Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE Il`lVENTION
1. Field of the Invention
This invention per~ains generally ~o multi-layer polymer
films and to methods for producing the same.
2. Description of the Prior ~rt
Polyolefin film such as polyethylene and polypropylene
are common packaging materials because of their relative low
cost, heat sealability, and moisture resistance. ~owever, these
polyolefins have a fairly high permeability to gases including
oxygen, so that, used alone, they are not adequate for packaging
oxygen sensitive Eoods and other materials that degrade in the
presence of oxygen or other atmospheric gases.
It is known that polymers and copolymers of vinyl alco-
hol, hereafter called vinyl alcohol films, have excellent resis-
tance to pérmeatian by gases, particularly to oxygen. Vinyl al-
coh~l films include polyvinyl alcohol and ethylene vinyl alcohol
copolymer. The desirable proper~ies of the vinyl alcohol films
degrade in the presence of moisture. Vinyl alcohol films also
tend to be structurally brittle, par~icularly when formed in re-
latively thin layers, i.e. in the range of 1 mil or less. Thus,
efforts have been made to laminate a vinyl alcohol web between
two outer webs of polyolefin which provide a moisture barrier and
structural support for the vinyl alcohol core. An example of
such a structure, a core layer of ethylene vinyl alcohol blended
wi~h another polymer and sandwiched between ~wo ou~er layers of
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polyolefin, is shown in the U.S. Patent to Nohara! et. al., No.
3,882,259. Since the oxygen ba~rier property of substantially
pure vinyl alcohol is superior to that of vinyl alcohol blends,
it is desirable to produce a film having a subs~antially pure
vinyl alcohol layer within polyolefin layers. However, substan-
tially pure ethylene vinyl alcohol and polyvinyl alcohol do not
bond well to many polymer films, particularly polyolefin films.
SUMMARY OF THE INVENTION
This invention provides a film structure in which a
substantially pure polyvinyl alcohol or ethylene vinyl alcohol
layer is joined with one or more polyolefin layers with adequate
adhesion to prevent delamination.
A multi-layer fllm in accordance with the invention in-
cludes a core layer of substantially pure vinyl alcohol polymer
or copolymer such as polyvinyl alcohol (PVOH) or ethylene vinyl
alcohol copolymer (EVOHj adhered to at least one layer of
chemically modified polyolefin having surprisingly strong
adhesion charac~eristics to the vinyl alcohol core layer. The
multi-layer film can be made by coextrusion. Because the core
layer is formed of substantially pure polyvinyl alcohol or
ethylene vinyl alcohol, the entire film may be made very thin
while still retaining high oxygen impermeability. The modified
polyolefin layer and the core may be coextruded with additional
layers of other polymers to provide an in~egrated film structure
having a composite of ~he characteristics of the various films in
the structure and which has sufficient adhesion between layers to
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resist delamination.
The modified polyolefins that have been found to have
suitable adhesion to the vinyl alcohol core layer are blends of a
polyolefin and a graft copolymer of high density polye~hylene
having an unsaturated fused ring carboxylic acid anhydride
grafted thereto. The polyolefin component of the blend may be
polyethylene or ethytlene copolymers such as ethylene vinyl ace-
tate. Such modified polyolefins have shown a marked ability to
adhere to vinyl alcohol materials, particularly tv ethylene vinyl
alcohol when coextruded therewith.
The atmospheric gas barrier layer is preferably ethylene
vinyl alcohol which is readily coextrusible with the modified
polyolefin layers and with additional layers. Ethylene vinyl
alcohol is approximately fifty times less permeable to oxygen
than extrusion grade saran (vinylidine chloride). Heretofor,
saran has been d~minant in the field v oxygen barrier materials
for films. The lower oxy~en permeability of EVOH allvws it tv be
used as a very thin layer. Polyvinyl alcohol also displays
superior barrier properties, but is difficult to coextrude
without the addition of water to the resin thereby at least
temporarily impairing its barrier properties. Polyvinyl alcohol
is better suited to laminated film structures wherein a polyvinyl
alcohol core web is ex~rusion coated or otherwise plied with the
adherent modified polyolefin layers.
- In a preferred process for the production of the multi-
layer film, resins of the modified polyolefin and the core bar-
rier material are melted and coextruded a$ joined concentric
tubes which are blown to the desired thickness. Addi~ional
layers of polymer material similarly may be c~extruded over the
modified polyolefin layers. Cast film coextrusion and water
quench coextrusion may also be utilized. All such coextrusion
processes yield a multi-layer film which has excellent adhesion
between the layers and high resistance to passage of gas and
moisture. Heat sealing of polyolefin outer layers is readily ob-
tained. Thus, the multi-layer film product is well adapted to
use in packaging food products as well as many non-food products
which require moisture and oxygen barrier packaging.
The coextruded multi-layer film can be processed further
in hydraulically lubrica~ed compression rolls to reduce the
thickness of the film, to improve its clarity, and to provide
improved physical properties by orientation.
Fur~her objects, features, and advantages of the inven-
tion will be apparent from the following detailed description
illustratiny preferred embodiments of the invention~
DESCRIPTION OF THE PREFERRED EMBOD-tMENTS
The multi-layer film of the invention includes a core of
substantially pure polyvinyl alcohol (PVOH), or ethylene vinyl
alcohol (EVOH). Excellent adhesion of the layers in the film is
obtained without the need to introduce bond promoting materials
into the core layer, and it is highly preferred that they ~e eli~
mina~ed from the core layer since introduction of such extraneous
materials may reduce the oxygen barrier properties of the core.
As a result, the core layer may be made quite thin, in the range
of 0.2 mil or less, while still obtaining adequa~e ox~gen barrier
properties.
Vinyl alcohol polymers and copolymers suitable for ex-
trusion are available commercially. Kuraray of Japan produces an
ethylene vinyl alcohol copolymer under the trade mark "EP-F" which
has 32 to 36% ethylene, a molecular weight of 29,500 and a melt-
ing point of 356~F. Nipon Gohsei of Japan produces ethylene vinyl
alcohol copolymers under the trade marks "GL-D" and "GL-E". GL-D
has about 29% ethylene, a molecular weight of ~2,000, and a melt-
ing point of 365F. GL-E has about 40% ethylene, a molecular
wieght of 26,000, and a melting point of 327F. The film struc-
tures reported in the tables which follow were made with these
commercial EVOH resins.
The modified polyolefins which form the layers adjacen~
the core layer are blends of polyolefin and a graft copolymer of
high density polyethylene (HDPE) with an unsaturated fused ring
carboxylic acid anhydride. The polyolefin component of the blend
may include a number of resins such as high, medium and low
density polyethylene (HDPE, MDPE, LDPE), and polyolefin copoly-
mers such as e-thylene vinyl acetate copolymer (EVA) and ethylene
acrylic acid (EAA). Modified polyolefin blends are disclosed in
U.S. Patents 4,087,5~7 and ~,087,588. Suitable modified polyole-
fin blends are available from the Chemplex Company of Rolling
Meadows, Illinois under the trade mar~ Plexar. Commercially
available grades of such modified polyole~ins are blends of the
graft copolymer with different polyolefins. These include Plexar-
I, a trade mark for a blend with ethylene vinyl acetate copoly-
mer; Plexar-II, a trade mark for a blend with high density poly-
ethylene; Plexar-II (a trade mark) further modified with an elas-
tomer such as polyisobutylene; and Plexar-III, a trade mark for
a blend with ethylene vinyl acetate polymer adapted for cast
film coextrusion. It has been found that these materials coex-
trude well with extrusion grades of ethvlene vinyl alcohol
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without need for adding m~terials to either the polyoleEin or
the core layers in order to obtain the desired level o~ adhesion
between layers.
.
Other materials used for layers overlyiny the modified
polyolefin layers, include by way of illustration, high, medium
and low density polyethylene, polypropylene, modified polyolefins,
polyolefin copolymers such as ethylene vinyl acetate copolymer
and ethylene acrylic acid copolymer blends two or more of the
foregoing polymers, nylon, and ionomers such as those sold under
t~e trade mark "Surlyn" by duPont. Five or more layers can be
coextruded to produce a film having a plurality oEdesired pro-
perties in a single pass.
Substantial improvements of a coextruded multi-layer
structure may be obtained by lubricated compression rolling of
the film. Such compression rolling of polymeric films is des-
cribed in U.S. Patents 3,504,075, 3~194,863 and Re. 27,~04. The
film resulting from the fluid compression roll technique is uni-
axially oriented and reduced in thickness, with improved clarity
or transparency and improved moisture barrier properties.
Examples of coextruded film structures in accordance
with the present invention which have been made and tested are
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reported in the following tables. Table I reports oxygen per-
meabi]ity ~or the various st~uctures. The permeability is ex-
pressed as milliIiters of oxygen which pass through a square
meter of the structure during a 24 hour period. The permea-
bilities are reported at different humidities. Table II reports
the adhesion between layers of various structures. The data was
obtained using an Ins~ron test device which reveals the force in
grams required to separate a 10 inch wide fil~ structure.
28
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In the tables "slip" means that a slip mat~rial such
as ~iO2 (Union Carbide 0291) has been added to the outer sur-
face layers to make the surface more slippery. Pigments such
as supplied under the trademark Ampacet 11078 have been added to
some layers. "DNS" means that -the sample did not separate in the
adhesion test.
Containers were made from certain of -the films of the
examples by heat sealing to form a pouch or by thermoforming to
make containers in which foods were packased and stored to eva-
luate the performance of the films. The containers were success-
ful
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