Note: Descriptions are shown in the official language in which they were submitted.
204746
BIAXIALLY ORIENTED SEALABLE POLYPROPYLENE
FILM HAVING IMPROVED BARRIER PROPERTIES
Background of the Invention
The present invention relates to a multilayer
polypropylene film which has been produced by
coextrusion and which has improved barrier
properties with respect to permeability to water
vapor and oxygen and simultaneously possesses
favorable slip properties and low shrink values, in
order to ensure good machine running properties.
EP-A-0,247,898 (= U.S. Patent No. 4,921,749)
describes a special polypropylene film which is
claimed to possess, in particular, high-strength
sealed seams and improved barrier properties.
DE-A-35 35 472 (= U.S. Patent No. 4,786,533)
is directed to polypropylene films where a certain
amount of resin is incorporated in the base layer.
However, the films disclosed therein possess, in
particular, inadequate barrier properties towards
water vapor, so that there has been a need for films
having improved properties in this respect.
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~047~6~
DE-A-38 14 942 describes polypropylene films
containing a resin proportion of 5 to 40% by weight
in their base layers, the resins having a softening
point in the range of 80 to 125°C. These films are
used as shrink-on labels, but exhibit disadvantages
with respect to the barrier properties towards water
vapor and oxygen.
Summary of the Invention
Accordingly, it is an object of the present
invention to provide a polypropylene film having
improved barrier properties with respect to
permeability to water vapor and oxygen, while at the
same time having favorable slip properties and low
shrink values.
Another object of the present invention is to
provide a process for producing the improved film.
In accomplishing the foregoing objectives,
there has been provided, in accordance with one
aspect of the present invention, a sealable film
comprising (i) a base layer comprising polypropylene
and a hydrocarbon resin having a softening point of
at least 140°C, and (ii) at least one top layer
comprising (a) an ethylene/propylene copolymer
having an ethylene content of not more than about
10% by weight, (b) a propylene/1-butene copolymer,
(c) a propylene/ethylene/alpha-olefin terpolymer, or
(d) a blend of two or more of (a), (b) and (c),
wherein at least one of said base layer and said at
least one top layer contains an anti-blocking agent
or lubricant.
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207469
In accordance with another aspect of the
present invention there is provided a process for
producing the foregoing film comprising the steps
of: producing by coextrusion through a slot die a
cast film comprising said base layer and at least
one said top layer; chilling said cast film on a
chill roll; and then orienting said film by biaxial
stretching in the longitudinal and transverse
directions.
In accordance with still another aspect of
the present invention there is provided a process
for producing a multilayer sealable film comprising
the step of incorporating in at least one layer of
said film a resin having a softening point of at
least 140°C.
Other objects, features and advantages of the
present invention will become apparent to those
skilled in the art from the following detailed
description. It is to be understood, however, that
the detailed description and specific examples,
while indicating preferred embodiments of the
present invention, are given by way of illustration
and not limitation. Many changes and modifications
within the scope of the present invention may be
made without departing from the spirit thereof, and
the invention includes all such modifications.
Detailed Description of the Preferred Embodiments
In comparison to the films according to DE-A-
38 14 942, the films according to the present
invention display reduced deposits on the rollers.
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~o~~~e~
Compared to the film disclosed in that
publication, the film of the instant invention is
distinguished by improved processability, in
particular with respect to machine running
properties on high-speed packaging machines and to
thermal blocking. The term "machine running
properties" refers to the ease of processing on
high-speed packaging machines. For this purpose, the
film must be neither too smooth nor too rough, for
this might cause jams during the slitting operation.
The stiffness of a film is also important in this
respect. The term "thermal blocking" relates to the
mutual adhesion of film-packaged goods, which is,
above all, caused by the action of heat. The lower
this mutual adhesion between two adjoining film
layers under the action of heat is, the better is
the thermal blocking behavior.
In comparison to the films acccording to DE
A-38 14 942, the films according to the present
invention display reduced deposits on the rollers.
The terpolymer described under (c) above is
preferably comprised of about 93.2 to 99.0% by
weight of propylene, about 0.5 to 1.9% by weight of
ethylene and about 0.5 to 4.9% by weight of the
alpha-olefin, the percentages relating to the total
weight of the terpolymer.
The base layer of the multilayer film is
comprised of a propylene polymer having a melting
point in the range of about 162 to 168°C. Isotactic
polypropylene having an n-heptane-soluble fraction
of 6% or less is preferably used. The polypropylene
of the base layer in general has a melt flow index
of about 1.5 to 5 g/10 min, measured under a load of
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~Q47469
21.6 N and at a temperature of 230°C, according to
D1N 53 735.
The low-molecular weight resin contained in
the base layer is a natural or synthetic resin
having a softening point of > 140°C, determined
according to DIN 1995-U4, corresponding to ASTM E-
28, and it is present in an amount of about 5 to 30%
by weight, preferably of about 10 to 20% by weight,
relative to the total weight of the base layer.
Surprisingly, it was found that as a result of
incorporating a resin having a softening point in
the range specified above (> 140°C) into the
polypropylene, the barrier properties of the films
towards water vapor are substantially improved, and
that at the same time the optical properties and
also the shrink behavior of the film are favorably
influenced. Moreover, the films according to this
invention do not lead to deposits on rollers, e.g.,
during the longitudinal stretching. It has also been
shown that as a result of their particular slip
properties, the films of this invention are
especially well-suited for use on high-speed
packaging machines. The film surface is smooth
enough to avoid so-called 'slip-stick' effects,
i.e., irregular running speeds. On the other hand,
the roughness of the film surface is sufficiently
high to avoid jams prior to the actual slitting
operation.
From among the numerous low-molecular weight
resins, preference is given to the hydrocarbon
resins, in particular to the petroleum resins,
styrene resins, cyclopentadiene resins and terpene
resins (these resins are described in detail in
-5-
2~~~46~
'Ullmanns Enzyklopadie der Technischen Chemie'
(Ullmann's Encyclopedia of Technical Chemistry), 4th
edition, vol. 2, pp. 539-553). The term 'petroleum
resins' defines hydrocarbon resins produced by
polymerization of deep-decomposed petroleum
components in the presence of a catalyst. These
petroleum materials usually contain a mixture of
resin-forming substances, such as styrene,
methylstyrene, vinyltoluene, indene, methylindene,
l0 butadiene, isoprene, piperylene and pentylene. The
styrene resins are low-molecular weight styrene
homopolymers or copolymers of styrene and other
monomers, such as alpha-methylstyrene, vinyltoluene
and butadiene. The cyclopentadiene resins are
cyclopentadiene homopolymers or cyclopentadiene
copolymers obtained from coal tar distillates and
fractionated petroleum gas. The resins are produced
by subjecting the cyclopentadiene-containing
materials to high temperatures over a prolonged
period of time. Depending on the reaction
temperature, dimers, trimers or oligomers are
obtained.
The terpene resins include polymers of
terpenes, i. e. , hydrocarbons of the formula CioH~b.
which are present in practically all etherial oils
or oil-containing vegetal resins, and also phenol-
modified terpene resins. Specific examples of
suitable terpenes include alpha-pinene, beta-pinene,
dipentene, limonene, myrcene, camphene and similar
terpenes. The hydrocarbon resins may also be chosen
from among the so-called modified hydrocarbon
resins. Modification is generally performed by
reacting the raw materials prior to polymerization,
-6-
204749
by introducing special monomers or by reacting the
polymerized product, whereby preference is given to
hydrogenations or partial hydrogenations.
Suitable hydrocarbon resins also include
styrene homopolymers, styrene copolymers,
cyclopentadiene homopolymers, cyclopentadiene
copolymers and/or terpene polymers, which in each
case have a softening point of > 140°C (among the
unsaturated polymers, preference is given to the
hydrogenated products). Particularly preferably, the
cyclopentadiene homopolymers having a softening
point of > 140°C are employed in the base layer.
If the top layer(s), too, are to contain a
hydrocarbon resin, the resins listed above for the
base layer can be used. In this case, it is,
however, also possible to employ hydrocarbon resins
having a softening point of < 140°C.
In order to further improve certain
properties of the film according to this invention,
effective amounts of suitable additives may be
incorporated both in the base layer and in the top
layer(s). Preferred additives include antistatic
agents and/or antioxidants.
Straight-chain and saturated, aliphatic,
tertiary amines, which possess a Clo to C2o aliphatic
radical and two 2-hydroxy-(CZ-C4)alkyl groups are
preferred antioxidants . N- ( Cla-CZO) -, and especially
N- ( Clz-Cl8) alkyl-N' , N"-bis- ( 2-hydroxyethyl ) -amines are
employed particularly preferably.
The antioxidants employed preferably are so-
called primary antioxidants, i.e., sterically
hindered phenols or secondary amines, but it is also
possible to use secondary antioxidants, such as, for
2~~7~69
example, thioethers or phosphites or phosphonites,
or synergistic mixtures of primary and secondary
anitoxidants. Antioxidants of this generic type are
described, for example, in Gachter/Miiller:
Kunststoff-Additive (Plastics Additives), Carl
Hanser Verlag, 2nd edition (1983). Below, the
structural formulae of a number of suitable
compounds are given:
R 0
R--C-R ,R = _Cg20~CHZCHz ~ ~ $
I
R
v
C H3
R' R'
R ' _ _Cg2 ~ ~ H
H3C ~ ~CH3
R'
/OCH2 CHzO
C18H370P /~~ ~OC1gH37
~OCHZ CH20 ~
Preferred lubricants include carboxylic acid
amides, such as erucic acid amide and stearic acid
amide, or polydiorganylsiloxanes.
_g_
20~~~~
Suitable anti-blocking agents include, for
example, organic polymers which are incompatible
with the raw material employed for the top layer(s),
such as polyamides, polyesters, polycarbonates and
the like, or inorganic substances such as silicates,
silicon dioxide and calcium carbonate. Inorganic
substances, in particular silicon dioxide, with an
average particle size of 1 to 6 ~cm, have found to be
most suitable. These anti-blocking agents are added
in amounts of about 0.1 to 1% by weight, preferably
of about 0.15 to 0.5% by weight, relative to the
weight of the top layer(s).
The thickness of the top layers) preferably
varies between about 0.4 and 1.0 ~Cm.
The parameters for producing the films
according to this invention are expediently selected
such that stretching in the longitudinal direction
is performed at a temperature between about 100 and
130°C, preferably between about 105 and 120°C, and
at a stretch ratio between about 1:4 and 1:6.
Stretching in the transverse direction is performed
at a temperature between about 120 and 160°C,
preferably between about 130 and 150°C. The
stretching ratio in the transverse direction is
higher than about 1:7.5, and preferably it is in the
range of about 1:8 to 1:11. Following the stretching
state in the transverse direction, the film is heat-
set. During this treatment the film is conveyed in
the tenter frame, optionally in a slightly
converging manner, at a temperature which is about
5 to 50°C below the stretching temperature.
preferably, a convergence range of about 5 to 15% is
set for the heat-setting treatment. Ready
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2~4'~469
printability of the film is achieved by subjecting
the film to one of the conventional treatments prior
to winding, such as, for example, to a flame
treatment or electrical corona treatment. Corona
treatment by means of any of the known methods is
expediently performed such that the film is passed
between two conductor elements which serve as
electrodes, whereby a voltage which is high enough
to cause spray or corona discharges is applied
between the electrodes. This usually is an
alternating voltage of about 10,000 v and a
frequency of about 10,000 Hz. As a result of these
spray or corona discharges, the air above the film
is ionized and reacts with the molecules of the film
surface, so that polar inclusions are obtained in
the essentially non-polar polymer matrix. The
treatment intensities are within the usual range;
preferably they are between 38 and 42 mN/m.
The invention will be illustrated in greater
detail by way of the Examples which follow. A
comparative survey is given in Table 1.
Example 1
A three-layered, transparent film having a
total thickness of 20 ~Cm was produced by coextrusion
and subsequent orientation by biaxial stretching.
The film had the layer build-up ABA, 'A' denoting
the top layers and 'B' denoting the base layer. Each
of the top layers was 0.6 ~cm thick.
The base layer was comprised of polypropylene
to which 10$ by weight of resin (EXCOREZ~ ECR 356,
supplied by Exxon, Darien, Connecticut, USA;
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~0~7~0~
softening point of the resin: 140°C), relative to
the total weight of the blend, had been added. The
polypropylene had a melt flow viscosity of 3.5 g/10
min, determined according to DIN 53 735, under a
load of 2.16 kg.
The top layers were comprised of a
propylene/ethylene copolymer having an ethylene
content of 4.8% by weight, to which 0.8% by weight
of polydimethylsiloxane, 0.13% by weight of a
phenolic stabilizer (Antioxidant 330, supplied by
Ethyl Corp., Brussels, Belgium, and Baton Rouge, LA,
USA), 0.075% by weight of calcium stearate and 0.33%
by weight of Si02 having an average particle size of
2 ~,m, had been added. The polydimethylsiloxane had
a kinematic viscosity of 30,000 mm2/sec; the
propylene/ethylene copolymer had a melt flow
viscosity of 6.0 g/l0 min, measured according to DIN
53 735, under a load of 2.16 kg.
Example 2
A film was produced as described in Example
1, except that the resin content of the base layer
was 20% by weight (same resin as in Example 1).
Exam~~le 3
A film was produced as described in Example
1, except that the resin content of the base layer
was 30% by weight (same resin as in Example 1).
-11-
.._ 204746
Example 4
A film was produced as described in Example
2, except that the layer build-up was ABC. Top layer
'A' had been corona-treated and did not contain any
polydimethylsiloxane, whereas in top layer 'C' the
polydimethylsiloxane content had been doubled. Layer
B was the base layer.
Example 5
A film was produced as described in Example
2, except that the layer build-up was ABC. Top layer
'C' was comprised of a blend comprising an
ethylene/propylene copolymer with an addition of 10~
by weight of the resin employed in Example 1.
Comparative Example 1 (C1)
A film was produced as described in Example
1, but without the addition of the resin.
Comparative Example 2 (C2)
A film was produced as described in Example
1, except that a resin having a softening point of
85°C was employed (ESCOREZ~ 5380, supplied by
Exxon) .
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204'469
Comparative Example 3 ~ C3)
A film was produced as described in Example
2, except that no SiOz was contained in the top
layers.
In Table 1 below, the properties of the films
described in the Examples and Comparative Examples
are expressed in numerical values or rated as
follows:
++ - very good or no resin deposition on the
l0 rollers
+ - good or hardly any resin deposition on
the rollers
- - poor or noticeable resin deposition on
the rollers
-- - unacceptable or severe resin deposition
on the rollers
Determination of thermal blocking
To determine the thermal blocking properties,
two wooden blocks (72 mm x 41 mm x 13 mm) , to one
surface of which a piece of felt had been glued, are
wrapped into a sample of the film to be tested and
sealed. The two blocks are stacked on top of one
another, with the felt-clad surfaces facing each
other, and loaded with a weight of 200 g. This
arrangement is put in an oven preheated to 70°C and
left there for two hours. Then the temperature is
reduced to room temperature (21°C) for 30 minutes
and the weight is lifted off from the wooden blocks.
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2047469
By means of a mechanical appliance the upper block
is removed from the lower block. Evaluation is
performed over the course of 4 individual measure-
ments, from which the maximum take-down force
(measured in N) is determined.
The requirements of the specification are
fulfilled if none of the individual measurements
exceeds 5 N.
Determination of haze
The haze of the film is determined by a
method similar to ASTM-D 1003-52, whereby a 1° slot
aperture is used instead of the 4° round aperture,
and the haze is indicated for four superimposed
films, because in this way measurement can be
performed within the optimum range. Haze is
evaluated as follows:
up to 15%: very good
15 to 25%: moderate
over 25%: unsatisfactory
Determination of gloss
The gloss of the films is determined
according to DIN 67 530. The reflector value is
measured as an optical quantity for the surface of
a film. In accordance with the ASTM-D 523-78 and ISO
2813 standards, the angle of radiation incidence is
adjusted to 20°. A light beam hits the planar test
surface at the set angle of incidence and is
-14-
2Q~7~~9
reflected or scattered by the test surface, The
light beams incident on the photoelectronic receiver
are indicated as a proportional electrical quantity.
The measured value is dimensionless and must be
given with the angle of incidence. The gloss (angle
of incidence 20°) is evaluated using the following
ratings:
down to 115: very good
115 to 100 . moderate, and
less than 100: poor
Determination of shrink:
The shrink of a film is defined as the
percental change in length (1°-1/1°). Square film
samples having a side length of 10 cm ( 1°) are heated
to a temperature of 120°C for five minutes. Then the
remaining length (1) is measured.
Determination of permeability to water vapor and
oxYQen
The permeability to water vapor is determined
in accordance with DIN 53 122, part 2.
The barrier effect towards oxygen is measured
according to draft standard D1N 53 380, part 3, at
an atmospheric moisture content of 53%.
The Table shows that with regard to the
desired combination of properties, the films
according to the present invention are superior to
the films according to the Comparative Examples.
-15-
_. - 2~4'~469
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