Note: Descriptions are shown in the official language in which they were submitted.
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BIAXIALLY ORIENTED POLYOLEFIN SLIP FILMS
WITH IMPROVED FLATNESS AND ADHESION PROPERTIES
SPECIFICATION
FIELD OF THE INVENTION
This invention relates generally to polyolefin films; more particularly to
biaxially
oriented polyolefin films, preferably polypropylene films, having a desired
flatness to
minimize processing problems during printing and/or laminating, desired slip
properties
to render the films machinable in labeling apparatus and desired adhesion
properties
to adhere to adhesives, particularly hot melt adhesives, employed in labeling
applications.
BACKGROUND ART
Biaxially oriented polypropylene films have been employed in the roll fed
label
and packaging industries. One of the key product attributes that must be
possessed
by these films is a sufficiently low coefficient of friction (COF) to allow
the film to easily
slide on various machine surfaces, thus resulting in efficient processing of
the films.
In addition, a second key attribute is that the surface having desired slip
properties
must also have sufficient adhesion properties to effectively adhere to
adhesives
employed in label applications.
Since the COF of polypropylene film is generally higher than that desired for
effective processing in labeling applications, various additives have been
incorporated
into the film to reduce the COF. Two common additives employed to reduce COF
are
organosilicone particulate materials, such as Tospearl T120, which is supplied
by G.E.
Toshiba Sales Americas in Waterford, New York, and fatty acid amides.
When using an organosilicone particulate material such as Tospearl, the
particles are typically included in a thin, coextruded surface layer of a
multi-layer film.
The particles tend to reduce the degree of contact between the polypropylene
film and
machine surface engaging the skin layer, and generally act like ball bearings
to reduce
the drag of the film on the machine surfaces. While this technology works well
in
reducing the degree of contact between the film and the machine surfaces, as
well as
lowering the measured COF, the resulting film surface has a roughness, or
uneven
contour, that has been shown to have a negative impact in certain labeling
applications.
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As noted above, fatty acid amides also have been employed as slip agents to
reduce COF. These amides generally are compounded with the polyolefin polymer
and are thus contained within the matrix of the polymer throughout the
orientation
process of the film. Since the polyolefin polymer and amide are generally
chemically
incompatible, over time the amide will migrate to the film's surface, thereby
providing
desired slip properties to the film. However, due to other steric,
thermodynamic and
practical considerations, the diffusion process often needs to be accelerated
through
the use of heat. Although heating the film does provide for a faster and more
effective
migration of the amide to the film surface, thereby lowering the COF, the
exposure of
the film to elevated temperatures often causes non-uniform shrinkage of the
film. This
can have an adverse affect on the desired flatness of the sheet, thereby
creating
problems during printing and or laminating processes. Moreover, excessive
migration
of the fatty acid amides to the surface of the film may adversely affect the
adhesion
properties of the film surface. This can present problems in utilizing the
film in labeling
applications.
Although coextruded, multilayer polyolefin films have included both an
antistatic
additive and a migratory amide in them, to the best of applicant's knowledge
no prior
art, coextruded multilayer film has included an antistatic agent in an outer
skin layer
and a migratory amide in the core layer for any purpose, let alone for the
purposes of
reducing the COF of the skin layer against metal machine surfaces, such as
metal
surfaces in labeling machines, and of providing desired adhesion properties to
adhesives employed in labels. Stating this another way, applicant is not aware
of any
prior art films in which the types and amounts of an antistatic additive in
the outer skin
and a migratory amide in the core are employed for any purpose, let alone to
achieve
the desired COF and adhesion properties desired for adhesive label
applications, as
in the present invention.
In view of the state of the art, a need exists in the labeling art for a
biaxially
oriented polyolefin film having a desired flatness to reduce, or minimize,
problems
during printing and/or laminating in the manufacture of labels, desired slip
properties
to render the film machinable without the use of particulate materials, such
as
organosilicone particulate materials, in the surface layer to reduce COF, and
also
without the need for heating the film to enhance the rate of migration of a
fatty acid
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amide from the core to the surface of the film for providing the desired COF,
and with
the desired "adhesion" (hereinafter defined) to adhesives employed in label
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applications, such as to hot melt adhesives. It is to such an improved
multilayer
polyolefin film that the present invention is directed.
SUMMARY OF THE INVENTION
The above and other objects of this invention are achieved in a biaxially
oriented, multilayer, polyolefin film including a core layer and at least one
outer skin
layer. The skin layer preferably is adhered to the core layer and has desired
slip
properties and adhesion properties in accordance with this invention. The
outer skin
layer preferably includes a blend that is predominately polypropylene
homopolymer,
by weight, with a minor weight percent of an additive that results in the
films of this
invention having the desired slip and adhesion properties as specified in
greater detail
hereinafter. In the most preferred embodiments of this invention the additive
includes
an antistatic agent. Most preferably the antistatic agent includes an
ethoxylated
alkylamine and/or an ethoxylated alkylamide, each by itself, or each possibly
either
physically blended with an ester or esterified by a chemical reaction with a
suitable acid
(e.g., stearic acid) or with another ester, e.g., glycerol monostearate,
sorbitol-based
esters, etc. Most preferably, the antistatic agent includes an ethoxylated
alkylamine
that is esterified with another ester or with a suitable acid, an ethoxylated
alkylamine
blended with an ester, or an ethoxylated alkylamine, by itself.
It is also within the scope of the broadest aspects of this invention to
employ a
glycerol monostearate, by itself, as the antistatic additive. However, the
benefits
achieved with this latter antistatic additive are not as good as the benefits
achieved
with an antistatic agent in the form of an ethoxylated alkylamine that is
esterified with
another ester or with a suitable acid, an ethoxylated alkylamine blended with
an ester,
or an ethoxylated alkylamine, by itself.
The core layer is predominantly a polypropylene homopolymer, by weight,
including a minor percent by weight of the core layer of a migratory amide.
The
additive in the skin layer, preferably including an antistatic agent, and the
migratory
amide in the core layer are present in amounts that provide a film/metal COF
of less
than 0.45 and preferably less that 0.35, and an adhesion of at least 50% to
hot melt
adhesives usable in the products of this invention; more preferably an
adhesion of at
least 70%; even more preferably an adhesion of at least 90% and most
preferably an
adhesion in excess of 90%. In order to achieve these COF and adhesion
properties
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the additive in the skin layer needs to be of a type that provides the desired
slip
properties without undesirably affecting adhesion properties and needs to be
present
in an amount to preclude the migratory amide in the core from migrating to the
surface
of the skin layer in an amount that impairs the adhesion properties at that
surface.
Similar{y, the migratory amide needs to be of a type and amount that does not
preclude the additive in the skin layer from providing its desired slip and
adhesion
properties.
In a preferred embodiment of this invention, the additive in the skin layer is
an
antistatic agent that is included in a particulate masterbatch based on
homopolymer
polypropylene, with the polypropylene being about 87 - 88% of the masterbatch
and
the antistatic agent being about 12 - 13% of the masterbatch. The preferred
~ TM
masterbatch Is POLYBATCH ASPA 2485, manufactured by A. Schulman, Inc., of
Akron, Ohio. The weight percent of the POLYBATCH ASPA 2485 masterbatch
employed in the skin layer preferably is at least 4% by weight of said skin
layer; more
preferably in excess of 4% and even more preferably about 8% or even more.
Thus,
the weight percent of the antistatic agent in the skin layer preferably is at
least about
0.48%, more preferably in excess of 0.48% and even more preferably in the
range of
0.96% to 1.04%, or even more.
As an alternative to employing the above-identified masterbatch, the
antistatic
agent may be an ethoxylated amine, such as a synthetic ethoxylated amine,
e.g.,
Atmer 163 sold by Uniqema in New Castle, Delaware. This ethoxylated amine can
be
employed by itself or in combination with a small amount of an ester, such as
glycerol
TM
monostearate, e.g., Atmer 129 sold by Uniqema in New Castle, Delaware.
Moreover,
for some applications it may be possible to employ an ester, such as glycerol
monostearate, by itself as the antitstatic agent.
In a preferred embodiment of this invention the migratory amide in the core is
behenamide and preferably is present in the core in an amount greaterthan
0.10% by
weight of the core layer and more preferably about 0.25% or more. Most
preferably
the behenamide is present in a range of about 0.25% to about 0.50%. At a level
of
about 1.00%, adhesive performance is diminished, and in some cases is
unacceptable. In a preferred embodiment of this invention, the migratory amide
is a
behenamide of the type supplied as Kemamide B by Witco, based in Greenwich
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Connecticut, and is included in the commercially available polypropylene
homopolymer, AristechTM FF038A2.
In accordance with the preferred embodiments of this invention, the adhesives
employed in the labels are hot melt adhesives having varying levels of
aggressiveness.
In this invention, these hot melt adhesives have a desired level of adhesion
to the
surface of the outer skin layer including the antistatic agent therein even if
the film is
exposed to adverse environmental conditions, e.g., as a result of being stored
and/or
transported in a hot environment for long periods of time. Under these adverse
conditions the surface of prior art films including behenamide in both the
core and in
the outer skin layer of a multilayer structure, or in a single monolayer
structure, will not
retain its desired adhesion properties. As noted earlier, the adhesion of the
hot melt
adhesives In accordance with this invention is at least 50%; more preferably
at least
70%; even more preferably at least 90% and most preferably in excess of 90%.
In accordance with the preferred embodiment of this invention, the film
surface
including the antistatic agent therein is free of direct oxidative treatment,
such as
corona or flame treatment. However, when the opposed surface of the film is
subject
to direct oxidative treatment, e.g., to improve its ink-receptivity or
adhesive-receptivity
properties, there is a spillover, or "backside treatmenY' of the film surface
including the
antistatic agent that inherently occurs. The opposed surface preferably is
oxidatively
treated. This backside treatment is substantially less than the direct
treatment of the
opposed surface, and still results in the formation of an acceptable product.
It has been found that the direct corona treatment of the film surface
including
the antistatic agent therein tends to decrease the adhesion of the hot melt
adhesive
to the surface layer, resulting in poor, or reduced label performance. The
reason for
this phenomena is not clearly understood. However, in some products within the
scope of this invention the film surface including the additive therein may be
directly
oxidatively treated. In fact, it has been determined that direct, '9ight"
corona treatment
of the slip layer still results in the formation of an acceptable product,
although the best
results are achieved when the film surface including the antistatic agent
therein is not
directly treated.
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DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
When reference is made herein to "polypropylene" it is intended to mean a
crystalline propylene homopolymer or a copolymer of propylene with another
olefin
having from 2 to 5 carbon atoms in an amount insufficient to have a
significant affect
on the crystallinity of the polypropylene. Typically this is ethylene in an
amount up to
about 2% by weight. The polypropylene employed in this invention preferably is
the
homopolymer.
Most preferably the polypropylenes employed in this invention are Ziegler-
Natta
catalyzed and are commercially available isotactic polypropylenes having a
melt-flow
rate between about 2 and 10 gm/10min at 230 C and 2.16 Kg. load and a DSC
melting
point of about 160 to 166 C. Suitable polypropylene homopolymers are available
from
numerous sources, such as Sunoco of Philadelphia, Pennsylvania, Basell, which
is
located in the Netherlands, Exxon Chemical Company of Baytown, Texas, and Fina
Oil and Chemical Company of Deerpark, Texas. The specific polypropylene
employed
in this invention is not considered a limitation on the broadest aspects of
this invention.
For example, and not by way of limitation, the polypropylene can be
metallocene
catalyzed, or a blend of Ziegler-Natta catalyzed and metallocene catalyzed
polypropylene.
In a preferred embodiment of this invention, the polyolefin film is a
multilayer,
biaxially oriented polypropylene film having a central core and at least one
outer skin
layer, and preferably opposed outer skin layers. Preferably the central core
is the
thickest component of the multilayer film and is sufficiently thick to be self-
sustaining
or supporting, without the inclusion of one or more outer skin layers.
In a representative, non-limiting embodiment of this invention, the film has a
thickness of 48 gauge, with the core layer being 42 gauge and opposed outer
skin
layers each being 3 gauge. At least one of the outer skin layers includes a
desired low
COF in accordance with this invention.
In accordance with the broadest aspects of this invention, the multilayer film
includes at least two layers; a thick core layer and a thin outer skin layer
having the
desired low, metal/film COF slip properties in accordance with this invention.
For ease
of description, this outer skin layer will sometimes be referred to as the
"slip layer" or
"outer slip layer." Moreover, reference throughout this application to
"coefficient of
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friction" or "COF" refers to a metal/film coefficient of friction determined
in a manner
to be described hereinafter.
Most preferably the multilayer film includes at least three (3) layers. In
particular, for printed label applications, a coextruded three (3) layer film
is clear, or
transparent, and includes the slip layer and core layer, as described above,
and, in
addition, an outer, lower skin layer in the form of a printable or bondable
layer, which
is on the side of the core layer opposed to the slip layer.
In printed labels employing the clear or transparent three layer structure
described in the immediately preceding paragraph, the lower skin layer
generally is
laminated through a suitable adhesive to another single layer or multilayer
film, which
can be either clear or opaque. The construction and/or use of this latter
single layer
or multilayer film does not constitute a limitation on the broadest aspects of
this
invention.
If the coextruded clear film of this invention is printed on the lower skin
layer, a
reverse printing process is employed so that the printed indicia will be
readable through
the upper surface of the slip layer, and the adhesive employed to laminate the
upper
layer of the additional film to the reverse printed label surface is
compatible with the
ink and the surfaces joined by the adhesive.
If the coextruded clear film of this invention is not printed on the lower
skin layer,
a direct printing process is employed to include printed indicia on the upper
surface of
the additional film, which is the surface that is laminated to the lower skin
layerthrough
a suitable adhesive. Therefore whether the lower skin layer is reverse
printed, or the
upper surface of the additional film is direct printed, the printing is in the
interior of the
label stock, and therefore is protected.
Moreover, in order to highlight, or enhance the visibility of the printed
indicia,
whether indirect or direct, the additional film laminated to the multilayer
film of this
invention preferably is a single layer or multilayer opaque film; thereby
providing a
white background for the printed material. However, if it desired to actually
view the
color or appearance of an edible product or beverage retained within a clear
bottle or
can, then the printed label employed on that bottle or can needs to be
transparent.
Therefore, the additional film laminated to the lower skin layer of a
transparent film of
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this invention also needs to belransparent, whether or not the additional film
is a single
layer or multilayer film.
In the completed label construction, which includes both a clear or
transparent
multilayer film of this invention and an additional film laminated to the
multilayer film
to protect the printed indicia on the label, an adhesive is applied to the
lower, exposed
surface of the additional film for use in attaching the label to a container,
such as a
bottle or can. Preferably, the adhesive is included on vertical edges of the
label and
most preferably is applied adjacent each vertical edge in the form of a narrow
adhesive
stripe, as is well known in the art.
In the most preferred embodiment of this invention, one of the vertical
adhesive
stripes on the label is adhered directly to the container, and the label is
then wrapped
about the container with the other vertical adhesive stripe overlapping and
adhesively
bonded to the edge of the outer slip layer of the label in a region overlying
said first
vertical adhesive stripe that directly adheres the lower surface of the label
to the
container. In this most preferred embodiment, it is extremely important that
the vertical
adhesive stripe secured to the slip layer provides effective adherence to
prevent the
label from separating in the overlapped, bonded region. Thus, the slip layer
of the film
of this invention, in addition to having a desired low coefficient of friction
to permit the
film to be handled in commercial labeling equipment, must be capable of
effectively
adhering the overlapping lower surface of the label thereto through the stripe
of
adhesive employed for that purpose.
In accordance with this invention, the slip layer of the multilayer film has a
metal/film COF of less than 0.45 and more preferably less than 0.35, as
determined
by a modified form of ASTM D1894. Specifically, in the standard ASTM procedure
a
piece of film is placed on both the sled and stainless steel plafform, and the
COF that
is determined is a film/film COF. In accordance with this invention, the film
of this
invention is placed only on the sled, and the COF is determined between the
slip layer
of this film and the stainless steel platform of the test apparatus, employing
all of the
other steps specified in ASTM D1894.
Most preferably, the adhesives employed in the labels of this invention are
hot
melt adhesives, which may have various levels of aggressiveness. In accordance
with
this invention, the slip layers need to permit "adhesion" of the hot melt
adhesives at a
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level of at least 50%; more preferably at least 70%; even more preferably at
least 90%
and most preferably in excess of 90%.
The test employed to determine "adhesion" forthe hot melt adhesives to the
slip
layers of the films of this invention is as follows:
A Krones labeling machine is employed to adhere a label to a metal can with
a hot melt type of adhesive. Krones, Inc., the supplier of Krones packaging
equipment
in the United States, is located at 9600 South 58'h Street, Franklin, WI. The
adhesive
is applied as a stripe to the inside, or lower, surface of the label (that
which will contact
the can) at the recommended application temperatures using the standard
operating
procedures for the labeler. The amount of glue applied corresponds to a
setting of 1.5
on the glue application wheel of the Krones labeling machine. Visually, this
corresponds to a point where the entire length (i.e., vertical height) of the
label (5-7/8
inches, in this case) is covered with adhesive, but less than the point where
excess
adhesive is squeezed from the overlap seal that is formed between the upper
slip layer
of the film of this invention and the insider, or lower, surface of the label
. The labeler
is run at an operating speed of 120 labels per minute (LPM).
In accordance with one test procedure, the label is an adhesive lamination of
an outer film in accordance with this invention to a printed, voided, opaque,
inner film.
Both are multi-layer, coextruded, oriented polypropylene films. The structure
of the
inner film can be varied; however, it is important that the hot melt adhesive
that is
employed be firmly adhered to the lower surface of the inner film in a manner
that will
not permit the hot melt adhesive to separate from this lower surface during
the peel
test for "adhesion." In other words, the peel test is for the purpose of
determining the
adhesion of the adhesive to the slip layer, and therefore the adhesive can not
separate
from the lower surface of the inner film prior to separating from the slip
layer or prior
to the film failing in some other manner, as described hereinafter.
After reaching steady-state conditions, the labeler is run at the conditions
described above and labels are applied to new, clean metal cans. As the can
exits the
machine, it is removed and the adhesion evaluated in the manner described
hereinafter. As noted above, the adhesion being investigated is the overlap
seal of the
adhesive to the slip layer of the film of this invention.
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An attempt is made to peel the overlap seal apart by lifting the outer,
overlapping edge of the label by hand along the length, or vertical dimension,
of the
can. If the adhesion of the hot melt adhesive to the surface of the slip layer
is 100%,
then the overlap seal will remain intact and the label will tear or destruct
at some other
interface. If the adhesion of the hot melt adhesive to the surface of the slip
layer is
0%, then the hot melt adhesive will peel cleanly away from the surface of the
slip layer.
If the pattern of adhesion is intermittent, with portions of the adhesive
remaining
adhered to the slip layer and other portions peeling apart cleanly, then the
percentage
of adhesion is the fraction of the total length of portions of the overlapping
edges of the
label that remain adhered, as evidenced by the length of portions of the
overlapping
edges that tear or destruct at an interface other than the adhesive-slip layer
interface,
to the total length of the overlap seal, reported as a percentage.
It is desirable to have a degree of adhesion as close to 100% as possible.
However, as noted above, adhesion of at least 50% is deemed acceptable with
the hot
melt adhesives utilized in this invention. More preferably the level of
adhesion is at
least 70%; even more preferably at least 90% and most preferably in excess of
90%.
If the adhesion level falls below 50%, then the degree of adhesion should be
considered a failure.
It should be noted that the test for adhesion generally is carried out within
one
(1) minute of removal of the can from the labeler, and the percentage of
adhesion
reported for the multilayer films of this invention relate to the adhesion
determined in
this manner. If the time is extended, then the degree of adhesion generally
increases.
However, due to the chemical nature of certain hot melt adhesives, the mode of
failure
can be cohesive (through the thickness of the hot melt itself). This can tend
to cloud
the adhesion results. In these latter cases, it has been determined that the
period of
time between production and evaluation of the seal must be altered to a point
that is
significantly greater than 1 minute. The exact time is dependant upon the
specific hot
melt adhesive being used and is that time that prevents cohesive failure.
In a preferred embodiment of this invention, the outer slip layer is a blend
of a
polypropylene homopolymer and a minor weight percent of an additive that
results in
the films of this invention having the desired slip and adhesion properties
specified
earlier herein. In the most preferred embodiments of this invention the
additive
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includes an antistatic agent. Most preferably the antistatic agent includes an
ethoxylated alkylamine and/or an ethoxylated alkylamide, each by itself, or
each
possibly either physically blended with an ester or esterified by a chemical
reaction with
a suitable acid (e.g., stearic acid) or with another ester, e.g., glycerol
monostearate,
sorbitol-based esters, etc. Most preferably, the antistatic agent includes an
ethoxylated alkylamine that is esterified with another ester or with a
suitable acid, an
ethoxylated alkylamine blended with an ester, or an ethoxylated alkylamine, by
itself.
It is also within the scope of the broadest aspects of this invention to
employ a
glycerol monostearate, by itself, as the antistatic additive. However, the
benefits
achieved with this latter antistatic additive are not as good as the benefits
achieved
with an antistatic agent in the form of an ethoxylated alkylamine that is
esterified with
another ester or with a suitable acid, an ethoxylated alkylamine blended with
an ester,
or an ethoxylated alkylamine, by itself.
In the most preferred embodiment of this invention, the additive is an
antistatic
agent including an ethoxylated alkylamine included in a particulate, or
pelletized,
masterbatch that is predominantly polypropylene homopolymer, by weight, with a
minor
percentage, byweight, of the ethoxylated alkylamine. In a representative
embodiment,
the additive is a blend including 87 - 88% by weight polypropylene homopolymer
and
12 - 13% by weight of an antistatic agent including an ethoxylated alkylamine
that
possibly is either physically blended with an ester or esterified by
chemically reacting
with another ester, e.g., glycerol monostearate, sorbitol-based esters, etc.,
or with a
suitable acid ,e.g., stearic acid.
The antistatic agent may be an ethoxylated amine, such as a synthetic
ethoxylated amine, e.g., Atmer 163 sold by Uniqema in New Castle, Delaware.
This
ethoxylated amine can be employed in small quantities by itself or in
combination with
a small quantity of an ester, such as glycerol monostearate, e.g., Atmer 129
sold by
Uniqema in New Castle, Delaware. For example, excellent results have been
achieved
by employing as the additive either 0.50% Atmer 163 or 1.00% Atmer 163, by
itself.
In addition, excellent results have been achieved by employing blends
including 0.50%
Atmer 129 with 0.50% Atmer 163; 0.25% Atmer 129 with 0.50% Atmer 163 and 0.50%
Atmer 129 with 0.25% Atmer 163. All percentages herein are percentages by
weight,
based on the weight of the skin layer.
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Although with less satisfactory results, 1.00% Atmer 129 (a glycerol
monostearate) may be employed by itself as the antistatic agent in the slip
layer. For
some applications this may be acceptable; however, it is not a preferred
formulation
in this invention. It has been determined that employing 0.50% Atmer 129 by
itself as
the antistatic agent in the slip layer does not provide a satisfactory product
in
accordance with this invention.
In accordance with the broadest aspects of this invention, other additives for
achieving the desired slip and adhesion properties may be employed; provided
that the
additives, in conjunction with the other components of the film, provide the
desired slip
and adhesion properties specified earlier in this application. The present
invention will
hereinafter be described in connection with the preferred embodiments
including an
antistatic agent as the desired additive to the slip layer.
Applicant has determined that in order to obtain the desired slip effect, or
low
metal/film COF, on the outer surface of the slip layer in accordance with this
invention,
the core layer needs to include a migratory amide therein. In a preferred
embodiment,
the migratory amid is behenamide, which is supplied as Kemamide B by Witco,
based
in Greenwich, Connecticut. However, other migratory amides are believed to be
usable in this invention to achieve the desired COF within the scope of this
invention.
In accordance with a preferred embodiment of this invention, application has
found thatthe core layer preferably is a polypropylene homopolymer containing
at least
0.1 % of the migratory amide, such as behenamide; more preferably a migratory
amide
in the range of about 0.2 - 0.5%, and more preferably a migratory amide in the
range
of about 0.2 - 0.3% based on the total weight of the core layer. In the most
preferred
embodiment, the behenamide is present in an amount of about 0.25% of the total
weight of the core layer. Although higher percentages of behenamide may be
usable
there does not appear to be any reason for using such higher percentages.
Moreover,
it has been determined that a level of behenamide as high as 1.00% adversely
affects
adhesion, and therefore films employing this latter lever of behenamide may be
unacceptable for various label applications.
Although some reduction in coefficient of friction was observed when the
percentage of behenamide was as low as 0.1 % by weight of the total weight of
the
core layer, more desirable coefficients of friction were achieved with the
level of
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behenamide being about 0.25% and greater.
Interestingly, it has been determined that Croda EBS, which is an ethylene bis-
stearamide, when employed in the core layer at levels of 0.25% and 0.50% of
the total
weight of the core layer, did not provide the same desired reduction in
coefficient of
friction as behenamide, which is classified as a primary amide. In addition,
it has been
determined that Erucamide, when employed in the core layer at a level of 0.25%
did
not provide desirable results in terms of processibility in Krones labeling
equipment.
However, the testing with Erucamide has been quite limited, and therefore it
is possible
that this amide may function acceptably for certain applications and/or at
higher
concentrations. Although Erucamide is not a preferred slip agent for use in
the most
preferred embodiment of this invention, its use is within the scope of the
broadest
aspects of this invention.
As noted earlier herein, in accordance with the most preferred aspects of this
invention the antistatic additive is an ethoxylated alkylamine and/or an
ethoxylated
alkylamide; most preferably an ethoxylated alkylamine. The ethoxylated
alkylamine
and/or the ethoxylated alkylamide may be employed individually, either alone,
or
physically blended with an ester or esterified by a chemical reaction with
another ester
or a suitable acid, as has been discussed in detail earlier in this
application.
In a preferred embodiment of this invention, the antistatic additive is
included
in a particulate, or pelletized masterbatch identified as POLYBATCH ASPA 2485,
which is supplied as a pelletized concentrate by A. Schulman, Inc. of Akron,
Ohio.
POLYBATCH ASPA 2485 comprises an antistatic agent including an ethoxylated
alkylamine that either is blended with an ester or is esterified by a chemical
reaction
with another ester or with a suitable acid. The POLYBATCH ASPA 2485 is a
masterbatch including about 87 - 88% polypropylene homopolymer and about 12 -
13% of the antistatic agent.
Most preferably, the POLYBATCH ASPA 2485 masterbatch is present in the
slip layer in a range of at least 4% by weight of said skin layer; more
preferably in
excess of 4% and even more preferably about 8% or even more. Therefore, the
antistatic agent in the slip layer, which constitutes 12-13% by weight of the
masterbatch, is present in the range of at least about 0.48% by weight based
upon the
total weight of the polymeric slip layer; more preferably in excess of 0.48%;
more
CA 02452026 2006-11-14
14
preferably in the range of about 0.96% to 1.04%, or even more. Although
greater
weight percentages of the antistatic agent can be employed, applicant has not
perceived any desired benefit of improved, iowercoefficient of friction by
employing,
for exampie,12% by weight of the particulate masterbatch, i.e., 1.44% by
weight of the
antistatic agent. Thus, although higher percentages of the antistatic agent
can be
employed, there is no practical reason for utiiizing such higher percentages.
On the
other hand, lowering the level of the POLYBATCH ASPA 2485 to 4% by weight of
the
total weight of the slip layer, although not providing a significant adverse
affect on the
coefficient of friction of the slip layer, did reduce the operating window on
Krones
labeling equipment. That is, when 4% by weight of the POLYBATCH ASPA 2485 was
employed in the slip layer closer control overthe vacuum level employed on the
cutting
drum and over the position of the cutting knife needed to be achieved in order
to
provide effective operation of the labeling equipment.
In accordance with this invention, the slip layer is capable of receiving hot
melt
adhesives having various levels of aggressiveness. with a desired level of
adhesion,
thereby making the films highly desirable for use in adhesive label
applications. Such
hot melt adhesives are available from H. B. Fuller Company of St. Paul, MN. as
well
as from other sources.
TM
When Fuller CLARITY HL-4157 hot melt adhesive was applied to films
containing behenamide both in the core and on the film surface (i.e., prior
art films)
there was a low level of adhesion near 0% immediately after application of the
adhesive. However, with a coextruded, biaxiaiiy oriented polypropylene film
having an
outer slip layer containing 8% of the POLYBATCH ASPA 2485 in accordance with
this
invention, the behenamide in the core was effectively blocked from the surface
of the
outer slip layer and the level of adhesion increased from approximately 0% in
the prior
art film to 70% - 98%.
When the film was heated at 55 C for twenty-four (24) hours prior to the
application of the Fuller CLARITY HL-4157 hot melt adhesive (to accelerate
migration
of the fatty acid amide to the surface), the level of adhesion of the film
without the
outer slip layer of the present invention remained poor, at approximately 0%,
while the
modified film in accordance with the present invention had high levels of
adhesion in
the range of 60% - 98%. With other Fuller hot melt adhesives having more
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WO 03/011593 PCT/US02/12818
aggressiveness than Fuller CLARITY HL-4157, i.e., Fuller CLARITY HL-4164 and
Fuller CLARITY HL-4165, adhesion levels of the "amide only" prior art film
went from
70% - 75%, without heating, to 0% - 5% after heating at the above-stated
conditions
(i.e., at 55 C for twenty-four (24) hours). However, with these latter
adhesives the
modified films in accordance with the present invention displayed adhesion
levels of
98% to 100% regardless of the thermal history of the film.
Quite surprisingly, applicant determined that directly oxidatively treating
the
outer slip layer of the films of the present invention, such as by corona
treatment,
actually reduces the level of adhesion of the hot melt adhesive to the surface
of the slip
layer. This was somewhat surprising in view of the fact that corona treatment
most
commonly is employed to improve the adhesion characteristics of a surface
layer of
a film to both printing inks and adhesives. Thus, in accordance with the most
preferred
embodiments of the present invention the outer slip layer is free of direct
oxidative
treatment, although it is believed that for some applications direct oxidative
treatment
of the outer slip layer, particularly if the treatment is "light" will still
result in the
formation of an acceptable product. Therefore, films in which the outer slip
layer is
directly oxidatively treated are within the scope of the broadest aspects of
the
invention.
EXAMPLE 1
A coextruded, multilayer, biaxially oriented polyolefin film in accordance
with this
invention is a 48-gauge thick, biaxially oriented polypropylene film having a
3-gauge
outer slip layer with the antistatic agent therein, a 42-gauge thick core
layer adhered
to the slip layer and a 3-gauge thick skin layer on the surface of the core
opposite that
of the slip layer and being receptive to printing and/or bonding applications.
In this
exemplary embodiment, the slip layer included 92% polypropylene homopolymer
(Aristech FFO35C) combined with 8% of the A. Schulman POLYBATCH ASPA 2485
concentrate. The core layer is a polypropylene homopolymer (Aristech FF038A2)
containing 0.25% behenamide (Kemamide B). The print layer opposite the slip
layer
is a isotactic polypropylene homopolymer containing 0.25% Tospearl T120 and
0.15%
Sylobloc 45, and is sold under the brand name Aristech FFO35W. The Tospearl
included in the print layer functions both as a slip and anti-block agent,
whereas the
Sylobloc 45 functions predominately as an anti-block agent.
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16
All of the polymers employed in this invention are polypropylene homopolymers
containing 0.068% sodium benzoate nucleator and 0.25% Irganox B225 stabilizer.
The inclusion of the sodium benzoate nucleator aids in maintaining the
crystallinity of
the polypropylene, thereby providing improved optical and strength properties
in the
films of this invention.
It should be noted that the POLYBATCH ASPA 2485 is advertised by A.
Schulman, Inc., as being usable to control dust and optimize antistatic
properties.
There is no recognition in any of the product literature known to applicant
that this
additive can or will function in any way to improve the slip properties of a
film, let alone
that it should or could be employed as in the present invention.
In accordance with this invention, the POLYBATCH ASPA 2485 including the
antistatic agent therein, in a minor amount, is blended with polypropylene
homopolymer to form an outer skin layer, and the migration of the antistatic
agent out
of the skin layer is believed to be precluded, or at least minimized, by the
presence of
a migratory amide in the core. At the same time, the composition of the slip
layer
precludes the migratory amide in the core from migrating to the surface of the
slip layer
in an amount that adversely affects the ability of the slip layer to receive
hot melt
adhesives with the desired degree of adhesion to the film. Moreover, because
of the
construction of the films of this invention, the films do not need to be
heated to achieve
migration of the slip agent to the film surface. Thus, the possible adverse
affect on the
flatness of the film resulting from heating of the film is avoided. Moreover,
if the films
of this invention are exposed to hot environments, e.g., during storage or
shipment, the
adhesion properties of the slip layer for the hot melt adhesives commonly
employed
in label applications in accordance with this invention will not be adversely
affected.
The above examples are given by way of illustration only, and the invention
should only be limited in accordance with the terms of the appended claims. It
should be understood that the present invention can be employed to form a wide
range of multilayer films of varying thickness and employing alternate polymer
types
and a variety of different additives. The specific types of additives which
can be
employed can be determined by routine experiment carried out by individuals
skilled in the art.
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17
Without further elaboration, the foregoing will so fully illustrate my
invention
that others may, by applying current or future knowledge, readily adopt the
same for
use under various conditions of service.
