U.S. patent number RE35,285 [Application Number 08/085,468] was granted by the patent office on 1996-06-25 for thermoplastic multi-layer packaging film and bags made therefrom.
This patent grant is currently assigned to W. R. Grace & Co.-Conn.. Invention is credited to Cesare Quacquarella, Richard S. Williams.
United States Patent |
RE35,285 |
Quacquarella , et
al. |
June 25, 1996 |
Thermoplastic multi-layer packaging film and bags made
therefrom
Abstract
A thermoplastic, multi-layer packaging film is disclosed which
comprises at least 5 layers in which one surface layer is a heat
sealing layer and one of the internal layers is a barrier layer.
The heat sealing surface layer comprises a copolymer of ethylene
and an alpha-olefin having 4 to 8 carbon atoms per molecule and
said copolymer comprises 90% to 75% ethylene and 10% to 25%
alpha-olefin having 4 to 8 carbon molecules and a density of less
than 915 kg/m.sup.2. The heat sealing layer may also comprise a
blend of the aformentioned copolymer with an ethylene/vinyl-acetate
copolymer or an ethylene/butyl-acrylate copolymer. The film can be
oriented to be heat shrinkable and can be formed into bags by heat
sealing portions of the sealing surfaces together. The heat seals
have superior cold strength and minimize seal failure when the bags
are loaded with relatively heavy products.
Inventors: |
Quacquarella; Cesare (Milan,
IT), Williams; Richard S. (Wyton, GB2) |
Assignee: |
W. R. Grace & Co.-Conn.
(Duncan, SC)
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Family
ID: |
27452817 |
Appl.
No.: |
08/085,468 |
Filed: |
June 30, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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648480 |
Jan 30, 1991 |
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Reissue of: |
911936 |
Sep 26, 1986 |
04801486 |
Jan 31, 1989 |
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Foreign Application Priority Data
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Sep 30, 1985 [IT] |
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22305/85 |
May 16, 1986 [IT] |
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20457/86 |
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Current U.S.
Class: |
428/34.9;
426/127; 428/349; 428/35.4; 428/476.3; 428/476.9; 428/516; 428/518;
428/520 |
Current CPC
Class: |
B32B
27/08 (20130101); Y10T 428/31757 (20150401); Y10T
428/3175 (20150401); Y10T 428/3192 (20150401); Y10T
428/31913 (20150401); Y10T 428/31928 (20150401); Y10T
428/1341 (20150115); Y10T 428/1328 (20150115); Y10T
428/2826 (20150115) |
Current International
Class: |
B32B
27/08 (20060101); B32B 027/08 (); B32B 027/30 ();
B32B 027/32 (); B29D 007/02 () |
Field of
Search: |
;428/34.9,347,349,516,518,476.9,476.3,520 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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533613 |
|
Dec 1980 |
|
AU |
|
553698 |
|
Oct 1984 |
|
AU |
|
0002606 |
|
Jun 1979 |
|
EP |
|
0120503 |
|
Oct 1984 |
|
EP |
|
52-135386 |
|
Nov 1977 |
|
JP |
|
58-82752 |
|
May 1983 |
|
JP |
|
58-102762 |
|
Jun 1983 |
|
JP |
|
58-37907 |
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Aug 1983 |
|
JP |
|
58-208435 |
|
Dec 1983 |
|
JP |
|
2066274 |
|
Jul 1981 |
|
GB |
|
1600250 |
|
Oct 1981 |
|
GB |
|
2097324 |
|
Nov 1982 |
|
GB |
|
Other References
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Extrusion," The Dow Chemical Co., copyright 1984. .
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Polyethylene," The Dow Chemical Company, Oct. 16, 1985. .
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.
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1984, p. 113. .
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Resistance," Plastics Technology, Oct. 1984, pp. 13 and 15. .
"Can `VLDPE` outperform EVA, other copolymers?", Modern Plastics
International, Oct. 1984, pp. 6, 8. .
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1984, p. 8. .
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World, Oct. 1984, p. 86. .
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.
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|
Primary Examiner: Sluby; P. C.
Attorney, Agent or Firm: Quatt; Mark B.
Parent Case Text
.Iadd.This application is a continuation of application Ser. No.
648,480 filed on Jan. 30, 1991, now abandoned, which in turn is a
reissue of U.S. Pat. No. 4,801,486. .Iaddend.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A multi-layer thermoplastic packaging film having improved heat
shrink and cold seal properties, said film comprising:
(a) a first surface layer which is a heat sealing surface, said
layer comprising:
(1) a copolymer of ethylene and an alpha-olefin having 4 to 8
carbon atoms per molecule; and,
(2) said copolymer comprising 90% to 75% ethylene and 10% to 25%
alpha-olefin having 4 to 8 carbon molecules and a density of less
than 915 kg/m.sup.3 ;
(b) a second surface layer, said layer comprising a polymer
selected from the group consisting of amide polymers,
ethylene/vinyl-acetate copolymers, the copolymer of the first
layer, and ionomers;
(c) a first core layer comprising an ethylene/vinyl-acetate
copolymer;
(d) a second core or barrier layer comprising a polymer selected
from the group consisting of copolymers of vinylidene chloride and
hydrolyzed copolymers of ethylene/vinyl-acetate;
(e) said core layers being internal film layers positioned between
said first and second surface layers;
(f) at least one adhesive layer to promote adhesion of the barrier
layer to another layer; and
(g) said film comprising at least five layers.
2. The film of claim 1 wherein the first surface layer is
cross-linked.
3. The film of claim 1 wherein the first surface layer comprises a
blend of the copolymer defined in (a) (1) and (2) with a copolymer
selected from the group consisting of copolymers of
ethylene/vinyl-acetate and copolymers of
ethylene/butyl-acrylate.
4. The film of claim 1 wherein the second core or barrier layer
comprises a vinylidene chloride copolymer and the second surface
layer comprises a polyamide.
5. A bag formed from the film of claim 1. .Iadd.
6. A multi-layer, heat-shrinkable thermoplastic packaging film
having excellent heat shrink and cold seal properties, said film
comprising:
(a) a first surface layer which is heat-sealing, said first surface
layer comprising a blend of (i) a linear copolymer of 90% to 75%
ethylene and 10% to 25% of an alpha-olefin having 4 to 8 carbon
atoms per molecule, said linear copolymer having a density of less
than 915 kg/m.sup.3 and at least 906 kg/m.sup.3, and (ii) up to 50%
by weight, based on the total weight of the first surface layer, of
a polymer compatible with said linear copolymer selected from the
group consisting of linear polyethylene copolymers having a density
above 915 kg/m.sup.3, low density polyethylene, ethylene
vinylacetate, acid-modified ethylene vinylacetate, polypropylene,
copolymers of ethylene and propylene, ionomeric polymers and
ethylene/alkylacrylate copolymers wherein the alkyl moiety has 1 to
8 carbon atoms; and
(b) a second surface layer comprising a polymer material selected
from the group consisting of said linear copolymer of the first
surface layer, ethylene-vinylacetate copolymers, polyamides and
ionomeric resins. .Iaddend. .Iadd.7. A multi-layer, heat-shrinkable
thermoplastic packaging film having excellent heat shrink and cold
seal properties as recited in claim 6, said film further
comprising:
(c) a first internal layer between said first surface layer and
said second surface layer, said first internal layer comprising a
core or barrier material. .Iaddend. .Iadd.8. A multi-layer,
heat-shrinkable thermoplastic packaging film having excellent heat
shrink and cold seal properties as recited in claim 7, said film
further comprising:
(d) a second internal layer between said first internal layer and
said second surface polymer layer, said second internal layer
comprising a core or barrier material. .Iaddend. .Iadd.9. A
multi-layer, heat-shrinkable thermoplastic packaging film having
excellent heat shrink and cold seal properties as recited in claim
8, wherein said core or barrier materials of said first and second
internal layers are selected from the group consisting of
copolymers of ethylene vinylacetate and polyvinylidene chloride
copolymers. .Iaddend. .Iadd.10. A multi-layer, heat-shrinkable
thermoplastic packaging film having excellent heat shrink and cold
seal properties as recited in claim 8, further comprising:
(e) an adhesive layer between said first surface layer and said
second surface layer. .Iaddend. .Iadd.11. A multi-layer,
heat-shrinkable thermoplastic packaging film having excellent heat
shrink and cold seal properties as recited in claim 10, wherein
said at least one adhesive layer contains acid-modified ethylene
vinylacetate. .Iaddend. .Iadd.12. A multi-layer, heat-shrinkable
thermoplastic packaging film having excellent heat shrink and cold
seal properties as recited in claim 8, wherein the core or barrier
material of said first internal layer contains a copolymer of
ethylene vinylacetate and the core or barrier material of said
second internal layer contains polyvinylidene chloride. .Iaddend.
.Iadd.13. A multi-layer, heat-shrinkable thermoplastic packaging
film having excellent heat shrink and cold seal properties as
recited in claim 12, further comprising:
(e) an adhesive layer between said first internal layer and said
second internal layer; and
(f) an adhesive layer between said second internal layer and said
second surface layer. .Iaddend. .Iadd.14. A multi-layer,
heat-shrinkable thermoplastic packaging film having excellent heat
shrink and cold seal properties as recited in claim 13, wherein
said adhesive layers contain acid-modified ethylene vinylacetate.
.Iaddend. .Iadd.15. A multi-layer, heat-shrinkable thermoplastic
packaging film having excellent heat shrink and cold seal
properties as recited in claim 7, wherein said first surface layer
is cross-linked. .Iaddend. .Iadd.16. A multi-layer, heat-shrinkable
thermoplastic packaging film having excellent heat shrink and cold
seal properties as recited in claim 6, wherein in the heat-sealing
blend, said polymer compatible with the linear copolymer is
selected from the group consisting of copolymers of ethylene
vinylacetate, acid-modified ethylene vinylacetate, copolymers of
ethylene and propylene, and ethylene-butylacrylate. .Iaddend.
.Iadd.17. A bag formed from the film according to claim 16.
.Iaddend. .Iadd.18. A multi-layer, heat-shrinkable thermoplastic
packaging film having excellent heat shrink and cold seal
properties, said film comprising:
(a) an inner layer comprising a heat-sealing blend of (i) a linear
copolymer of ethylene and x to y % by weight, based on the linear
copolymer, of an alpha-olefin having 4 to 8 carbon atoms per
molecule, wherein x=0.5n.sub.C +8 and y=1.25n.sub.C +15 with
n.sub.C being the number of carbon atoms in the alpha-olefin, said
linear copolymer having a density of less than 915 kg/m.sup.3 and
at least 906 kg/m.sup.3, and (ii) up to 50% by weight, based on the
total weight of the heat-sealing inner layer, of a polymer
compatible with said linear copolymer selected from the group
consisting of linear polyethylene copolymers having a density above
915 kg/m.sup.3, low density polyethylene, ethylene vinylacetate,
acid modified ethylene vinylacetate, polypropylene, copolymers of
ethylene and propylene, ionomeric polymers and
ethylene/alkylacrylate copolymers wherein the alkyl moiety has 1 to
8 carbon atoms; and
(b) an outer layer comprising a polymer material selected from the
group consisting of said linear copolymer of the inner layer,
ethylene-vinylacetate copolymers, polyamides and ionomeric resins.
.Iaddend. .Iadd.19. A multi-layer, heat-shrinkable thermoplastic
packaging film having excellent heat shrink and cold seal
properties as recited in claim 18, wherein in the heat-sealing
blend, said polymer compatible with the linear copolymer is
selected from the group consisting of copolymers of ethylene
vinylacetate, acid-modified ethylene vinylacetate, copolymers of
ethylene and propylene, and ethylene-butylacrylate. .Iaddend.
.Iadd.20.
A bag formed from the film according to claim 18. .Iaddend.
.Iadd.21. A thermoplastic multilayer packaging film having superior
heat shrink and cold seal properties comprising at least an outer
polymer layer and an inner heat sealing layer, said heat sealing
layer comprising a linear copolymer of ethylene and x to y % by
weight, based on the copolymer, of an alpha-olefin with 4 to 8
carbon atoms per molecule, wherein x=0.5nc+8 and y=1.25nc+15 with
nc being the number of carbon atoms in the alpha-olefin, said
copolymer having a density of less than 0.915 g/cc and at least
0.906 g/cm.sup.3. .Iaddend.
Description
The invention relates to thermoplastic multi-layer packaging films
and bags or pouches made therefrom. In particular this invention
relates to films and bags having superior heat shrink and cold seal
properties, which give good seal strength even through
contamination like grease or blood, and which allow the application
of higher shrink temperatures due to their improved seal strength
at elevated temperatures.
Shrinkable and heat sealable thermoplastic films are being used in
packaging of non-food and food products like meat, cheese, poultry
and the like. Many attempts have been made to combine good shrink
properties with satisfactory sealability and good seal strength
both at room temperature and at higher temperatures but there is
still room for improvement.
A laminate film known from U.S. Pat. No. 3,741,253 comprises a core
layer of a vinylidene chloride copolymer between a layer of a
cross-linked ethylene-vinyl acetate copolymer used as a sealant and
an outer layer of ethylene-vinylacetate copolymer which is not
cross-linked. The ethylene-vinylacetate copolymer has improved
properties over the previously used polyethylene, especially when
it has been cross-linked by irradiation.
As disclosed in U.S. Pat. No. 4,064,296 the core layer may also
consist of a hydrolized ethylene-vinyl acetate copolymer. It has
similar oxygen barrier properties as vinylidene chloride copolymers
and often the advantage that it may be irradiated without
discoloration.
Certain advantages have been achieved by using an ionomeric resin
as sealing layer. They result in a higher seal strength at elevated
temperatures thus allowing higher shrink temperatures. However,
their cold seal strength is not fully satisfactory. This latter
property is important in modern automated bag packaging processes
in which the seals are subjected to substantial stress when the
bags are opened and loaded with the article to be packed.
More recently low density linear polyethylenes have become
commercially available and have also been proposed for packaging
applications. Actually the so called linear low density
polyethylenes are copolymers of ethylene and varying amounts of
higher .alpha.-olefins with e.g. 5 to 10 carbon atoms per molecule
(U.S. Pat. No. 4,076,698) or 3 to 8 carbon atoms per molecule
(published European patent application No. 120503). Depending on
their density these materials are referred to as linear low density
polyethylene (LLDPE) or very low density polyethylene (VLDPE), the
separation line being at a density of about 0.910 g/cm.sup.3. Some
properties of VLDPE have been described in Plastics Technology,
September 1984, page 113 and October 1984, page 13 as well as in a
company brochure published in February 1984 by DSM in the
Netherlands and entitled "Stamilex PE". Their properties are said
to be a unique combination between those of standard polyethylene
and polyolefmic rubbers. Their sealability and their compatibility
with other polymers has been mentioned.
It is an object of the present invention to provide a multi-layer
laminated packaging film and bags made therefrom which have
improved shrink characteristics over the materials used in the
past. This means that at a given temperature the shrink in percent
(the mean between the shrink in the longitudinal and transversal
direction) should be higher and the tolerated maximum shrink
temperature should also be higher. The maximum shrink temperature
is dependent on the seal strength at elevated temperatures.
It is a further object of this invention to provide a packaging
material with improved cold seal strength and thereby a reduced
risk of breakages when bags made of the film material are utilized
in automated loading processes.
It is yet another object of the present invention to provide a
packaging material which gives reliable seals even when
contaminated. Especially in the packaging of meat it frequently
occurs that the sealing areas are contaminated with grease or
blood, both of which tend to impair the sealing between hitherto
used sealing layers.
Finally and most importantly it is an object of this invention to
provide a material combining the above advantages, i.e. improved
shrinkability, increased maximum shrink temperature, good cold seal
strength and sealability through contamination.
It has been found that the above-mentioned objects are achieved by
the present invention which is directed to a thermoplastic
multi-layer packaging film comprising at least an outer polymer
layer, an inner heat sealing layer and optionally an oxygen barrier
layer between said inner and said outer layer, and which is
characterized in that the heat sealing layer comprises a copolymer
of ethylene and x to y% by weight, based on the copolymer, of an
.alpha.-olefin with 4 to 8 carbon atoms, whereby x=0.5n.sub.C +8
and y=1.25n.sub.C +15 will n.sub.C being the number of carbon atoms
in the .alpha.-olefin, said copolymer having a density of less than
920 kg/m.sup.3.
Preferably the heat sealing layer comprises a copolymer of ethylene
and 12 to 20% by weight, based on the copolymer, of an
.alpha.-olefin with 4 to 8 carbon atoms.
It has surprisingly been found that both the heat shrink properties
and the cold seal strength of the laminated packaging film depend
on the comonomer content of the ethylene copolymer used as the
sealing layer. In order to achieve optimal results the comonomer
content must increase when going from the C.sub.4 -.alpha.-olefin
(butene(1) to the C.sub.8 -.alpha.-olefin (octene(1). It is
particularly surprising that the films of the present invention
exhibit improved shrink properties both in irradiated
(cross-linked) and non-irradiated condition and that these
properties as well as the seal strength are a function of the
comonomer content of the linear low and very low density
"polyethylene".
When using a copolymer of ethylene and butene (1) the comonomer
content should be between 10 and 20% by weight, based on the
copolymer. Such copolymers have a density of less than 915
kg/m.sup.3. When employing a copolymer of ethylene and octene (1)
the comonomer content should preferably be increased to between 12
and 25% by weight, based on the copolymer. Such copolymers have a
density of less than 920 kg/m.sup.3.
According to the invention it is also possible to blend the
ethylene/.alpha.-olefin copolymer with up to 50% by weight, based
on the sealing layer composition, of a polymer which is compatible
with said ethylene/.alpha.-olefin copolymer. Such further polymer
may preferably be selected from the group comprising linear low
density polyethylene (LLDPE) with a density above 920 kg/m.sup.3,
linear high density polyethylene (LHDPE), low density polyethylene
(LDPE), ethylene vinyl acetate (EVA), acid modified EVA,
polypropylene, ethylene/propylene copolymer, ionomeric polymers and
ethylene/alkylacrylate copolymers wherein the alkyl moiety has 1 to
8 C atoms, in particular ethylenemethylacrylate (EMA),
ethylene-ethylacrylate (EEA) and ethylene-butylacrylate (EBA).
The ethylene/alkylacrylate copolymer which can be blended with the
ethylene/.alpha.-olefin in the heat sealing layer can comprise 3 to
30% by weight of alkylacrylate.
According to one embodiment of the invention the ethylene copolymer
forming the sealing layer may be cross-linked by irradiation in
order to improve its shrink characteristics and mechanical
properties. However, such cross-linking is not necessary since a
very satisfactory packaging film is also obtained without
irradiation. If polyvinylidene chloride is used as core layer
providing an oxygen barrier the irradiation should take place prior
to the lamination of the core layer to the sealing layer since PVDC
may suffer when exposed to high energy irradiation. The entire
multi-layer film may be irradiated if the PVDC core layer is
substituted by a core layer of partly hydrolyzed ethylene
vinyl-acetate copolymer (EVOH). PVDC and EVOH are the preferred
core layers since they show outstanding oxygen barrier
properties.
The films are normally oriented in longitudinal and transversal
direction to obtain the desired shrink properties. If a tubular
film is produced by extrusion the orientation may be effected using
the known air bubble method and further stretching the film by
pulling. Alternatively, orientation may be achieved by deep drawing
of a planar film.
The outer layer of the multi-layer packaging film of the invention
may be of the same ethylene copolymer as the sealing layer, i.e. an
ethylene/.alpha.-olefin copolymer comprising 10 to 25% by weight,
based on the copolymer, of an .alpha.-olefin with 4 to 8 carbon
atoms, said copolymer having a density of less than 920 kg/m.sup.3
ethylene vinyl acetate and polyamides, and ionomeric resins. It may
frequently be of advantage if an adhesive layer and/or an adhesion
promoting layer is interposed between said other layers.
If bags are made from the film material the film will normally be
folded so that the fold forms the bottom of the bag whereafter both
sides are sealed so that the side seals close both sides of the
bag. Alternatively a bag may be formed from a tubular film in that
the bottom of the bag is formed by a transverse seal across the
flattened tube and the mouth is formed by severing the tube at a
pre-selected distance from said transverse seal. Pouches can be
obtained by sealing two film sheets at three edges.
The invention will be further illustrated by reference to the
following examples without being limited thereto.
The following test methods were used to investigate the properties
of the packaging films and to compare them with other
materials,
A. VARIABLE PRESSURE HOT BURST TEST (VPHB)
This is a test to determine the seal quality of shrinkable
materials at different temperatures. The approach with this test is
to immerse total seal areas into hot water and after a
predetermined dwell time, the pressure inside the bag is increased
at a constant rate of approx. 25.4 mm of water per second until the
seal fails.
The mm of water pressure, at the level at which the seal fails, are
recorded, and there are minimum specifications expressed in mm for
each bag width. The highest is the number, the better is the seal
quality at that temperature.
B. PERCENT SHRINK
The percent shrink is measured immersing for a couple of seconds in
hot water a specimen and measuring the percent of retraction, both
in the longitudinal and transversal direction. The highest is the
percent of shrink, the better is the performance of the material in
terms of package presentation.
In practice, the maximum achievable percent of shrink is that
measured at the temperature at which the material meets the minimum
VPHB requirements expressed in mm of water pressure (see point A).
To predict the shrink behaviour it is therefore useful to plot in
the same diagram the % of shrink and the VPHB as a function of the
temperature.
C. COLD SEAL STRENGTH
The cold seal strength represents the seal quality at room
temperature and predicts the seal performance when the bag is
loaded with a product either manually or by means of automatic
loaders, which operate with a pusher, which pushes the product into
the bag. The cold seal strength is measured with a modified pusher
of a commercial loader, equipped with a system to record the
pressure necessary to open the bottom seal. The head of the pusher
operates on a 6 cm length of seal and the cold seal strength is
expressed in kg/6 cm. The maximum pressure recordable with this
systems is 40 kg/6 cm.
The reported results are the average of 10 measurements.
The following raw materials were used in the examples:
__________________________________________________________________________
RAW MATERIALS ABBREVIATION COMPOSITION MELTING POINT DENSITY
(g/cm.sup.3)
__________________________________________________________________________
E/15% B ethylene-butene copolym., 15% butene 118.degree. C. 0.906
E/8% B ethylene-butene copolym., 8% butene 121.5.degree. C. 0.918
E/6% VA ethylene-vinylacetate copolym., 6% VA 105.degree. C. 0.926
E/9% VA ethylene-vinylacetate cop., 9% VA 96.degree. C. 0.929 E/14%
VA ethylene-vinylacetate copolym., 14% VA 90.degree. C. 0.932 E/18%
VA ethylene-vinylacetate cop., 18% VA 91.degree. C. 0.940 Ionomer
(Na) Ionomeric resin (Na salt) 90-92.degree. C. 0.940 E/19% O
ethylene-octene copolym., 19% octene 124.degree. C. 0.911 E/13% O
ethylene-octene copolym., 13% octene 124.degree. C. 0.915 E/10.5% O
ethylene-octene copolym., 10.5% octene 123.degree. C. 0.920 Acid
modif. EVA acid modified Ethylene-Vinylacetate (55.degree. C.)
0.948 PA Polyamide (Nylon 6,12 copolymer) 140-147.degree. C. 1.06
EMA (20% MA) ethylene-methylacrylate copolym., 20% MA 85-90.degree.
C. 0.942 EBA (7% BA) ethylene-butylacrylate copolym. 7% BA
98-108.degree. C. 0.923 P/4.5% E propylene-ethylene copolym., 4.5%
E 135.degree. C. 0.900
__________________________________________________________________________
COMPARATIVE EXAMPLES 1 TO 4 AND EXAMPLES 1 TO 3
Multi-layer packaging films of the composition summarized below in
table I were prepared in the following manner. The first two layers
A and B were coextruded through a conventional tubular extruder to
form a tube. After leaving the die the substrate was cooled and
flattened. It was then sent through an irradiation vault where it
was irradiated by high energy electrons to a dosage of about 4.5
MR. Depending on the characteristics desired this dosage could vary
from 2 to 20 MR. After leaving the irradiation vault the substrate
was again inflated and sent through a first tubular extrusion
coating die where it received a coating of vinylidene chloride
copolymer. Thereafter the still inflated and now triple-walled film
passed through a second tubular extrusion coating die where it
received a layer of ethylene-vinyl acetate copolymer. After the
final coating the film was cooled, collapsed and rolled-up. This
tape was subsequently unrolled, fed through a bath of hot water,
held at a temperature of about 80.degree. to 98.degree. C., and as
it left the water it was inflated and blown into thin tubing with a
total wall thickness of about 59 microns. This so-called "trapped
bubble" technique is well known in the art. The film was then
rapidly cooled to set the orientation and then rolled up for
further processing.
The polyvinylidene chloride and the ethylene-vinylacetate copolymer
can alternatively be coated onto the substrate using a two-ply
coextrusion die.
Thee test results are summarized in the following table II.
TABLE I
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A B D F Exampe No. sealing layer .mu.m core layer .mu.m barrier
layer .mu.m outer layer .mu.m
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Comp. 1 E/9% VA 37 -- PVDC B E/9% VA 14 Comp. 2 Ionomer (Na) 15
E/9% VA 22 PVDC 8 E/9% VA 14 Comp. 3 E/8% B 15 E/9% VA 22 PVDC 8
E/9% VA 14 Comp. 4 E/10.5% O 15 E/9% VA 22 PVDC 8 E/9% VA 14 1
E/15% B 15 E/9% VA 22 PVDC 8 E/9% VA 14 2 E/13% O 15 E/9% VA 22
PVDC 8 E/9% VA 14 3 E/19% O 15 E/9% VA 22 PVDC 8 E/9% VA 14
__________________________________________________________________________
TABLE II
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Example Comparison 1 Comparison 2 Comparison 3 Comparison 4 Shrink
Shrink Shrink Shrink VPHB % VPHB % VPHB % VPHB % ##STR1## ##STR2##
##STR3## ##STR4## ##STR5## ##STR6## ##STR7## ##STR8## ##STR9##
__________________________________________________________________________
60.degree. C. 4 2 2 70.degree. C. 11 6 7 80.degree. C. 663 21 856
25 855 12 930 14 85.degree. C. 652 30 782 795 841 90.degree. C. 520
41 608 45 644 24 695 28 95.degree. C. 318 57 438 540 552
100.degree. C. 268 63 420 42 432 49 COLD 24,5 15,8 16,1 17,3 SEAL
STRENGTH (kg/6 cm) (10% no. breakage)
__________________________________________________________________________
Example Example 1 Example 2 Example 3 Shrink Shrink Shrink VPHB %
VPHB % VPHB % ##STR10## ##STR11## ##STR12## ##STR13## ##STR14##
##STR15## ##STR16##
__________________________________________________________________________
60.degree. C. 3 2 4 70.degree. C. 10 7 9 80.degree. C. 1040 27 1051
16 1074 25 85.degree. C. 940 928 990 90.degree. C. 783 48 745 33
802 49 95.degree. C. 647 558 621 100.degree. C. 398 64 429 53 363
67 COLD 22,5 19,4 23,5 SEAL STRENGTH (kg/6 cm)
__________________________________________________________________________
When comparing the results it should be borne in mind that the
shrink temperature should not exceed a value at which the hot-burst
pressure drops below 650 mm/H.sub.2 O. For some food products the
maximum shrink temperature may be more limited but a higher
hot-burst will then give additional safety. It is clear from the
data in table II that comparison 1 gives good cold seal strength
but the hot seal strength and the percent shrink leave something to
be desired. Further, this material results in poor seal strength
through contamination like gease or blood. Comparision 2 shows
improved shrink characteristics but the cold seal strength is
considerably lower and this material cannot be sealed reliably
through blood contamination.
Examples 1, 2 and 3 show that excellent shrink characteristics and
a very good seal strength both at room temperature and at elevated
temperatures are being obtained. It has further been found that the
seal strength is not significantly affected by contamination like
grease or blood. Comparisons 3 and 4 show that a very significant
decrease both in seal strength and percent shrink occur when the
comonomer content in the ethylene .alpha.-olefin copolymer drops
below a certain level, i.e. the minimum comonomer content is an
essential feature if the combination of desired properties is to be
achieved.
COMPARATIVE EXAMPLES 5 AND 6, EXAMPLES 4 TO 10
Multi-layer packaging films of the composition summarized in the
following table III were produced by conventional techniques, i.e.
by coextruding a tubular film of layers A to F and subsequent
stretching and orientation, e.g. in the blown-up bubble, so as to
obtain a heat shrinkable film. No irradiation was employed in this
case. The test results obtained are tabulated in the following
table IV.
TABLE III
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C D E F Example A B adhesion barrier adhesion outer No. sealing
layer .mu.m core layer .mu.m promotor .mu.m layer .mu.m promotor
.mu.m layer .mu.m
__________________________________________________________________________
Comp. 5 Ionamer (Na) 25 E/18% VA 9 + acid modif. 6 PVDC 8 acid
modif. 6 PA 23 E/14% VA 43 EVA EVA Comp. 6 E/8% B 25 E/18% VA 9 +
acid modif. 6 PVDC 8 acid modif. 6 PA 23 E/14% VA 43 EVA EVA 4
E/15% B 25 E/18% VA 9 + acid modif. 6 PVDC 8 acid modif. 6 PA 23
E/14% VA 43 EVA EVA 5 E/13% O 25 E/18% VA 9 + acid modif. 6 PVDC 8
acid modif. 6 PA 23 E/14% VA 43 EVA EVA 6 E/19% O 25 E/18% VA 9 +
acid modif. 6 PVDC 8 acid modif. 6 PA 23 E/14% VA 43 EVA EVA 7 75%
E/13% O + 25 E/18% VA 9 + acid modif. 6 PVDC 8 acid modif. 6 PA 23
25% EBA E/14% VA 43 EVA EVA 8 90% E/13% O + 25 E/18% VA 9 + acid
modif. 6 PVDC 8 acid modif. 6 PA 23 10% acid modif. E/14% VA 43 EVA
EVA EVA 9 50% E/13% O + 25 E/18% VA 9 + acid modif. 6 PVDC 8 acid
modif. 6 PA 23 50% E/8% VA E/14% VA 43 EVA EVA 10 50% F/15% B + 25
E/18% VA 9 + acid modif. 6 PVDC 8 acid modif. 6 PA 23 50% P/4.5% E
E/14% VA 43 EVA EVA
__________________________________________________________________________
TABLE IV
__________________________________________________________________________
Example Comparison 5 Comparison 6 Example 4 Example 5 Example 6
Shrink % Shrink % Shrink % Shrink % Shrink % ##STR17## ##STR18##
##STR19## ##STR20## ##STR21## ##STR22## ##STR23## ##STR24##
##STR25## ##STR26## ##STR27##
__________________________________________________________________________
60.degree. C. 15 4,5 13,5 5 9 70.degree. C. 25 9,5 21.5 11 19,5
80.degree. C. 993 37 951 15,6 1084 33,5 1043 19 1007 33 85.degree.
C. 877 892 753 90.degree. C. 617 43 857 30,6 835 45,0 710 36 560 43
95.degree. C. 381 534 431 100.degree. C. 319 46 599 42,0 546 55,0
436 51 338 51 COLD SEAL 17,6 25,6 32,7 29,6 28 STRENGTH (70% no.
(kg/6 cm) breakage)
__________________________________________________________________________
Example Example 7 Example 8 Example 9 Example 10 Shrink % Shrink %
Shrink % Shrink % ##STR28## ##STR29## ##STR30## ##STR31## ##STR32##
##STR33## ##STR34## ##STR35## ##STR36##
__________________________________________________________________________
60.degree. C. 5 6 10 4 70.degree. C. 12 13 16 10 80.degree. C. 1020
18 980 20 940 20 1150 18 85.degree. C. 850 780 720 810 90.degree.
C. 700 35 670 35 610 34 700 34 95.degree. C. 525 510 470 560
100.degree. C. 410 52 430 48 320 52 490 44 COLD SEAL 32,0 34 32 25
STRENGTH (kg/6 cm)
__________________________________________________________________________
The results are essentially similar to those obtained with the
irradiated material. Only the films of the present invention
combine excellent shrink characteristics with outstanding cold and
hot seal strength. In addition thereto they may be sealed without
impairing the seal strength through contamination like grease or
blood which are always encountered in the packaging of meat
products. Further, the non-irradiated structures show the same
effect, namely a strong dependency of the shrink and seal
characteristics on the comonomer content of the
ethylene/.alpha.-olefin copolymer.
* * * * *