Note: Descriptions are shown in the official language in which they were submitted.
FIELD OF THE INVENTION
This invention relates to thermoplastic, heat shrinkable
packaging films, particularly, polypropylene films having improved heat
sealing characteristics. These films can be used to package a wide
variety of food and non-food items.
BACKGROUND OF THE 'INVENTION
Oriented polypropylene films are useful and widely accepted
packaging films because of their good moisture'barrier, stiffness, high
strength, and optical properties. However, films of polypropylene do
not, in general,"exhibit good heat sealing properties which is an
important consideration in packaging applications. This is because
polypropylene films have very narrow sealing temperature ranges.
The sealing temperature range for a thermoplastic film can be
considered as beginning at that point where upon application of heat and
pressure the surfaces of the t'ilm will seal or bond together. As
temperature is increased further, a point is reached where the thermo-
plastic melts and flows quite readily thus making it difficult to
achieve a seal in a specific area thus marking the upper end of the
sealing range. Also at the upper end of the sealing range the film may
tend to burn and char leaving weak and unsightly sèais. For propylene
homopolymer film, as temperature is increased, the film does not become
sealable but rather suddenly flows undesirably as its melting point is
approached making it quite difficult to achieve smooth, continuous,
satisfactory seals. This type of sealing performance can not be tolerated
on high speed packaging equipment.
In orde~. to get good heat sealability, it has been the practice
to apply various types of coatings to ~he films. Coatings have conventionally
been applied in separate coating operations, such as from emulsions,
extrusion coating, and so forth, but such operations are costly and
require additional handling of the film. Coextrusion has been used
success~ully to put a heat sealable coating on polypropylene and produce
a film of good seal strength, but until the present inYention coextruded
polypropylene film laminates had a very narrow heat seal range. and were
expensive to produce. Accordingly, it is a~principal object of the
present invention to provide a polypropylene film having a wide sealing
range.
Another object of the present invèntion is to provide a poly-
propylene film having acceptable seal strength for shrink packaging
applications.
Typical patents which disclose coatings for polypropylene film
are U.S. Patent No. 3,285,766 which issued on November 15, 1966 to
Edward Barkis et al which discloses a method of extruding a coat~ng
containing at least 65% ethylene onto a sheet of polypropylene, U.S.
Patent No. 3,671,383 which issued on June 20, 1972 to Rikita Sakata et al
which discloses a biaxially oriented po1ypropylene film laminated to a
uniaxially oriented ethylene-propylene copolymer ~ilm with at least 75%
by weight polypropylene; U.S. Patent No. 4~132,050 which issued on
January 2, 1979 to Roger N. Young and which discloses a film having a
substrate formed of a blend of 87.5 to 60 parts of polypropylene and
12.5 to 40 parts of ethylene-propylene block copolymer with a heat
sealable layer; and, U.S. Patent No. 4,148,972 which issued on April 10,
1979 to Toshi Hiro Yamane et al which discloses a polypropylene film
laminate made by laminating a polypropylene layer having 1 to 8 weight
percent of an ionomer with an ionomer layer.
SUMMARY OF THE INVENTION
It has been surprisingly discovered that the principal objects of
the present invention, namely, increasing the seal temperature range o~ a
multi-layer polypropylene film and providing acceptable seal strength for such
a film, can be achieved by a heat-sealable, multi-layer ilm which comprises a
base layer of propylene homopolymer and a sk~in layer affixed to one surface of
the base layer wherein the skin layer comprises a blend of 60~ to 80% by
weight of propylene-ethylene copolymer with 40% to 20% by weight of propylene
homopolymer. In a particularly desirable embodiment, the skin layer has 70~
10 by weight of propylene-ethylene copolymer blended with 30% by weight of prop-ylene homopolymer and the copolymer contains 2 to 6% by weight of ethylene, or
even more preferably, 3% to 4% ethylene by weight. In addition, the ratio of
the thickness of the skin layer to the thickness of the base layer is prefer-
ably 4:1 or greater.
In another aspect, the subject invention is a method of increasing
the sealing temperature range of propylene homopolymer film comprising the
steps of providing a base layer of propylene homopolymer; and, laminating a
skin layer to at least one side of the propylene homopolymer film wherein the
skin layer comprises a blend of 60 to 80% by weight of propylene-ethylene
copolymer having 2 to 6% by weight o~ ethylene with 40% to 20% by weight of
propylene homopolymer; and, biaxially stretching the laminated film to a final
total film thickness in the range of 0.25 to 2.5 mils, thereby producing a
heat sealable, heat shrinkable polypropylene homopolymer film. Such a film
will have a sealing range of at least 20C and will generally produce a seal
strength of greater than 300 gms/inch. A preferred method of producing the
laminated film is to coextrude the skin layer and the base layer.
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Still another aspect of the.subject invention is a.method of
producing a.heat sealed and shrunken package of polypropylene film
comprising the steps of provid;ng a base layer nf propylene homopolymer
film, laminating a skin layer to at least one side of said propylene
homopolymer film, said skin layer comprising a blend of 60% to 80% by
weight of propylene-ethylene copolymer havi;ng 2 to 6% by weight of
ethylene; biaxially stretching the laminated film to a final total film
thickness in the range of 0..25 to 2.5 mils..thereby producing a heat
sealable, heat shrinkable film, providing.a product be wrappedi over-
wrapping the product with fllm and bringing skin layer surfaces into
contact with each other at points to be heat sealed; applying heat and
pressure at the points to be sealed thereby sealing said film at
said points, the sealing temperature range of said applied heat being in
the range of 135C to 160C, the heat seals of the resulting package
having a minimum strength of 300 gms/inch; and~ applying heat in the
range of 110C to 135C to said package for time sufficient to shrink
said film about said product.
In all aspects of the subject invention, the film may be
perforated.
PREFERRED EMBODIMENT
The preferred embodiment of the present invention is a two
layer film made by blending 70% by weight.of propylene ethylene copolymer
having 3.5 to 4.2% by weight of ethylene with 30% by weight of propylene
homopolymer for the skin or outside l.ayer. This mixture is blended in a
Banbury blender and then fed into the hopper of an extruder which feeds
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nto an annular d;e for extruding a tube having two layers. The inside
or base layer comprises propylene homopolymer which is fed into the
hopper of a second extruder which also feeds the annular die to provide
the inside layer.
In the annular die the two tubular extrudates are jo;ned to
become a multi-layer tubular laminate. This tubular laminate is rapidly
cooled and then collapsed. Afterwards, the tubing is inflated and
heated to the orientation temperature range of polypropylene which is in
the ran~e of 135C to 150C. The heating takes place in an oven, and
as the flattened film emerges from the oven, it passes through pinch
rolls and is opened to a bubble by the well known trapped bubble technique
where it is ;s stretched about five times in both vertical and hori~ontal
directions which reduces the film to a thickness in the range of 0.60 to
0.80 mils. Prior to stretching the typical tubing would have a thickness
of about 18 mils with the thickness ratio of the polypropylene layer to
the blended layer being 5:1. A description of one such bubble technique
for biaxially orie!nting polypropylene is given in U.S. Patent No. 3,260,776
whicil issued on July 17, 1966 to C. A. Lindstrom et al.
After the film has been expanded into the bubble and the
bubble cooled, it is collapsed, slit, and wound up.
In some instances it is desirable to perforate the film so
that an array of perforations or small holes are made in the film.
Depending on the desired application, the number of holes may be from
eight to ten per square inch of film up to several hundred holes per
square inch and the holes may run anywhere from 2 to 20 mils in diameter.
A typical apparatus for perforating the film will employ a cooled grid
over which the film is passed while ;t is exposed to a jet of hot gas
which will melt the film in the uncooled areas. Such an apparatus and
method are described in U.S. Patent No. 3,038,19~ which issued on
June 12, 1962 to C. H. Schaar.
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To determine the sealing range and seal strength of the
preferred film two sheets of the film were superposed with the skin
layer surfaces in contact. An electrical resistance heated flat sealing
bar 5 mm. wide was used to press the sheets together against a backing
surface with a pressure of 1 kg./sq.cm. for 0.5 second and the temperature
of the sealing bar was recorded. After the sealing had been completed a
one inch strip was cut perpendicular to and across the seal made by the
bar, and the ends of the respective strips were placed in the 3aws of an
Instron testing machine with the seal locàted at approximately the mid
point between the gripping jaws. Force was applied by driving the jaws
apart until the seal either peeled or broke. Seals having a minimum
strength of approximately 300 gms/inch, the minimum considered sufficient
for packaging applications, were achieved by the preferred embodiment at
a seal bar temperature as low as 135C and satisfactory seals continued
to be made as temperature increased up to about 160C for a sealing
range of approximately 25C. ~Hereinbelow, field tests indicate that
the operable range is 30C.) ~ maximum seal strength of about 1,400
gms/inch was achieved.
As stated previously, a monolayer polypropylene film will have
an essentially "zero" width sealing range. ~ecause seal bars are cooled
by contact with film, in packaging machinery it is the necessary
practice with polypropylene to set the temperature o~ the seal bars
somewhat above the melting point so that a sealed package can be obtained
but seals made in this way have a poor appearance and burned zones.
With a film such as that of the present invention, setting the seal bars
and temperatures near the top of the sealing range does not run the risk
of melting the film or burning it plus at high speeds where the film
tends to cool the seal bars, the seal bars will still maintain suf~icient
heat to seal the film and not be cooled below the sealing range.
7~
A number of products can be packaged in the film of the
present invention and the various machines use different techniques. In
some instances, the product is placed on one sheet of film and then a
second sheet is laid over the product and a complete perimeter seal ;s
then made; and, in some instances the sealing operation also severs the
excess film to trim the package. After sealing, the product passes
through a heat tunnel which may have a temperature as low as 110C but
is usually set in the range in which the film was stretched. In the
tunnel the heat causes the stretch tensions to release and causes the
film to shrink around the product. In other packaging processes the
product may be placed between the folds of a center folded film and a
seal on the remaining three unsealed sides is made~ Many well known
methods of over-wrapping a product in shrink film are understood by
those skilled in the art.
Products such as bread, soft bakery products including rolls,
pizzas, and the like may be packaged with the film of the present invention.
Also food products requiring breathing-through the film such as fruits
and vegtables may be packaged using the perfbrated film.
EXAMPLES
In order to solve the problem presented by the virtually non-
existent heat sealing range of polypropylene, a number of blended
sealing layer compositions were tried. Listed below in Table I are the
two layer films having a polypropylene hompolymer base or substrate
layer which has been coextruded with the specifically designated sealing
layer composition. Only those compositions which could be successfully ,
processed by the trapped bubble technique into film are shown in Table I.
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The footnotes immed;ately below the table explain the abbreviat;ons forthe polymers, and sealing range and seal strength were determined as
explained hereinabove. Haze, of course~ i.s the percentage of trans-
mitted light which in passing through the specimen deviates from the
incident beam more than 2.5 ~ on the.average and which is determined by
ASTM Method D1003. The measurement of thickness reported is of the
total film laminate.
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Examples 3, 10~ and 11 are expressed as ranges because these
samples were run on actual equipment in the field as well as having
their sealing range measured in the laboratory and, in the field, about
a 5~C increase in sealing temperature range was observed thus increasing
the upper seal temperature to about 165C. These three samples were run
as perforated films packaging rolls and crusty bread. Each of these
films had good machinability, i.e. they were stiff enough to perform
well in standard packaging equipment.
The film according to Example ll also performed well in the
field for packaging crescent rolls and pizzas with non-perforated film.
Looking at Table I it is evident that Examples 10, 11, and 12
have desirably wide sealing ranges and each of these films has good
machinability and the haze is not objectionable. However, above approxi-
mately ~0% propylene-ethylene copolymer in the blend processability
problems in the trapped bubble process become pronounced as the sealing
layer tends to flow and distort at the high temperature needed to
stretch and orient the polypropylene base layer. In other words, the
propylene-ethylene copolymer is beginning to melt at the orientation
temperature of the propylene homopolymer and the sealing layer belng on
the outside of the tubing tends to stick to pinch rolls after being
heated but prior to being blown into a bubble. Thus, the preferred
range for the propylene-ethylene copolymer in the sealing layer blend is
from 6Q to 80% by weight and this g;ves a sealing temperature range
which varies from 20C to 30C. The composition of Example 11 is the
best all around combination of seal temperature range, seal strength,
processability on present state of the art film manufacturing equipment,
and machinability. On commercial packaging machinery 50 to 80 packages
per minute can be made and the indications are -that these rates can be
successfully exceeded.
1 1
By coextruding the base layer and skin as a flat sheet from a
slot die, rapidly cooling the sheet, and using a tenterframe to stretch
the flat sheet when heated to orientation temperature continuous manu-
facture of a film in which the skin layer blend is greater than 80%
could be achieved but trim wastage and other factors make the tenter-
frame method less attractive. In any event, it should be recognized
that the widening of the heat seal range begins noticeably with a skin
layer blend of at least 50% copolymer and continues up to 100% copolymer.
The propylene homopolymer used in the present invention i5
predominately isotatic polypropylene of film grade available from a
number of well known resin suppliers. Likewise, the propylene-ethylene
copolymer is a random copolymer of film grade also available from well
known resin producers and the ethylene weight percentage is in the range
of 2% to 6% which will perform satisfactorily. With a higher percentage
of ethylene in the copolymer it is to be expected that the percentage of
the copolymer in the blend may be increased.
The layer thicknesc. ratios of the multi-layer film oF this
invention are those which give the optimum combination of machineability,
sealability and`processability. If the sealing layer thickness is
increased much beyond the 4:1 ratio processing problems are incurred as
explained above at the orientation temperature of polypropylene. Thus,
the base layer to sealing layer ratio should be greater than 4:1 and as
noted above, a 5:1 ratio gives quite satisfactory performance. In
addition, a total film thickness in the range of 0.25 to 2.5 mil produces
the most satisfactory combination of film characteristics.
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Also within the scope of this invention are three layer films
where the central layer is the.propylene hompolymer layer which is
oriented and the sealing layers are on both sides of the oriented
hompolymer layer to give it balance. This is to prevent curl which can
occur in structurally unbalanced~ multi-layer films.
Having described by invention, T claim:
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