystalline form of polypropylene.
EP 0 865 910 and EP 0 865 912, both of Davidson et al., disclose biaxially
oriented polyolefin opaque films having a thickness of not more than 50 m and
having a layer of a polypropylene-based resin with microvoids therein. The
microvoids are formed by stretching a web containing the beta-crystalline form
of
polypropylene at an area stretch ratio of at least 15:1.
EP 0 865 911 of Davidson et al. discloses biaxially oriented polyolefin
films containing a heat seal layer and a layer having microvoids formed
therein by
stretching the polypropylene-based resin of the layer, which contains the beta-
crystalline form of polypropylene. The heat seal becomes transparent upon
heating.
EP 0 865 913 of Davidson et al. discloses biaxially oriented, heat-
shrinkable polyolefin films having a layer of a polypropylene-based resin with
microvoids therein. The microvoids have been formed by stretching a web
containing the beta-crystalline form of polypropylene. The film has a
shrinkage
after 10 minutes at 130 C of at least 10% in at least one direction.
EP 0 865 914 of Davidson et al. discloses biaxially oriented, high gloss
polyolefin films having a layer of a polypropylene-based resin with microvoids
therein and at least one olefin copolymer outer layer thereon. The microvoids
have been formed by stretching a web containing the beta-crystalline form of
polypropylene.
U.S. Patent 6,444,301 to Davidson, et al. discloses polymeric films
including a layer of propylene resin having microvoids therein, the microvoids
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having been formed by stretching a web containing the beta-form of
polypropylene.
U.S. Patent 5,594,070 to Jacoby, et al. discloses oriented microporous
films prepared from polyolefin resin compositions comprising an ethylene-
propylene block copolymer having an ethylene content of about 10 to about
50 wt. %, a propylene homopolymer or random propylene copolymer having up to
about 10 wt. % of a comonomer of ethylene or an a-olefin of 4 to 8 carbon
atoms,
and components selected from a low molecular weight polypropylene, a beta-
spherulite nucleating agent and an inorganic filler. The microporous films are
said to have improved breathability, strength, toughness and break elongation.
However, the films of Jacoby have a tendency to exhibit pink color when red
dye
(beta-spherulite nucleating agent) concentration is higher than 50 ppm. If the
concentration of red dye (beta-spherulite nucleating agent) is lower than 50
ppm,
then it is difficult to obtain consistent opacity due to poor dispersion
uniformity.
However, films cavitated using only a beta-crystalline nucleating agent,
such as films from the various Davidson publications noted above, are single
component cavitated films.
The polymer films market is also expanding due to growth in the
permeable films segment.
However, permeable polyolefin films that presently are commercially
available include those made with an embossable polyolefin loaded with a high
concentration of a filler, polyolefin films with high concentrations of a
filler and
low MD orientation, polyolefin films which are compounded with plasticizer and
later have the plasticizer extracted therefrom, and spun-bond fibers. These
types
of permeable films are either expensive or difficult to process.
For example, U.S. Patent 4,777,073 to Sheth discloses a breathable
polyolefin film prepared by melt embossing a highly filled polyolefin film to
impose a pattern of different film thickness therein and by stretching the
melt
embossed film to impart greater permeability in the areas of reduced thickness
in
comparison to the areas of greater thickness.
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U.S. Patent 6,002,064 to Kobylivker, et al. discloses a stretch-thinned
polymeric film formed from a mixture of a polymer matrix including a low
crystallinity propylene polymer having not more than about 30% crystallinity,
with a particulate filler.
5 U.S. Patent 6,045,900 to Haffner, et al. discloses a breathable barrier
laminate having a first film layer comprising a microporous breathable barrier
film; a second film layer comprising a breathable filled film which comprises
about 50% to about 70% by weight of a filler and an amorphous polymer such as
an elastomeric ethylene polymer having a density less than 0.89 g/cm3; and a
third
fibrous layer comprising a breathable outer layer, such as a nonwoven web of
spunbonded fibers. The multiple layers are thermally laminated wherein the
laminate has a peel strength in excess of 200 grams and a WVTR in excess of
300
g/m2/day.
U.S. Patent 6,106,956 to Heyn, et al. discloses a polymer film comprising
at least first and second contiguous and coextruded portions, wherein the
first
portion is extruded from a first polymer composition containing a filler
material in
an ainount sufficient to increase the water vapor permeability of the first
portion
relative to the second portion, and the second portion is extruded from a
second
polymer composition such that a tensile strength of the second portion is
greater
than the tensile strength of the first portion.
Finally, U.S. Patent 6,534,166 to Pip, et al. discloses films having
particular water vapor transmission rates (WVTR) produced by methods including
adherently superimposing at least one layer of a WVTR-controlling material to
a
base layer including a polyethylene and a cavitating agent, and subsequently
biaxially orienting the composite polyethylene sheet to yield a film having
the
desired WVTR. The base layer has a porous microstructure and a WVTR
substantially higher than the desired WVTR.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a film structure that is highly
permeable to both gas and water vapor.
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It is also an object of the invention to provide a permeable film structure,
which has uniform opacity, a low density and improved mechanical properties,
that is economically advantageous.
It is additionally an object of the invention to provide a permeable film
structure, which has uniform opacity, a low density and improved mechanical
properties, that is particularly suited for labeling applications.
There is provided a permeable film structure containing at least three layers,
the
core layer comprising a propylene polymer matrix that has been cavitated by a
two-component cavitation system, wherein the first component of the two-
component cavitation system is a beta-nucleating agent to produce the beta-
crystalline form of polypropylene, and the second component is a filler.
In preferred embodiments, there is provided a film structure that is highly
permeable to both gas and water-vapor, comprising: a core layer comprising a
propylene polymer, a beta-nucleating agent, and a filler; a first skin layer
on a first
side of the core layer, the first skin layer comprising a propylene polymer;
and a
second skin layer on a second side of the core layer, the second skin layer
comprising a propylene polymer; and wherein the film structure is opaque and
has
been biaxially oriented. In particularly prefeired enlbodiments, each of the
core
layer, first skin layer, and second skin layer is cavitated.
There is also provided a label comprising the permeable film structure
containing at least three layers, the core layer comprising a propylene
polymer
matrix that has been cavitated by a two-component cavitation system, wherein
the
first component of the two-component cavitation system is a beta-nucleating
agent
to produce the beta-crystalline form of polypropylene, and the second
component
is a filler.
There is furthermore provided a method of manufacturing a permeable
film structure, comprising: forming a melt comprising a propylene polymer, a
beta-nucleating agent and a filler; cooling the melt to form a film layer; and
stretching the film layer to form voids therein.
The invention takes advantage of a previously unknown synergy between
the beta-nucleating agent and the filler, which, when combined with a tailored
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combination of the respective amounts of the propylene polymer beta-nucleating
agent and the filler in the core layer, as well as the selection of first and
second
skin layers, provides a film structure that is highly permeable to both gas
and
water vapor, insofar as it has a water vapor transmission rate greater than
300
g/m2/day and a Gurley air permeability less than 3,000 s/10 cc.
DETAILED DESCRIPTION OF THE INVENTION
"Core layer" as used herein refers to the only layer of a monolayered film
or the thickest layer of a multilayered film. In general, the core layer of a
multilayer structure will usually be the innermost, central layer of the
structure.
It will be understood that when a layer is referred to as being "directly on"
another
layer, no intervening layers are present. On the other hand, when a layer is
referred to as being "on" another layer, intervening layers may or may not be
present.
"A filler" as used herein encompasses both the use of a single filler or any
combination of fillers.
The permeable film structure comprises a core layer.
The core layer comprises a polymeric matrix comprising a propylene
polymer. The term "propylene polymer" as used herein includes homopolymers
as well as copolymers of propylene, wherein a copolymer not only includes
polymers of propylene and another monomer, but also terpolymers, etc.
Preferably, however, the propylene polymer is a propylene homopolymer.
The propylene polymer of the core layer preferably has an isotacticity
ranging from about 80 to 100%, preferably greater than 84%, most preferably
from about 85 to 99%, as measured by 13C NMR spectroscopy using meso
pentads. A mixture of isotactic propylene polymers may be used. Preferably,
the
mixture comprises at least two propylene polymers having different m-pentads.
Preferably, the difference between m-pentads is at least 1%. Furthermore, the
propylene polymer of the core layer preferably has a melt index ranging from
about 2 to about 10 g/10 minutes, most preferably from about 3 to about 6 g/10
minutes, as measured according to ASTM D1238 at 190 C under a load of 5 lbs.
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Commercially available propylene polymers for the core layer include 3371,
which is an isotactic polypropylene homopolymer sold by Total Petrochemicals
USA, Inc., and PP4612E2, an isotactic propylene homopolymer, available from
ExxonMobil Chemical Company (Houston, Texas).
The core layer also comprises a beta-crystalline nucleating agent. Any
beta-crystalline nucleating agent ("beta-nucleating agent" or "beta-
nucleator")
may be used.
U.S. Patent 4,386,129 to Jacoby and U.S. Patent 4,975,469 to Jacoby
disclose processes of forming a film containing nucleating agents to produce
beta-
form spherulites and then selectively extracting the beta-spherulites. Both
Jacoby
patents disclose quinacridone compounds, bisodium salts of
o-phthalic acids, aluminum salts of 6-quinizarin sulfonic acid and isophthalic
and
terephthalic acids as beta-nucleating agents.
U.S. Patent 5,681,922 to Wolfschwenger, et al. discloses the use of
dicarboxylic acid salts of metals of the second main group of the Periodic
Table as
beta-nucleating agents.
A two-component beta-nucleator may be used as the beta-nucleating agent
of the invention. For example, U.S. Patent 5,231,126 to Shi, et al. discloses
the
use of a mixture of a dibasic organic acid and an oxide, hydroxide or salt of
a
metal of group IIA of the Periodic Table. A two-component beta-nucleator is
not
to be confused with the two-component cavitation method of the invention. A
two-component beta-nucleator still makes up only one component of the present
two-component cavitation method for producing the cavitated opaque polymer
films of the invention.
U.S. Patents 5,491,188; 6,235,823; and EP 0 632 095; each of Ilceda, et al.,
disclose the use of certain types of amide compounds as beta-nucleators.
U.S. Patent 6,005,034 to Hayashida, et al. discloses various types of beta-
nucleators.
U.S. Patents 4,386,129; 4,975,469; 5,681,922; 5,231,126; 5,491,188;
6,235,823; and 6,005,034; as well as EP 0632095, are herein incorporated by
reference to the extent not inconsistent with the disclosure herein.
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Preferably, the beta-nucleating agent is a two-component beta-nucleator
formed by the mixing of Components A and B. Component A is an organic
dibasic acid, such as pimelic acid, azelaic acid, o-phthalic acid,
terephthalic and
isophthalic acid and the like. Component B is an oxide, hydroxide or an acid
salt
of a Group II metal, e.g., magnesium, calcium, strontium and barium. The acid
salt of Component B may come from inorganic or organic acid such as carbonate,
stearate, etc. Component B may also be one of the additives of polypropylene
that
already is present in the polypropylene material. The proportion of component
A
may be in the range of 0.0001-5% by weight, based on the total weight of
polypropylene, most preferably 0.01-1 wt %, whereas the proportion of
component B is 0.0002-5% by weight, based on the total weight of
polypropylene,
most preferably 0.05-1%, during mixing.
Preferably, the beta-nucleating agent is not a red dye.
Preferably, the propylene polymer and beta-nucleating agent are brought
together to form the core layer via a masterbatch.
For example, in some embodiments, the core layer may comprise BEPOL
022SP, a masterbatch of isotactic propylene homopolymer and beta-nucleating
agent, available from Sunoco Chemicals. In other embodiments, the core layer
may coinprise an impact propylene copolymer masterbatch with a beta-crystal
nucleator of polypropylene or the core layer may comprise an impact propylene
copolymer masterbatch with a beta-crystal nucleator of polypropylene an.d an
isotactic polypropylene. In still other embodiments, the core layer may
comprise:
an (isotactic propylene)-ethylene heterophasic copolymer masterbatch with a
beta-
crystal nucleator of polypropylene and an isotactic polypropylene; an impact
polypropylene masterbatch with a beta-crystal nucleator of polypropylene and a
metallocene isotactic polypropylene; or an (isotactic propylene)-ethylene
heterophasic copolymer, ethylene-propylene-ethylidene norbornene elastomer,
isotactic polypropylene masterbatch with a beta-crystal nucleator of
polypropylene and an isotactic polypropylene that has a different m-pentad
than
the isotactic polypropylene in the isotactic polypropylene masterbatch.
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One type of impact copolymer which may be used in the invention
comprises a polymer matrix with a dispersed rubbery copolymer phase. The
matrix is a homopolymer or random copolymer matrix. The rubbery copolymer
phase is a reactor blend of an amorphous rubber, a rubber-like polymer which
is
5 normally an ethylene-propylene copolymer (rubber), and a semicrystalline
ethylene copolymer.
By mixing the propylene polymer of the core layer, which predominantly
contains the alpha-crystalline form of polypropylene, with the beta-nucleating
agent of the core layer, high concentrations of the beta-crystalline form of
10 polypropylene are induced after the melting and subsequent cooling steps of
the
film-making process. The beta-crystalline form of polypropylene has a lower
melting point and a lower density than the common alpha-crystalline form of
polypropylene.
The core layer furthermore comprises a filler. Preferably, the filler is an
inorganic filler. Most preferably, the filler is selected from the group
consisting of
calcium carbonate (CaCO3), barium carbonate (BaCO3), clay, talc, silica, mica,
titanium dioxide (Ti02) and mixtures thereof
Although in broader embodiments the filler of the invention encompasses
an organic filler, preferably the filler is not an organic filler. Organic
fillers tend
to plate-out, which results in manufacturing downtime. Also, the cavitation
quality from the use of organic fillers is sensitive to the viscosity change
from the
polypropylene reclaims and output rate variations.
The amount of a filler to be included in the core layer may range from 1 to
50 wt%, based on the total weight of the core layer. Preferably, the core
layer
contains from 5 to 35 wt% of a filler, most preferably from 5 to 25 wt%, based
on
the total weight of the core layer.
The amount of beta-nucleator to be included in the core layer should be
enough to obtain the desired degree of void formation upon stretching. The
amount of beta-nucleator may also be used to control the degree of opacity.
Preferred amounts of beta-nucleator are from 50 ppm to 1,000 ppm based on the
amount of propylene polymer in the core layer, more preferably from 50 to 500
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ppm, more preferably from 50 to 300 ppm, and most preferably from 100 to 200
ppm.
Generally, the remainder of the core layer is made up of the propylene
polymer(s) mentioned above, after the filler, beta-nucleator, and any optional
additives have been taken into account.
The core layer is typically the thickest layer of the film structure.
Preferably, the core layer thickness is at least 70% of the whole film
thickness.
The permeable film structures of the invention may be multilayer film
structures wherein another layer or layers besides the core layer has been
cavitated
or otherwise provided with permeability enhancement, such as by
microperforation or embossing. In some preferred embodiments, at least one
other layer besides the core layer of the permeable film structure is
cavitated.
More preferably, at least two other layers besides the core layer of the
permeable
film structure are cavitated. Most preferably, each layer of the permeable
film
structure is cavitated and/or provided with enllanced permeability. In
particular,
the optimum combination of low density, low light transmission and high
permeability to both gas and water vapor is attained when each layer of the
permeable film structure is cavitated.
The other cavitated layer(s) of the permeable film structure may be
cavitated via the two-component cavitation method of the invention. For
example, another layer(s) of a multilayer film structure according to this
invention
may comprise each of the same cavitating components as the core layer.
Alternatively, the other cavitated layer(s) of the permeable film structure
may be cavitated without using the two-component cavitation method of the
invention. For example, the other cavitated layer(s) of the permeable film
structure may be cavitated by use of a single component cavitation method,
e.g., a
filler only or a beta-nucleator only. Indeed, the other cavitated layer(s) of
the
permeable film structure may also or separately be provided with enhanced
permeability by any marnler known in the art, including microperforation of
the
outer skin layer or layers, or by embossing the outer skin layer or layers.
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The permeable film structures of the invention comprise a first skin layer
on one side of the core layer. The first skin layer may be provided on or
directly
on a side of the core layer. The first skin layer may be cavitated by the two-
component cavitation method of the invention or by any manner known in the
art.
In general, the first skin layer may comprise a polymeric matrix
comprising any of the film-forming thermoplastic polymers. Examples of
suitable
film-forming thermoplastic polymers include the polyolefins, such as propylene
polymers and ethylene polymers.
In preferred embodiments, the first skin layer comprises a propylene
polymer selected from the group consisting of isotactic propylene homopolymer,
syndiotactic propylene homopolymer, isotactic propylene impact copolymer, and
syndiotactic propylene impact copolymer. The propylene homopolymer and
propylene impact copolymer may contain a beta-nucleating agent. For example,
the impact copolymer may be TI-4040-G, an impact propylene copolymer
available from Sunoco Chemicals. TI-4040-G contains 17% ethylene-propylene
rubber content. The ethylene-propylene rubber content of
TI-4040-G can, upon stretching, cause cavitation of the layer comprising the
TI-4040-G.
In other embodiments, the first skin layer will be a sealable skin layer,
such as a heat-sealable skin layer. For example, the first skin layer may
comprise
propylene-ethylene copolymer, propylene-ethylene-butene-1 terpolymer (such as
XPM7510, an ethylene-propylene-butene-1 terpolymer, available from Chisso
Company, Japan), propylene-a-olefin copolymer, or metallocene-catalyzed
ethylene-a-olefin copolymer.
In still other embodiments, the first skin layer is a sealable skin layer
comprising a polymer selected from the group consisting of an an (isotactic
propylene)-a-olefin copolymer, a (syndiotactic propylene)-a-olefin copolymer,
an
ethylene-vinyl acetate copolymer (EVA), an ethylene-methacrylic acid copolymer
(EMA), an ethylene-acrylic acid copolymer (EAA), an ethylene-methylacrylate-
acrylic acid terpolymer (EMAAA), an ethylene-alkyl acrylate copolymer, an
ionomer such as ethylene-alkyl acrylate-acrylic acid Zn salt or Na salt, a
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metallocene-catalyzed plastomer, a very low density polyethylene (VLDPE), for
example, having a density of 0.89 to 0.915 g/cc, an ethylene-(methyl acrylate)-
(glycidyl methacrylate) terpolymer, and an ethylene-(glycidyl methacrylate)
copolymer.
The first skin layer may also comprise a mixture of any of the foregoing
polymers.
As mentioned, the first skin layer may be provided directly on a side of the
core layer or on a side of the core layer with one or more intermediate layers
therebetween.
An intermediate, or tie layer of the invention may comprise a polymeric
matrix comprising any of the fihn-forming polymers. Suitable film-forming
polymers for forming the polymeric matrix of the optional intermediate
layer(s)
include polyolefins, such as polypropylene, syndiotactic polypropylene,
polypropylene copolymers, low density polyethylene (LDPE), linear low density
polyethylene (LLDPE), medium density polyethylene (MDPE), high density
polyethylene (HDPE), ethylene copolymers, nylons, polymers grafted with
functional groups, blends of these, etc. For example, an intermediate layer
may
comprise a polyolefin grafted with a functional group, such as ADMER 1179, a
maleic anhydride-grafted polypropylene available from Mitsui Petrochemical
Industries Ltd. (Tokyo, Japan). The intermediate, or tie layer may be
cavitated by
the two-component cavitation method of the invention or by any manner known in
the art.
Permeable film structures of the invention further comprise a second skin
layer on a side of the core layer opposite the first skin layer.
The film-forming material for the second skin layer may be independently
selected from the same film-forming materials noted above for the first skin
layer.
As with the first skin layer, the second skin layer may be provided directly
on the
side of the core layer or on the side of the core layer with one or more
intermediate layers therebetween. The intermediate layer between the core
layer
and second skin layer may comprise a polymeric matrix comprising any of the
film-forming polymers. For example, the film-forming material for an
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intermediate layer between the core layer and second skin layer may be
independently selected from the same film-forming materials noted above for an
intermediate layer between the core layer and first skin layer.
Both the second skin layer and a.ily intermediate, or tie layer between it
and the core layer may be cavitated by the two-component cavitation method of
the invention or by any manner known in the art. At least one of the first
skin
layer and the second skin layer is a cavitated layer and the cavitating agent
of at
least one of the first skin layer and the second skin layer comprises at least
one of
a beta-nucleating agent and a filler. Films according to the present invention
may
thus comprise one or more intermediate layers between the core layer and the
first
skin layer and/or between the core layer and the second skin layer.
One or both outer surfaces of the overall film structure may be surface
treated. In the case of a monolayer structure, the outer surfaces of the
structure
would simply be the outer surfaces of the core layer. If the structure
consists of a
core layer and first skin layer, the outer surfaces would be the surface of
the first
skin layer opposite the core layer and the surface of the core layer opposite
the
first skin layer. If the structure contains a core layer and at least first
and second
skin layers, the outer surfaces would be the surfaces of the first and second
skin
layers that are respectively opposite the core layer.
The surface treatment may be effected by any of various techniques,
including, for example, flame treatment, corona treatment, and plasma
treatment.
In certain embodiments, the outer surface or surfaces may even be metallized.
Metallization can be effected by vacuum deposition, or any other metallization
technique, such as electroplating or sputtering. The metal may be aluminum, or
any other metal capable of being vacuum deposited, electroplated, or
sputtered,
for example, gold, silver, zinc, copper, or iron.
One or both outer surfaces of the overall film structure may be coated with
a coating, such as a primer coating, e.g., a polyvinylidene chloride (PVdC),
acrylic, or silicon oxide (SiO,) coating, a water-based coating, or a coating
comprising inorganic particles, such as clay, calcium carbonate, or titanium
oxide,
dispersed in a binder, such as an iminated butyl acrylate copolymer. Coatings
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may be used to provide advantages such as enhanced gloss and enhanced
compatibility with manufacturing processes and machinery. In certain
embodiments, priming the first skin layer can render it more receptive to
printing.
The films may also be coated with an adhesive, such as a cold glue adhesive or
a
5 pressure-sensitive adhesive.
In order to modify or enhance certain properties of the overall film
structure, it is possible for one or more of the layers to contain dispersed
within
their respective matrices appropriate additives in effective amounts.
Preferred
additives include anti-blocks, anti-static agents, anti-oxidants, anti-
condensing
10 agents, co-efficient of friction (COF) modifiers (slip agents), processing
aids,
colorants, clarifiers, foaming agents, flame retardants, photodegradable
agents,
UV sensitizers or UV blocking agents, crosslinking agents, ionomers and any
other additives known to those skilled in the art.
For exainple, in certain embodiments, it may be desirable to include a
15 coloring agent, such as a pigment or dye, in one or more of the layers,
including
the first skin layer or the tie layer between the core layer and first skin
layer.
As another example, in certain embodiments, and especially certain label
embodiments, the polymer matrix of a skin layer may include dispersed therein
one or more anti-block agents to prevent "grabbing" of the label on machine
surfaces, one or more slip agents to provide better slip on heated metal
surfaces,
and/or one or more anti-static agents to maximize sheetability. Specific
examples
of anti-block agents include coated silica, uncoated silica and crosslinked
silicone.
Specific examples of slip agents include silicone oils. Specific examples of
anti-
static agents include alkali metal sulfonates, tertiary amines and the like.
The invention provides permeable film structures that have been tailored
for label applications. A preferred label structure comprises a core layer
comprising a polymeric matrix comprising a propylene polymer, a beta-
nucleating
agent, and a filler, and first and second skin layers. The first and second
skin
layers may be provided on or directly on the respective sides of the core
layer.
Preferably, a label according to the invention will comprise an adhesive
provided on an outer surface of the first or second skin layer. The type of
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adhesive to be employed is not particularly limited. As an example, the
adhesive
may be a water-based adhesive, such as a cold glue adhesive or a
polyvinylidene
chloride latex. Cold glue adhesives are natural or synthetic adhesives, such
as
HENKEL 7302, available from Henkel Adhesives, or OC 363-20, available from
O.C. Adhesives Corp. As another example, the adhesive may be a pressure-
sensitive adhesive. Adhesives suitable for labels are well-known in the art.
Alternatively, a label according to the invention can be an in-mold label
used in the making of hollow blown articles such as containers. Methods of
manufacturing hollow blown articles which include the use of an in-mold label
are
well-known in the art, and any in-mold labeling method may be used to apply an
in-mold label according to the invention onto a container. As just one
example,
reference is made to U.S. Patent 5,855,838, which is herein incorporated by
reference to the extent not inconsistent with the disclosure herein.
There is also provided a method of manufacturing a permeable film
structure. For example, a melt(s) corresponding to the individual layer(s) of
the
film structure may be prepared. The melt(s) may be cast-extruded or coextruded
into a sheet using a flat die or blown-extruded or coextruded using a tubular
die.
The sheets may then be oriented either uniaxially or biaxially by known
stretching
techniques. For example, the sheet may be uniaxially oriented from four to
eight
times of orientation ratio.
While the films may be made by any method, preferably the films are
made by coextrusion and biaxial stretching of the layer(s). The biaxial
orientation
may be accomplished by either sequential or simultaneous orientation, as is
known in the art. In particularly preferred embodiments, the film structure is
oriented from four to six times in the machine direction and from four to ten
times
in the transverse direction.
During the manufacturing process, if the cast temperature is set too low,
i.e., quick quenching, the alpha-crystalline form will dominate and the beta-
crystalline form will be in the minority. Therefore, films according to the
invention are preferably manufactured by setting the cast roll temperature at
above
85 C, more preferably from 90 C to 100 C. The nip roll against the cast roll
is
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preferably set to a range of from 93 C to 120 C. At these settings, beta-
crystalline formation is maximized. Though the films can be cast with or
without
a waterbath, preferably the film is cast without a waterbath.
In comparison to single component cavitated films, the two-component
cavitated films of the invention have a low density of from 0.20 to 0.45
g/cm3,
preferably from 0.25 to 0.45 g/cm3, more preferably from 0.25 to 0.40 g/cm3.
The film density values reported herein were measured by a method of first
measuring the yield of the film. Specifically, 80 pieces of film from a film
sample
are cut, each having a diameter of 4 inches (10.16 cm). The total area of the
80
pieces is then calculated. The weight of the 80 pieces (in grams) is then
measured. The yield of the film (cm2/gram) will equal the total specimen area
(Cm2) over the specimen weight (gram).
After measuring the film yield, the film thickness is measured with a laser
beam. In particular, the film thickness (mil) is measured with a Model 238-20,
available from Beta LaserMike Company. The thickness unit value is converted
from mils to centimeters. This non-contact method for measuring film thickness
is especially suited for microvoided film because it avoids the error that
arises
from mechanical compression on the film from a conventional micrometer.
Finally, the density (gram/em3) is calculated from the inverse (1/X) of the
film
yield (cm2/gram) times the film thickness (cm).
Opacity represents a substrate's light blocking ability and may be defined
in terms of light transmission, as measured by ASTM D1003. The two-
component cavitated films of the invention are opaque and have more uniform
opacity in comparison to single-component cavitated films. Light transmission
is
the percentage of incident light that passes through a film and for films
according
to the present invention may range from about 0% to about 50%, preferably from
about 5% to about 35%, and more preferably from about 15% to about 35%.
Preferably, the light transmission of the film is less than 35%, more
preferably
less than 30%, and most preferably less than 25%.
Films according to the invention are ideal for label applications, including
cut & stack labeling, patch, pressure-sensitive adhesive, and in-mold
labeling.
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Their excellent stiffness allows them to endure any labeling and bottling
application.
For example, the permeable films of the invention can be used as a label
facestock laminated to a silicone release liner with pressure-sensitive
adhesive.
The pressure-sensitive label stock can be run through a die-cutter to produce
labels affixed to a continuous release liner. As another example, the
permeable
films can be used as cut & stack labels to replace paper-based labels.
Traditional
cut & stack labels are paper labels coated with cold glue and applied on glass
or
plastic containers.
The permeable films of the invention may also be used with particular
advantage for the manufacture of opaque packages for various materials, such
as
light-sensitive foodstuffs, particularly where moisture permeability is
desired.
Additionally, the permeable films may be used for other packaging purposes
where opaque polymeric films are required. In general, the films of the
invention
can be useful for any thick film application that requires superior stiffness.
Due to the high gas and moisture transmission rates of the film structures,
they may be used for medical applications, where breathable films are
required.
Indeed, any development using a water-activated coating would take particular
advantage of the breathable permeable films of the invention. Other
opportunities
are wall paper, high speed water-based ink printing or water-based coatings,
water-vapor and gas permeable films for TYVEK's home wrap, stucco wrap, and
commercial wrap, a moisture-permeable film for bakery packaging, and air- and
moisture-permeable films for garments.
Total thickness of a film according to the invention is not particularly
limited. For certain applications, the overall thickness should be greater
than 20
m for poly-gauge. Preferably, the film has an overall thickness of 30 m to
110
m for poly-gauge. Preferably, the thickness of each layer, as measured for the
poly-gauge, ranges from 15 m to 80 m for the core layer; from 0.5 m to 5 m
for the first outer layer (if present); from 0.5 m to 5 m for the second
outer
layer (if present); and from 1 m to 10 m for an intermediate layer (if
present).
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The present invention will be further described with reference to the
following
nonlimiting examples. For each example, the thickness values represent poly-
gauge thickness.
EXAMPLES
Test Procedures
Gas permeability results for the example films of the invention are set
forth below in terms of Gurley air permeability (s/10 cc). Gurley air
permeability
was calculated by using a Teleyn Gurley Model 4190 Porosity Tester with
sensitivity attachment in accordance with the following procedure:
(a) a strip of film (-2" wide) was cut across the entire web width;
(b) the film sample to be tested was inserted between the orifice plates;
(c) the sensitivity adjustment was set to "5;"
(d) the inner cylinder was turned so that the timer eye was vertically
centered below the 10 cc silver step on the cylinder;
(e) the timer was reset to zero; and
(f) the spring was pulled clear of the top flange and the cylinder was
released.
When the timer stopped counting, the test was completed. The resulting
value was "Gurley seconds per 10 cc."
Moisture barrier properties were evaluated by determining the water vapor
transmission rate (WVTR) of the films according to ASTM F1249 methods.
Example 1
A three layer opaque film is cast, without waterbath, at 93 C and oriented
via
tenter-frame sequential orientation at five times in the MD and eight times in
the
TD. The film had an A/B/A structure, as follows:
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First outer layer TI4040G; 2.5 m
65 wt% PP4612E2 + 25 wt% Bepol
Core layer 022SP + 10 wt% HDPE/CaCO3
masterbatch (60 wt% CaCO3
concentration); 37.5 m
Second layer TI4040G; 2.5 m
The film of Example 1 had a light transmission of about 7.1 % and a film
density of about 0.358 g/cm3.
5 Example 2
A three layer opaque film is cast, without waterbath, at 93 C and oriented
via tenter-frame sequential orientation at five times in the MD and eight
times in
the TD. The film had an A/B/A structure, as follows:
First outer layer TI4040G; 2.5 in
75 wt% PP4612E2 + 15 wt% Bepol
Core layer 022SP + 10 wt% PP/CaCO3
masterbatch (70 wt /o CaCO3
concentration); 37.5 m
Second layer T14040G; 2.5 m
10 The film of Example 2 had a light transmission of about 7.5% and a film
density of about 0.362 g/cm3.
Example 3
A three layer opaque film is cast, without waterbath, at 93 C and oriented
via tenter-frame sequential orientation at five times in the MD and eight
times in
15 the TD. The film had an A/B/A structure, as follows:
First outer layer TI4040G; 2.5 m
55 wt% PP4612E2 + 25 wt% Bepol
Core layer 022SP + 20 wt% PP/CaCO3
masterbatch (70 wt/o CaCO3
concentration); 40 m
Second layer TI4040G; 2.5 m
The film of Example 3 had a light transmission of about 5.4% and a film
density of about 0.304 g/cm3.
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Example 4
A three layer opaque film is cast, without waterbath, at 93 C and oriented
via tenter-frame sequential orientation at five times in the MD and eight
times in
the TD. The film had an A/B/A structure, as follows:
First outer layer TI4040G; 2.5 m
45 wt% PP4612E2 + 25 wt% Bepol
Core layer 022SP + 30 wt% PP/CaCO3
masterbatch (70 wt/o CaCO3
concentration); 45 m
Second layer TI4040G; 2.5 m
The film of Example 4 had a light transmission of about 4.3% and a film
density of about 0.270 g/cm3.
Comparative Example A
A three layer opaque film is cast, with waterbath, at 38 C and oriented via
tenter-frame sequential orientation at five times in the MD and eight times in
the
TD. The film had an A/B/A structure, as follows:
First outer layer TI4040G; 2.5 m
85 wt% PP4612E2 + 15 wt%
Core layer HDPE/CaCO3 masterbatch (60 wt%
CaCO3 concentration); 37.5 m
Second layer TI4040G; 2.5 m
The film of this comparative example had a light transmission of about
21.0% and a film density of about 0.555 g/cm3. Conventional polypropylene
(without a beta-nucleating additive) is typically cast at around 38 C with a
waterbath in order to facilitate orientation.
Comparative Example B
A three layer opaque film is cast, without waterbath, at 93 C and oriented
via tenter-frame sequential orientation at five times in the MD and eight
times in
the TD. The film had an A/B/A structure, as follows:
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First outer layer XOM 4712; 2.5 ~Lm
Core layer 85 wt% PP4612E2 + 15 wt% Bepol
022SP; 32 m
Second layer XOM 4712; 2.5 m
Thus, the core layer of this comparative film had beta-nucleating agent but
no filler, e.g., no CaCO3. This comparative film had a light transmission of
about
16.7% and a film density of about 0.56 g/cm3.
Example 5
A three layer opaque film is cast, without waterbath, at 93 C and oriented
via tenter-frame sequential orientation at five times in the MD and eight
times in
the TD. The film had an A/B/C structure, as follows:
First outer layer XPM7510; 2.5 m
50 wt% PP4612E2 + 20 wt% Bepol
Core layer 022SP + 30 wt% PP/CaCO3
masterbatch (70 wt% CaCO3
concentration); 42 m
Second layer XOM 4712; 2.5 gm
XOM 4712 is a propylene homopolymer, available from
ExxonMobil Chemicals.
The film of Example 5 had a film density of about 0.280 g/cm3.
The film was used as a label facestock by laminating it to a release liner
with a water-based pressure-sensitive adhesive. In particular, the second
outer
layer was coated with the pressure-sensitive adhesive, which contacted the
silicone surface of the release liner after lamination. The laminated label
stock
was run through a label-converting machine to make labels.
Example 6
A cold glue coating, Henkel 7302, was applied to the film from Example
1, and the film with cold glue thereon was applied onto a beer bottle. The
Henkel
7302 cold glue coating was applied on the outside surface of the second outer
layer.
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Example 7
The outer surface of the second layer of the film of Example 2 was
vacuum-metallized with aluminum and used as a metallized-paper replacement.
Example 8
The outer surfaces of the first and second layers of the film of Example 4
were coated with a coating comprising clay particles dispersed in an iminated
butyl acrylate copolymer at a coating weight of 2.6 g/m2. The coated film was
converted into cut-and-stack labels with a guillotine machine.
Example 9
A three layer opaque film was cast, without waterbath, at 93 C and
oriented via tenter-frame sequential orientation at five times in the MD and
eight
times in the TD. The film had an A/B/A structure, as follows:
First outer layer TI4040G; 2.5 m
73 wt% PP4612E2 + 15 wt% Bepol
Core layer 022SP + 12 wt% PP/CaCO3
masterbatch (70 wt% CaCO3
concentration); 37.5 m
Second outer layer TI4040G; 2.5 m
The film of Example 1 had a Gurley air permeability of 1,875 s/10 cc and
a WVTR of 617 g/m2/day.
Example 10
A three layer opaque film was cast, without waterbath, at 93 C and
oriented via tenter-frame sequential orientation at five times in the MD and
eight
times in the TD. The film had an A/B/A structure, as follows:
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First outer layer TI4040G; 2.5 gm
65 wt% PP4612E2 + 15 wt% Bepol
Core layer 022SP + 20 wt% PP/CaCO3
masterbatch (70 wt% CaCO3
concentration); 40 m
Second outer layer TI4040G; 2.5 m
The film of Example 2 had a Gurley air permeability of 1,055 sI10 cc and
a WVTR of 876 g/m2/day.
Example 11
A three layer opaque film was cast, without waterbath, at 93 C and
oriented via tenter-frame sequential orientation at five times in the MD and
eight
times in the TD. The film had an A/B/A structure, as follows:
First outer layer TI4040G; 2.5 m
55 wt% PP4612E2 + 15 wt% Bepol
Core layer 022SP + 30 wt% PP/CaCO3
inasterbatch (70 wt/o CaCO3
concentration); 40 m
Second outer layer TI4040G; 2.5 m
The film of Example 3 had a Gurley air permeability of 343 s/l0 cc and a
WVTR of 3,054 g/m2/day.
Comparative Example C
A three layer opaque film was cast, with waterbath, at 38 C and oriented
via tenter-frame sequential orientation at five times in the MD and eight
times in
the TD. The film had an A/B/A structure, as follows:
First outer layer TI4040G; 2.5 m
85 wt% PP4612E2 + 15 wt%
Core layer HDPE/CaCO3 masterbatch (60 wt%
CaCO3 concentration); 37.5 m
Second outer layer TI4040G; 2.5 m
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The film of this comparative example had a Gurley air permeability of >
18 hours/2.5 cc and a WVTR of 5.4 g/m2/day. Conventional polypropylene
(without a beta-nucleating additive) is typically cast at around 38 C with a
waterbath in order to facilitate orientation.
5 While the invention has been described in detail and with reference to
specific embodiments thereof, it will be apparent to one of ordinary skill in
the art
that various changes and modifications can be made therein without departing
from the spirit and scope of the invention. The Examples recited herein are
demonstrative only and are not meant to be limiting.
