Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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NOVEL SLIP AGENTS AND POLYPROPYLENE
FILMS PREPARED THEREFROM
SPECIFICATION
BACKGROUND INFORMATION
In the manufacture of polypropylene films, it is common practice to include,
in
the polymer formulation, additives referred to as slip agents. These additives
migrate
to the surface of the film and decrease the coefficient of friction between
the film and
the metal rollers over which it is passed during processing, thus facilitating
the
processing of the film. They also decrease the coefficient of friction between
layers
of the film when it is wound into rolls thereby facilitating unwinding of the
rolls for
further processing.
The slip additives employed in most commercial polypropylene films are
relatively high molecular weight fatty acid amides. The most widely used fatty
acid
amides are erucamide, an unsaturated 22 carbon amide (13-docosenamide) and
behenamide (docosanamide), the saturated analogue of erucamide. Both of these
compounds are readily available, naturally occurring materials. They are
normally
provided as mixtures containing a small amount of other amides containing
about 18
to 20 carbons.
Erucamide works quite well as a slip agent, but it is not favored by some
producers and converters as it is relatively volatile and extra care is
required during
film manufacture and conversion to avoid having quantities of the additive
escape
from the film and plate out on processing equipment, thereby causing a clean-
up
problem. For this reason, some producers prefer to use behenamide as the slip
agent.
As slip agents, erucamide and behenamide appear to perform substantially
equally
well but behenamide is less volatile and therefore films containing behenamide
are
easier to handle during processing since the behenamide does not escape and
plate
out on the processing equipment.
Despite the inconveniences encountered with erucamide-containing film,
erucamide is the fatty acid amide slip agent of choice for those who wish to
print the
film using the well known rotogravure printing technique. In the rotogravure
process,
ink is applied to the print surface from a gravure cylinder containing the
desired image
as depressions on its surface. Ink is applied to the cylinder and the surface
of the
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cylinder is wiped by a doctor blade to remove excess ink. The film to be
printed is then
contacted by the gravure cylinder and the image is thus transferred from the
cylinder
to the film. While the film is in contact with the cylinder, a small amount of
erucamide
is deposited on the raised portion of the cylinder. Since erucamide is soluble
in the
alcohol or ketone ink solvent, it is dissolved by the solvent in the next
application of
ink and is removed by the next swipe of the doctor blade. Thus, no build-up of
erucamide occurs on the doctor blade or on the gravure cylinder.
Behenamide, on the other hand, lacking the carbon-carbon unsaturation found
in erucamide, is substantially less soluble in the alcohols and ketones found
in printing
inks. Thus, when, in the course of rotogravure printing, some of the
behenamide is
deposited on the printing cylinder, it is not dissolved in the printing ink
and wiped off
when the next application of ink and wiping by the doctor blade takes place.
As a
result, the behenamide is merely wiped off by the next swipe of the doctor
blade and
substantially all of it remains on the blade. After multiple swipes of the
blade, a
behenamide build-up forms on the doctor blade and on the printing cylinder,
eventually reaching a point at which the blade is prevented from wiping the
surtace of
the printing cylinder clean. This build-up causes streaks to form on the
printed film,
requiring an interruption of the job in order to clean the doctor blade.
It will be immediately apparent that a slip additive that does not cause the
film
processing problems associated with erucamide but which allows the film to be
rotogravure printed without causing the problems associated with behenamide
would
be a welcome contribution to the polypropylene film art. In accordance with
this
invention, there has been discovered a slip additive that accomplishes this
objective.
It is an objective of this invention to provide oriented polypropylene films
that
exhibit improved properties in rotogravure printing as compared to films
presently
known to the art. It is likewise an objective of this invention to provide
film-forming
compositions for preparing such films.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with this invention, it has been found that certain bis-fatty
acid
amides exhibit a combination of properties that makes them highly satisfactory
as slip
agents in oriented polypropylene films. In particular, this combination of
properties
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make them highly satisfactory as slip agents in films that are to be printed
in
rotogravure printing operations.
Briefly stated, the invention is a composition comprised of a polymer or
copolymer of propylene and an N,N'-bis-alkylene fatty acid amide having the
general
structural formula R-CO-NH-(CH2)~ NH-CO-R wherein R and R~ are the same or
different alkyl or alkenyl groups having about 15 to 21 carbon atoms -CO- is a
carbonyl group and n is an integer from 2 to 4 said N,N'-bis alkylene fatty
acid amide
being present in an amount from about 0.05 to about 0.5 % by weight based on
the
combined weight of said amide and the polypropylene.
In another aspect, the invention contemplates an oriented film comprised of a
polymer or copolymer of propylene and an N,N'-bis-alkylene fatty amide having
the
general structural formula R-CO-NH-(CH2)~ NH-CO- R wherein R and R are the
same
or different alkyl or alkenyl groups having about 15 to 21 carbon atoms, n is
an integer
from 2 to 4 and -CO- is a carbonyl group said N,N'-bis alkylene fatty acid
amide being
present in an amount from about 0.05 to about 0.5 % by weight based on the
combined weight of said amide and the polypropylene.
A preferred embodiment of the invention is a composite film comprised of a
polypropylene core having, on at least one of its surfaces, a film layer
comprised of a
composition comprising polypropylene and up to about 0.5% of an N,N'-bis-
alkylene
fatty amide having the general structural formula R-CO-NH-(CH2)~ NH-CO- R
wherein
R and R are the same or different alkyl or alkenyl groups having about 15 to
21 carbon
atoms, n is an integer from 2 to 4 and -CO- is a carbonyl group, said N, N'-
bis alkylene
fatty acid amide being present in an amount from about 0.05 to about 0.5 % by
weight
based on the combined weight of said amide and the polypropylene.
DETAILED DESCRIPTION OF THE INVENTION
As used in the description of this invention and in the attached claims, the
term
"film" refers to a stand-alone film, i.e. a film of a sufficient thickness to
have the
strength and other characteristics required to have utility in packaging and
other
applications without having to be united with another film to support it.
Stand-alone
monolayer films are seldom seen in commerce. However, the compositions of the
invention are suitable for use in such films.
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Oriented polypropylene films of commerce are usually composite, i.e.
multilayer,
structures in which a core layer of a thickness sufficient to impart stand-
alone
properties to the overall structure carries one or more thin functional layers
on its
surface(s). The term "film" will also be used to refer to these thin
functional layer films
although they do not have sufficient thickness to stand alone.
Composite polypropylene films are typically comprised of a homopolypropylene
core having one or more functional layers on its surfaces. Functional layers
can act,
e.g., as heat or cold seal layers, as receiving layers for printing or other
decorative
material or as barrier coating receiving layers. In these cases, it is not
required that
the polymer in the surface layer be the same as that employed in the core,
although
it may be the same polymer formulated differently to serve the functional role
that it is
to serve in the completed film. Frequently, the surface layer will be of a
different
polymer that has been found to perform the desired function more
satisfactorily than
would the polymer employed in the core.
When reference is made herein to polypropylene, it is intended to indicate a
crystalline (isotactic) propylene homopolymer or a copolymer of propylene with
ethylene or an a-olefin having 2 to 5 carbon atoms in an amount insufficient
to have
a significant effect on the crystallinity of the polypropylene. Typically, the
comonomer
will be ethylene in an amount of 6% or less.
Suitable polypropylenes are the commercially available isotactic
polypropylenes
having a melt flow rate between about 2 and about 10 dg/10 min at 230°
C and 2.16
Kg. load and a DSC melting point of about 160 to 166°C. One
polypropylene that can
be used is the isotactic homopolymer having a melt flow rate of about 3.5
dg/10 min
at 230° C and 2.16 Kg. load, available from Aristech Chemical
Corporation, Pittsburgh,
PA. Suitable polypropylenes are also available from Montell, Inc. Wilmington,
DE,
Exxon Chemical Company, Baytown, TX and Fina Oil and Chemical Co., Deer Park,
TX.
Since different layers are formulated differently, it is possible that
different slip
agents may be used in the different layers or that some layers contain no slip
agent.
Thus, it is possible and desirable, in cases where problems are encountered
when
using conventional slip agents, e.g. in the rotogravure printing process as
described
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above, to limit the use of the bis-amides of the invention to the functional
layer where
its presence yields the desired advantage. In such cases, where in the past
the
conventional slip agents have been added to the core layer and allowed or
forced to
migrate into the functional layers, it is found that the presence of the bis
amides almost
completely prevents the migration of the conventional slip agent , e.g.
erucamide or
behenamide from the core to the functional layer where if can cause a problem.
The slip additives employed in the compositions and oriented films according
to this invention are N,N'-bis-fatty acid amides having the general structural
formula
R-CO-NH-(CH2)"NH-CO-R wherein R and R are the same or different alkyl or
alkenyl
groups having about 15 to 21 carbon atoms, n an integer from 2 to 4 and -CO-
is a
carbonyl group. The compounds employed in the compositions of this invention
are
characterized by having a relatively low degree of volatility and a relatively
high
degree of solubility in alcohols, ketones and esters.
Compounds meeting the above definition and description include, by way of
example, N, N~ bis-ethylene stearamide, N,N'-bis ethylene oleamide, N,N'-bis
ethylene
behenamide, N,N'-bis ethylene erucamide, N,N'-bis propylene stearamide, N,N'
bis
butylene oleamide.
The N,N~-bis-alkylene fatty acid amide is present in the compositions of the
invention in a concentration of about 0.05 to about 0.5% by weight based on
the total
weight of the composition. Preferably, the concentration of the slip additive
is between
about 0.05 and about 0.25% and most preferably between about 0.5 and about
0.2%.
The slip agents of the prior art such as erucamide or behenamide are
sufficiently volatile that a measurable portion of them always migrates from
the interior
of the film and forms a layer of slip agent on the film's surface, whether the
film is a
composite or a monolayer. The amount present on the surface can be measured by
a technique referred to as Attenuated Total Reflectance Infra-Red Spectroscopy
(ATRIR). When the ATRIR spectrum of a film sample stored at room temperature
is
compared to that of a film stored for 16 hours at 55°C, an increase in
the amount of
a prior art slip agent is detected. (Such heat aging is commonly applied to
simulate
film aging conditions.) The amount of the slip agents of this invention that
migrates
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to the surface of a film cannot be measured by ATRIR. Their presence on the
film
surface is demonstrated only by their efficacy as slip agents.
The presence of conventional slip agents on the surface of a film can also be
detected by rinsing the film surtace with a solvent and testing for the slip
agent by High
Pressure Liquid Chromatography. Similarly, no accumulation of the slip agent
of the
invention can measured on the film surtace by this technique, whereas it is
readily
detected in the solvent employed to wash the surtace of a prior art film.
In prior art practice, slip agent is frequently incorporated into the core
layer of
composite films, which are then heat treated to force it to migrate to the
surface layers.
In other cases, if the slip agent is present only in a surface layer, it tends
to migrate
into the core layer although, as a rule, it is not needed there. In either
case, this
migratory tendency results in slip agent being present in locations where it
is not
needed. Moreover, it also requires the use of greater quantities of slip agent
than are
needed in order to assure that sufficient quantities are present in the area
where the
real need exists.
It has been found that the slip agents of this invention exhibit a very low
tendency to migrate between layers or even within a layer of the film. Thus, a
slip
agent incorporated in a surface layer does not migrate into the core layer,
nor does it
migrate and accumulate on the surface of a layer as do the slip agents of the
prior art.
When tested by the ATRIR test or the surface rinse procedure mentioned above
no
accumulation of the slip agent is detected on the surfaces. The low migratory
character of these slip agents causes them to remain in the layer into which
they are
incorporated.
Despite the absence of measurable amounts of the slip agent on the film
surface, the slip agents of the invention are substantially equal to those of
the prior art
in decreasing the coefficient of friction (COF) of the film surface. Thus, the
COF
measured on the surtace of a prior art film containing erucamide as the slip
agent is
about 0.15 to 0.35. That measured on a prior art film containing behenamide is
about
0.15 to 0.35. The COF measured on the surface of a film of this invention is
generally
between about 0.15 and about 0.45.
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The low migratory character of the slip agents of the invention also leads to
other favorable effects on film processing. For example, the problem of
vaporization
encountered with erucamide and to a lesser extent with behenamide is
eliminated.
Also, as will be apparent from the above, in the preparation of composite
films, it is not
always necessary to have a slip agent in the core layer. Since the slip agent
of the
invention will not migrate into the core layer, one can put only the amount
actually
needed to give the desired slip properties in the surface layer and it will
not migrate
into the core layer and reduce the level in the surface layer below that
required to give
the surface of the film the proper slip quality.
As indicated hereinabove, the compositions of the invention have particular
utility in the manufacture of rotogravure printable oriented polypropylene
film thanks
to the unique combination of good alcohol, ketone and ester solubility and low
volatility
exhibited by the N,N~-bis-alkylene fatty acid amides. Due to their low degree
of
volatility, these compounds do not vaporize from the surface of the film
during
manufacturing and conversion. Due to their relatively high degree of
solubility in the
solvents that are normally employed as solvents for printing inks, whatever
small
amounts these compounds that might be on the film surface does not form a
build-up
on the doctor blade during rotogravure printing. Films prepared using these
compositions thus combine the favorable properties of conventional films
containing
behenamide during the manufacture and conversion of the film with the
favorable
properties of those containing erucamide for use in rotogravure printing.
In yet another aspect, the slip agents employed in the films of the invention
are
found to be improved candidates for use as cold seal films. Cold seal films
are
prepared by treating the films with a cold sealable adhesive such as a rubber
cement
and many applications for printed films are applications in which cold seals
are
preferred over more conventional heat seals. In prior art usage, particularly
when
erucamide is employed as the slip agent, it is found that the high level of
the slip agent
on the film surface weakens or even, in extreme cases, destroys cold
sealability.
Using the slip agents of this invention at the levels specified, very little
or no harmful
effect on cold seal is seen.
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In addition to the slip agents of this invention, conventional additives, in
conventional amounts, can also be included in the films. Suitable conventional
additives include, by way of example, antioxidants, pigments, orientation
stress
modifiers, flame retardants, antistatic agents, antiblocking agents, and
antifoggants.
Any such additives that do not interfere with the functioning of the slip
agent can be
present.
In any embodiment of the films of the invention, the film can be either clear
and
transparent or it can be opaque. Typically, polypropylene films are rendered
opaque
by loading the core layer with a void forming opacifier such as calcium
carbonate.
Such opacifiers cause opacity by forming microvoids in the polymer matrix
during the
drawing operation. Other types of opacifiers which cause opacity simply by
increasing
the optical density without voiding can also be employed. Typical of such an
opacifier
is titanium dioxide.
The total film thickness, for either the single or multiple layer films, is
preferably
in the range of about 0.25 to about 1.5 mil, i.e. about 25 to about 150 gauge.
In the
multilayer embodiment, the core layer preferably has a thickness of about 23
to about
40 gauge, while the other layers preferably each have a thickness of about 2
to about
gauge.
To prepare a printable surface, the surface of the film that is to be printed
is
subjected to an oxidation treatment to impart a degree of polarity to the
surface, which
ensures good adhesion of the printed information to the film. Suitable
oxidation
treatments include, by way of example, corona discharge, flame and acid
etching. The
preferred treatment is with corona discharge or with flame. The use of flame
and
corona, seriatim, is also known and frequently employed.
The oxidation treatment is preferably carried out to a degree sufficient to
create
a surface tension on the film surface equal to about 35 to about 60 dyneslcm.
A
preferred surface tension is about 35 to about 40 dynes/cm.
As stated hereinabove, composite films can have, in addition to the printable
surface, other functional layers on the surface opposite the print layer. If
the layer on
the side opposite the print layer is a barrier coating receiving layer, that
layer will also
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be subjected to an oxidation treatment to approximately the same degree as is
applied
to the print layer.
While the invention has been described with emphasis on its utility in
rotogravure printing, it will be apparent that other types of films, including
functional
layers, can also benefit from the low volatility and the lower degree of
migratory
tendency of the slip agents of the invention. One type of such film is
metallizable film
where the greater migratory tendency and volatility of conventional slip
agents tends
to result in a greater amount of slip agent on the film surface, which can
sometimes
interfere with bonding of the metal coating to the film surface. The same
effect is also
noted with cold seal coatings applied to the film surface.
Polypropylene films according to the invention can be prepared using film
forming and orientation methods and techniques well known to the plastic film
art.
Thus the films can be prepared using the tubular or bubble process wherein a
tube of
film is extruded through a circular die and is drawn and inflated
simultaneously to
effect biaxial orientation. Likewise, they can be prepared by the tenter
process in
which a flat sheet of the film is drawn from the die at a multiple of the
lineal rate of
extrusion to effect longitudinal orientation and is thereafter drawn laterally
by a
preselected multiple of its original width to effect lateral orientation. As a
rule, films
prepared by the bubble process are drawn uniformly 6X by 6X or 7X by 7X. When
the
tenter process is employed, the draw is usually carried out to 5X in the
machine
direction and 10X in the cross direction. These draw ratios are also suitable
for the
films of the invention, although other ratios can be employed if desired.
In the examples that follow, a series of biaxially drawn three layer composite
films were prepared on a 2.5 meter tenter. The films were cast from a three
layer die
at 265°C onto a chill roll maintained at 40°C. The cast films
were drawn 4.8X in the
machine direction at 120°C and then 10.4X in the cross direction at
120°C. Finally,
they were heat set at 160°C to a relaxation of about 8%. All samples
were then
corona treated on a covered roll corona treater at 35°C and 20 amps.
Example 1
A three layer composite film comprised of a homopolymer core of 0.69 mil
thickness
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containing 0.07% by weight behenamide, a 0.02 mil homopolymer layer on one
surtace containing 0.25% crosslinked methyl sesquioxide particles and, on the
other
surface, a 0.04 mil layer of a copolymer of polypropylene and 4.5% ethylene
(Fina~
8573 from Fina Oil and Chemical Ca., Deer Park, TX). This latter layer,
intended to
serve as a print layer, contained 0.70% by weight of N,N'-ethylenebis
oleamide.
Example 2 '
Example 1 was repeated except that the concentration of N,N' ethylenebis
oleamide was 0.05% by weight.
Example 3
Example 1 was repeated except that the concentration of N,N' ethylenebis
oleamide was 0.20%.
Example 4
Example 1 was repeated except that the concentration of behenamide was
increased to 0. 9 0%.
Example 5
Example 1 was repeated except that the print layer contained no slip agent.
Example 6
Example 4 was repeated except that the concentration of the N,N' ethylenebis
oleamide was 0.05°~.
Example 7
Example 1 was repeated except that the concentration of the behenamide was
0.05%.
Example 8
Example 7 was repeated except that the 0.4% copolymer layer contained no
N,N ethylenebis oleamide.
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The samples of Examples 1 to 8 were subjected to a number of physical tests
with the results reported in the following table.
Example %Slip gent COF~ %Gloss2 %Haze3
A
----- Core Skin Static Kinetic ----- -----
1 0.07 0.10 0.34 0.22 89.9 1.3
2 0. 07 0, 0.36 0.16 91. 8 1
05
3 0.07 0.2 0.35 0.16 90.8 1
4 0.1 0.10 0.38 0.16 89.8 1.3
0.10 0 0.35 0.18 91.3 1.1
6 0.10 0, 0. 36 0.16 91. 8 1. 0
05
7 0.05 0.10 0.27 0.18 93.0 0.5
8 0.05 0 0.4 0.17 93.0 0.8
~: cur accoramg to Ha i m una~
2: Gloss according to ASTM D2457
3: Haze according to ASTM D1003
Film samples from Examples 1 to 8 were evaluated for their printing quality on
an 11 color Cerutti press (North American Cerutti Corporation, Pittsburgh, PA)
Examples 1 to 6 were printed with Manders inks (Manders Premium, Inc., Niles,
IL)
thinned to a #22 Zahn cup with a 1:1 ethyl alcohol/n-propyl acetate solvent.
Examples
7 and 8 were printed with Sun NULAM and Sun SUNPLI inks thinned with Sun SL-02
and SL-05 solvent systems. (Here again we should have more informative
descriptions).
All samples were evaluated on commercial quality print jobs and evaluated
against commercially accepted controls as standards. In both evaluations,
Examples
5 and 8, which contained no N,N'-ethylenebis oleamide, exhibited unacceptable
streaking and/or ghosting. The films of Examples 1,2,3,4,6 and 7 which
contained the
N,N'-ethylenebis oleamide, exhibited no streaking or ghosting.
Two aspects of the cold seal properties of the film samples were examined. In
order for the film to be able to be unwound from a roll where the cold seal
layer is in
contact with the other surface of the film on which no such adhesive is
present, it must
release readily from that layer. Then, when a cold seal is formed by bringing
two
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12
areas of the cold seal layer together, it must not separate without
application of a
significant pulling force. These two aspects are referred to as release
strength,
which must be low, and the cold seal strength, which must be high.
To test these two parameters, a typical commercially available adhesive,
Findlay0 C1099 (Findlay Adhesives, Wawatosa, WI) was applied to the print
layer
of a specimen of the film using a #7 Meyer rod. The coated film was then dried
at
93°C for 30 seconds. The dried film was then covered with a release
film and cut
into six individual samples. These were placed in a blocking jig at 100 psi
and
blocked at 100 psi for 16 hours at 50°C.
To test cold seal release strength, the release film was pulled from the
specimen in an fnstron tensile tester and the force required to remove the
film was
measured.
To test cold seal adhesion strength, a specimen was cut in two along its
longitudinal axis and the two halves were brought together, coating to
coating, and
pressed together at 80 psi for 1/2 second. Within one minute following
sealing, the
two pieces of film were pulled apart on the Instron tensile tester. Triplicate
measurements were made for each specimen.
The cold seal release strength and the cold seal adhesion are both found to
be comparable to that of conventional commercial films prepared with
behenamide
and erucamide slip agents.
