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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to laminar thermo-plastic film structures
which are formed by coextrusion of the molten laminar layers through a
single die orifice and particularly such laminar structures that are
solidified after extrusion and passed onto an orientation or stretching
operation to produce a biaxially oriented laminate film. The film is
characterized by having good heat seal strengths, broad heat temperature
ranges and excellent impact resistance.
2. Description of the Prior Art
Oriented polypropylene films have become useful and widely employed
packaging films primarily because of their good moisture barrier
properties, stiffness, improved machine-ability, high strength
characteristics and excellent optical properties such as high gloss and
minimal haze. Such biaxially oriented polypropylene films are quite
difficult to seal because of the tendency of such films to deorient,
pucker or tear at the requisite sealing temperatures. Consequently, in
order to achieve satisfactory heat sealability, coatings of various types
have been applied to the polypropylene film surface to lower the requisite
heat seal temperatures. Generally, such coatings are applied in a separate
operation after the film has been formed and oriented. Many times the
coatings must be applied from solution in an organic solvent media. This
separate operation for application of relatively expensive coatings which
dictate employment of solvent recovery systems is quite costly. Examples
of such coatings which have been employed in the past to impart heat
sealability and other desirable characteristics to the polypropylene film
include saran and acrylic multipolymer coatings.
An alternate method for imparting improved heat sealability to oriented
polypropylene film, and one that is less costly than post orientation
coating, comprises the coextrusion of surface layers of a lower melting
resin onto the surface of the higher melting polypropylene core prior to
orientation. Following orientation a product is obtained which has a
relatively thick polypropylene core sandwiched between two relatively thin
layers or skins of the low melting resin. However, as practiced in the
prior art, this procedure has associated with it certain deficiencies. For
example, in an instance where it is desirable to form a laminar structure
comprising a thermoplastic core sandwiched between layers of a lower
melting point material such as an ethylene propylene copolymer to provide
heat sealability, the resulting laminar structure exhibits desirable high
heat seal strengths but because of the relatively high melting point of
such a copolymer layer, the heat sealing range, defined by the temperature
at which usable seal strengths are formed and that temperature where
undesirable film shrinkage occurs, is narrow.
In other instances in the prior art when a coextruded film comprising a low
melting skin material is employed as the coating or skin layer, e.g., a
medium density polyethylene resin produced by the high pressure, free
radical catalyst process, a product is produced which exhibits a much
wider sealing range than that of the hereinabove described coextruded
film, however, its seal strengths are undesirably low. Additionally, since
very low machine direction stretch ratios must be employed, such films are
oriented only to a slight degree in the machine direction and as a result
the film's toughness or impact strength is not as high as would be
desired. Improvement of the toughness characteristics of the film, by
imparting a greater machine direction stretch to the film while
maintaining the heat seal advantages of the polyethylene skin could not be
achieved because the higher temperatures which would be required to permit
greater machine direction stretching would also cause undesirable sticking
of the polyethylene skin to the stretching rolls.
SUMMARY OF THE INVENTION
The present invention is directed to the manufacture of a biaxially
oriented film that is heat sealable over a wide range of temperatures and
which has high heat seal strength. Such films are obtained by the
coextrusion of a surface layer of a polyethylene resin or predominantly
ethylene based copolymer surrounding a core of a predominantly
propylene-based polymer, the core composition being selected so that the
temperature required for machine stretching, i.e. orientation in the
machine direction, will not be so high as to cause sticking of the laminar
film skin surface to the surface of the orientation or stretching rollers.
Additionally, the laminar film structures show exceptionally high heat
seal strength when contrasted to prior art film laminations, superior
optical properties includng haze and gloss, and high strength and
toughness characteristics. Suitable core compositions include either (a) a
random ethylene/propylene copolymer, (b) a block ethylene/propylene
copolymer, (c) a mixture of polypropylene with a glassy, compatible resin
of low softening temperature, or (d) a mixture of a random or a block
ethylene/propylene copolymer with a glassy, compatible resin of low
softening temperature. The glassy compatible resins of low softening
temperature may be those resins which are described in U.S. Pat. Nos.
3,865,903 and 3,937,762, the disclosures of those patents beng
incorporated herein by reference. It will be noted that such glassy resin
compositions as disclosed in those patents comprise essentially an
aliphatic diene material such as pentadiene which has been copolymerized
with at least one other olefinically unsaturated comonomer. These glassy
resins serve essentially as high temperature plasticizers. Since they are
highly compatible with the core resin they do not impair the optical
properties of the laminar product and in most instances they have been
found to improve the optics of the final laminar product.
The present invention includes a method for the production of the
hereinabove described heat sealable coextruded films, which comprises the
coextrusion of a high melting point core layer which is coated on at least
one or both surfaces with a lower melting, relatively thin skin layer
based on polyethylene. The laminate structure is subsequently oriented in
both the machine and transverse direction and the resultant laminar film
may be heat sealed at temperatures below which disorientation of the core
material occurs.
The preferred stretch ratios employed in the present invention to obtain
satisfactory machine direction orientation levels are from about 3.0:1 up
to about 10.0:1 and preferably from about 4.0:1 up to about 7.0:1.
Uncoated, homopolymer, polypropylene resins which are machine direction
oriented to the desired levels hereinabove defined must be heated to
temperatures on the order of from about 285.degree. F. up to about
305.degree. F. Hence, if an unmodified polypropylene core were coated with
a low melting point resin, e.g. low density polyethylene and stretched at
ratios high enough to achieve satisfactory levels of machine direction
orientation (necessitating temperatures on the order of 285.degree. to
305.degree. F.) the lower melting polyethlyene casting melts and sticks to
the stretching rollers making high level machine direction stretch ratios
with such a laminar combination impossible to achieve.
The method of the present invention includes the employment of a core or
central resin layer which has a composition such that machine direction
orientation temperatures can be employed which, while permitting a
desirable degree of machine direction orientation to be carried out, will
not be so high as to cause sticking of the lower melting film surface
layers to the machine direction stretching rollers. Although such machine
direction stretching temperature may vary dependent upon the exact
compositions of the laminar core and surface or skin layers, temperatures
on the order of from about 200.degree. F. up to about 240.degree. F. have
been found to be generally suitable, the preferred machine direction
stretching temperature range being on the order of from about 215.degree.
F. up to about 235.degree. F.
Accordingly the thermoplastic film laminates of the present invention
comprise a single core layer, the core layer being coated with relatively
thin skin layers on one or both surfaces thereof, the core layer
comprising a member selected from the group consisting of (1)
ethylene-propylene copolymers; (2) blends of ethylene-propylene copolymers
with an aliphatic diene copolymer; and (3) blends of polypropylene
homopolymer with an aliphatic diene copolymer; said skin or surface layers
comprising polyethylene and copolymers of ethylene with olefinically
unsaturated comonomers.
In the case of the laminar film constructions of the present invention the
lower melting point skin or surface layers are considerably thinner than
the central or core layer, each of the skin layers constituting from about
1% up to about 10% of the overall thickness of the laminate. The
polyethylene homopolymer or copolymer which comprises the skin layer
should have a density of from about 0.910 up to about 0.939 and a melt
index range of from about 0.3 up to about 20.0 and preferably from about
3.0 to about 6.0. In the case of the employment of ethylene copolymers as
a surface or skin layer the ethylene content of the copolymer should be at
least about 80% and preferably 90% or more. Typical examples of such
copolymers include ethylene copolymerized with lower alkyl acrylates,
butene, pentene, hexene, octene, .alpha.-.beta. monoethylenically
unsaturated carboxylic acids including acrylic and methacrylic acids,
methyl pentene, vinyl acetate and the like.
DESCRIPTION OF SPECIFIC EMBODIMENTS
In accordance with the present invention, the laminar film structures
thereof are made by the coextrusion of the skin resin and the core
composition simultaneously, utilizing any of the prior art methods of
coextrusion. The extrudate is solidified by cooling it in a water bath or
on a casting roll. The solid base sheet is reheated and stretched in the
machine direction utilizing a series of rotating draw rollers, in
accordance with the prior art and finally the machine direction oriented
film is stretched in the transverse direction by employing well known
transverse direction stretching apparatus for film orientation such as a
tenter. Specific examples of the present invention are described
hereinbelow and are presented for illustrative purposes only and,
accordingly, should not be construed in a limitative sense with respect to
the scope of the present invention.
EXAMPLE 1
A three layer coextrudate was produced comprising a core of a random
ethylene/propylene copolymer having a melt flow rate of 5.4; a density of
0.90 and an ethylene content of 3.0% by weight, and surface layers of a
copolymer of ethylene and 4-methylpentene-1, identified by the
manufacturer as a linear low density resin. The copolymer was produced by
the utilization of a low pressure polymerization process and is
characterized by having a 4-methylpentene-1 content of 1.07 mole percent,
a melt index of 3.0 and a density of 0.935. Each surface layer constituted
approximately 6% of the overall thickness. The base film was quenched on a
casting roll having a surface temperature of 110.degree. F. and was
subsequently reheated to 218.degree. F. The preheated base sheet was drawn
5 times in the machine direction between heated rollers which were driven
at an approximate speed differential and subsequently tentered or
stretched in the transverse direction 7.5 times at a temperature of about
285.degree. F.
The machine direction orientation assembly comprised a series of 4
sequentially positioned preheating rollers which preheated the film to
about 218-220.degree. F., followed by a set of two stretching rollers, the
second of which is driven at a speed higher than the first or slow roller.
Film stretching or orientation in the machine direction occurs between the
closely positioned stretching rollers. In the present example in order to
achieve a stretch ratio of 5:1 the surface speed of the first stretching
roll as 10 ft./min. while the second or fast roller had a surface speed of
50 ft./min.
EXAMPLE 2
The procedure of Example 1 was followed except that the surface layer or
skin material comprised an ethylene/4-methylpentene-1 copolymer skin resin
having a 4 methylpentene content of 2.0 mole percent, a density of 0.925
and a melt index of 3.0.
EXAMPLE 3
The procedure of Example 1 was followed except that the surface layers
comprised a polyethylene homopolymer resin produced by the high pressure,
free radical catalyzed process, and having a density of 0.935 and a Melt
Index of 3.0.
EXAMPLE 4
The procedure of Example 3 was followed except that the core material
comprised a mixture of 84% polypropylene homopolymer and 16% of a glassy,
compatible, random interpolymer resin. The interpolymer resin was prepared
by anhydrous aluminum chloride catalyzed interpolymerization in toluene of
a mixture comprising by weight 55.3% of a piperylene concentrate; 9.7% of
a mixture comprising dipentene and .beta.-phellandrene present in a weight
ratio of about 2:1, respectively; and 35.0% of .alpha.-methylstyrene.
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Melting Point, .degree. C.
79 - 80
(Ball and Ring)
Molecular Weight 1442
(Weight Average) 1034
(Number Averge)
Bromine No. 6 - 10
Iodine No. 75 - 80
Acid Value <1
Specific Gravity 0.978 - 0.980
Percent Crystallinity 0
Tg (Glass Transition Temperature)
32.degree. C.
Saponification No. <1
Viscosity f to g
(in toluene -- 70%)
Color Gardner 5 - 7
(50% toluol solution)
Decomposition Temperature
205.degree. C.
(in nitrogen)
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The resinous interpolymer, when heated under nitrogen at a rate of
10.degree. C., per minute, had an initial decomposition temperature of
205.degree. C.; a 0.0% weight loss at 200.degree. C. a 12.8% weight loss
of 300.degree. C.; and a 90.0% weight loss at 400.degree. C.
The polypropylene homopolymer resin employed was identified by the
manufacturer as Tenite-612, having a melt flow of 4.0 to 5.0 and a
molecular weight of 340,000 to 380,000 (weight average) and 34,000 to
39,000 (number average). The polypropylene resin was also characterized by
the following physical properties:
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Crystalline Melting Point (.degree. F.)
330 - 340
Inherent Viscosity 1.4 - 1.6
Density 0.910 - 0.890
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EXAMPLE 4-A
The procedure of Example 4 was followed except that the surface layers
comprised an ethylene-vinyl acetate copolymer having a vinyl acetate
content of 4.5 mole percent; a melt index of about 1.0; and an apparent
density of 0.927.
EXAMPLE 5
For comparison purposes a multilayer laminate was prepared utilizing the
process as shown in Example 1, except that the skin comprised an
ethylene/propylene random copolymer, having an ethylene content of
approximately 3% by weight, and the core comprised a polypropylene
homopolymer. The ethylene-propylene copolymer and the polypropylene
homopolymer resins employed were the same as those identified in Examples
1 and 4 respectively. The Example 1 process was modified to the extent
that the present temperature of the laminar base film prior to machine
direction orientation was increased to 275.degree. F.
EXAMPLE 6
For comparison purposes another coextruded film was prepared in a similar
fashion to that of Example 1 except that it was only drawn 1.4 times in
the machine direction prior to stretching it 7.5 times transversely. The
film comprised a polypropylene homopolymer core resin (as described in
Example (4) having surface skins consisting of a mdeium density
polyethylene resin that was produced by the high pressure free radical
cartalyzed process and that was additionally characterized by having a
density of 0.935 and a melt index of 3.5. It is noted that attempts to
achieve a higher machine direction draw with this laminar combination,
i.e. by increasing the machine direction preheat temperature up to about
275.degree. F. resulted in the polyethylene skin layers sticking to the
machine direction orienting rollers whereby production of satisfactory
film was not possible.
The heat seal properties of the laminates which were prepared in accordance
with Examples 1 through 6 are set forth in the following Table I. It can
be seen that the heat seal characteristics of the laminar films of the
prior art, i.e. films made in accordance with Examples 5 and 6, are
inferior to the laminates of the present invention as described in
Examples 1, 2, 3, 4 and 4-A. While the prior art film of Example 5
provides a high heat seal strength, it only does so at a high sealing
temperatur eand therefore the temperature range over which high seal
strengths can be attained with this lamination is narrow. As can be seen
from the data set forth in th following Tables, the laminar film
structures of the present invention consistently exhibit wide heat seal
ranges and toughness which are superior to the prior art film laminates.
Such wide sealing ranges and toughness characteristics are essential in
the packaging end use applications for the laminar film constructions of
the present invention. Additionally in certain packaging applications,
high heat seal strength characteristics are particularly desirable.
Utilizing the film laminates of this invention, in addition to the
hereinbefore described toughness properties and wide sealing ranges, in
many instances high heat seal characteristics are also obtainable.
Additionally, as illustrated in Table 2, the laminated films of the
present invention, in contrast to those of the prior art, i.e. Examples 5
and 6, provide improved optical properties, including reduced haze and
higher gloss.
The beneficial effect on film toughness, e.g., impact strength which is
achieved with the higher machine direction orientation which is made
possible by the provisions of the present invention is shown by the data
as set forth in Table 3. It will be seen that the prior art film of
Example 6 is substantially less tough than the films of Examples 1 through
4-A.
TABLE 1
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Film Laminates Produced
In Accordance With:
Sealing Temper-
Seal Strength (grams/linear inch)
ature .degree. F.
240 250 260 270 280 290 300
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Example 1 0 20 300 475 530 555 --
Example 2 65 235 590 730 805 680 --
Example 3 0 0 145 125 125 150 --
Example 4 60 180 180 200 270 340 --
Example 4-A 200 230 200 205 250 220 --
Example 5 0 0 0 0 0 200 600
Example 6 0 0 190 260 245 305 --
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TABLE 2
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Laminates Produced In
Accordance With: Haze* Gloss**
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Example 1 2.4 79.8
Example 2 1.3 83
Example 3 1.2 79.5
Example 4 2.0 85
Example 4-A 2.0 80
Example 5 4.4 73.6
Example 6 6.5 60
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*ASTM - D 1003-61
**ASTM - D2457-70
TABLE 3
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Laminates Produced In Ball Burst
Accordance With: cm-kg./mil
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Example 1 27.0
Example 2 28.1
Example 3 20.9
Example 4 20.0
Example 4-A 20.0
Example 5 22.0
Example 6 3.0
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Although the present invention has been described with preferred
embodiments, it is to be understood that modifications and variations may
be resorted to, without departing from the spirit and scope of this
invention, as those skilled in the art will readily understand. Such
modifications and variations are considered to be within the purview and
scope of the appended claims.
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