High barrier shrink film

There is provided a heat shrinkable, high barrier laminate film which includes a hot blown, melt oriented high oxygen barrier film, of relatively high crystallinity and correspondingly relatively low ultimate elongation, laminated to a stretch oriented base film having a shrink tension substantially greater than that of said barrier film, the extent of orientation of said base film being sufficient such that its ultimate elongation is reduced so as to be commensurate with that of said barrier film. In one preferred mode, the barrier film is a layflat tubular film having its interior layflat surfaces laminated together. Representatively, the high barrier layer of the barrier film is vinylidene chloride copolymer having at least about 85% by weight vinylidene chloride content, preferably at least about 90%. Associated methods for making the laminate films are also provided.

BACKGROUND OF THE INVENTION 
This invention relates generally to a laminate film suitable for packaging, 
especially food packaging, and relates more particularly to a heat 
shrinkable laminate film having especially high oxygen barrier properties. 
In the packaging of food in thermoplastic film, it is frequently desirable 
that the film provide an oxygen barrier, i.e. have a low permeability to 
oxygen. For example, film made of a copolymer of 65 to 95% by weight of 
vinylidene chloride and 5 to 35% of a vinyl comonomer such as vinyl 
chloride, acrylonitrile or methyl acrylate, generally referred to as 
saran, is known to give a good oxygen barrier. The term "high barrier 
vinylidene chloride copolymer" as used herein is intended to refer to a 
saran having at least about 85% by weight of vinylidene chloride. The 
oxygen barrier property of vinylidene chloride copolymers tends to 
increase with increasing content of vinylidene chloride. 
Additionally, it is frequently desired in the packaging of food that the 
packaging film be heat shrinkable so that the film may be shrunk snugly 
about a contained product thereby presenting an attractive package 
appearance. In the manufacture of heat shrinkable film, it is commercially 
advantageous to produce packaging film in tubular form, e.g. by the 
conventional blown bubble method, and then to stretch orient such film to 
render it heat shrinkable by the conventional trapped bubble method. In 
many cases, such film is irradiatively crosslinked before orientation to 
enhance orientability. Representatively, such processes are disclosed in 
U.S. Pat. No. 3,741,253 issued June 26, 1973 to Brax et al. 
The term "high barrier" film as conventionally used refers to an oxygen 
barrier thermoplastic material composed predominately of the barrier 
constituent to the extent that the material is brittle and relatively 
difficult to stretch orient. Generally, the higher the barrier 
constituent, the higher the barrier property, but the higher the degree of 
crystallinity which tends to embrittle the film to impart corresponding 
low impact strength. 
One approach to making a high barrier, heat shrinkable multilayer film has 
been to melt-form the composite film and then to stretch orient the film 
before crystallization of the high barrier material proceeds to an adverse 
extent. An alternative approach provides for lamination of 
stretch-oriented barrier film to a heat shrinkable, base film, i.e. the 
high barrier film is stretch oriented after melt forming and before 
crystallization proceeds to an adverse extent. Apparently, stretch 
orientation of nascent high barrier material reduces the size of 
crystalline regions of the microstructure so that the material does not 
become brittle. In this alternative approach, it would be desirable to 
eliminate the step of stretch orienting the nascent high barrier film. 
This is the problem with which the present invention is concerned. 
Of general interest are the disclosures of U.K. Pat. No. 1,591,423 for 
"Laminate Film of Polyethylene and Vinylidene Chloride Copolymer" and U.K. 
Pat. No. 1,591,424 for "Process of Heating Food in a Package of Polymeric 
Laminate Films" by Thompson published June 24, 1981, which disclose a 
laminate film formed by coating vinylidene chloride copolymer, other than 
by melt extrusion, onto the surfaces of a heat shrinkable film. 
SUMMARY OF THE INVENTION 
It is a primary object of the invention to provide a high oxygen barrier 
laminate film that is heat shrinkable in spite of the use of relatively 
brittle high barrier thermoplastic material. It is a secondary object to 
provide for redundant barrier plies in an exemplary embodiment. 
Accordingly, there is provided a heat shrinkable, high barrier laminate 
film which includes a hot blown, melt oriented high oxygen barrier film 
laminated to a stretch oriented base film having a heat shrink tension 
substantially greater than that of said barrier film, the ultimate 
elongation of said barrier film being substantially less than that of said 
base film before orientation and the extent of orientation of said base 
film being sufficient such that its ultimate elongation is reduced so as 
not to substantially exceed that of said barrier film, and preferably is 
commensurate therewith. In one preferred mode, the barrier film is a 
layflat tubular film having its interior layflat surfaces laminated 
together by self-welding. Preferably, the barrier ply of the barrier film 
is vinylidene chloride copolymer having a vinylidene chloride content of 
at least about 85% by weight, most preferably at least about 90%. 
Thus, by selective stretch orientation of the base film, trouble-some 
stretch orientation of the high barrier film is avoided. 
The term "melt oriented" is intended to refer to a relatively low extent of 
orientation that is imparted to a hot blown film incidental to the hot 
blowing formative process. The relatively low extent of orientation is 
manifested by a low heat shrink tension relative to that obtained with 
stretch orientation. 
The term "high barrier" is intended to refer to oxygen barrier 
thermoplastic copolymeric material composed predominately of the barrier 
constituent to such an extent that the material is brittle due to a high 
degree of crystallinity which is manifested by the material having low 
impact strength relative to that of the material when stretch oriented. 
Additionally, there is provided a method for making a heat shrinkable, high 
barrier laminate film which includes the steps of forming a hot blown, 
melt oriented high oxygen barrier film; stretch orienting a base film 
further provided that said barrier film has an ultimate elongation 
substantially less than that of said base film before orienting and said 
base film after orienting has a heat shrink tension substantially greater 
than that of said barrier film, the extent of orientation being sufficient 
such that its ultimate elongation is reduced so as not to substantially 
exceed that of said barrier film and preferably is commensurate therewith; 
and then laminating said barrier film to said base film. Preferably, the 
method further includes providing the barrier film as a layflat tubular 
film and laminating together its interior layflat surfaces by self-welding 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, there is shown a preferred high barrier, heat shrinkable film 10 
of the invention wherein a layflat tubular barrier film 11 is laminated to 
a heat shrinkable planar base film 12. It is a required feature of the 
invention that barrier film 11 be a hot blown film thereby being melt 
oriented, i.e. having low heat shrinking potential relative to that 
typical of stretch orientation as in base film 12. The layflat tubular 
film 11 is joined together at its internal layflat surfaces as indicated 
at interface 13. One external layflat surface is laminated to heat 
shrinkable component film 12 at interface 14. Optionally, the surface of 
heat shrinkable component film 12 that is joined to the layflat tube may 
be printed with labeling matter so that upon lamination a trapped label 
results. The multilayer structure of layflat film 11 includes an internal 
oxygen barrier layer of high barrier material, as further discussed below. 
The internal layflat surfaces of layflat film 11 are of a polymeric 
material of the type characterized as self-welding so that upon passing 
layflat tube 11 through a set of heated press rollers, self-welding of the 
internal layflat surfaces occurs to form heat sealed interface 13. The 
external surface 15 of layflat tube 11 is of a polymeric material that is 
rendered pressure sensitive adherable to compatible polymeric surfaces by 
electrostatic conditioning, such as conventional corona discharge 
treatment. A representative thickness of the tubular film is about one mil 
overall. A representative thickness of the heat shrinkable component film 
is about 0.5-0.75 mil. 
Heat shrinkable base film 12 provides a stretch-oriented heat shrinkable 
substrate upon which hot blown layflat tube 11 is laminated at interface 
14. Heat shrinkable film 12 has an interior layer that initiates heat 
shrinking and a surface layer at interface 14 that preferably is rendered 
pressure sensitive adherable to the outer surface 15 of layflat tube 11 by 
electrostatic conditioning. In a less preferred mode, lamination at 
interface 14 can be accomplished by application of a suitable conventional 
adhesive. In general, lamination at interface 14 is conducted without the 
application of heat so as not to cause any significant preshrinkage of 
heat shrinkable base film 12. The exposed surface 16 of base film 12 is of 
a polymeric material that is preferably heat sealable to the exterior 
surface 15 of layflat tube 11, so that the laminate film may be used in 
form/fill/seal applications wherein a longitudinal lap heat seal is made 
to define a product containing envelope within the enclosure of the film. 
Summarizing the surface requirements of the various components of laminate 
film 10, the internal layflat surfaces of the layflat tube are of a 
self-welding polymeric material, the external surface of the layflat tube 
is preferably responsive to corona discharge treatment and is 
heat-sealable to one surface of the heat shrinkable substrate, and the 
other surface of the heat shrinkable substrate is preferably responsive to 
corona discharge treatment. 
The laminate film 10 may be characterized as heat shrinkable since the 
stretch oriented base film 12 will initiate heat shrinkage of the laminate 
film upon elevation of the laminate film to the appropriate shrink 
temperature and provides substantial shrink tension. The hot blown high 
barrier layflat tubular film, having relatively low shrink tension, will 
follow the heat shrinkage of base film 12 in compatible fashion. It is a 
required feature of the invention that the base film be stretch-oriented 
to a selected extent such that its ultimate elongation property in the 
oriented condition is about equal to or less than the ultimate elongation 
property of the melt-oriented barrier film, and preferably is commensurate 
therewith. It is believed that this feature accounts for the 
nontransference of the relatively brittle nature of the high barrier film 
into the composite film. The term "ultimate elongation" is used in the 
conventional sense to refer to percent elongation at tensile failure. For 
example, a typical heat-shrinkable film of irradiatively crosslinked 
polyethylene being stretch oriented to an elongation of about 400-500% 
will have an ultimate elongation capacity of about 90%, which is 
commensurate with the ultimate elongation capacity of hot blown, melt 
oriented high barrier saran film. The shrink tension of the base film will 
be substantially greater than that of the barrier film and therefore will 
be shrink controlling. Thus, by selective stretch orientation of the base 
film, troublesome stretch orientation of the high barrier film is avoided. 
The laminate film 10 is characterized as high barrier film according to two 
aspects. First, a high oxygen barrier material, as defined above, is used 
such as vinylidene chloride copolymer having a relatively high vinylidene 
chloride content, representatively about 85% by weight or greater and 
preferably at least about 90%. Additionally, the multilayer structure of 
layflat tube 11 has an internal oxygen barrier layer so that upon 
collapsing tube 11 to the layflat configuration two separate spaced apart 
barrier layers will result in the layflat configuration, one barrier layer 
appearing above interface 13 at the self-welded internal layflat surfaces 
and one barrier layer appearing below interface 13, in the regions 
indicated by 17 and 18 respectively. The two separate spaced apart barrier 
layers provide redundant barriers against minor manufacturing defects such 
as pin holes, carbon particle inclusions, and the like as may randomly 
occur in a large scale manufacturing process. 
In a less preferred embodiment of the invention, the barrier film may be a 
planar film similar in all respects to the foregoing except that, for 
example, the tubular barrier film 11 is slit into two mirror image films, 
each of which are laminated into a composite structure as above. Of 
course, in this configuration redundant barrier plies will not be present 
since, in effect, only one side of the slit tubular film is being used. 
In FIG. 1A, there is shown an alternative embodiment relative to that of 
FIG. 1 being similar in all respects but having a second heat shrinkable 
base film 12a adhered to the other of the exterior layflat surfaces of 
layflat tube 11. Base films 12 and 12a are identical in terms of 
multilayer structure and are arranged so that they sandwich layflat tube 
11 and so that their multilayered structures are in mirror image symmetry 
about layflat tube 11. Interfaces 14 and 14a are preferably responsive to 
corona discharge treatment thereby providing the means by which heat 
shrinkable films 12 and 12a are laminated to the exterior layflat surfaces 
of layflat tube 11, respectively. Further, the exposed surfaces of 
shrinkable films 12 and 12a as indicated at 16 and 16a are self-welding to 
facilitate use of the laminate film in form/fill/seal applications as 
discussed above. 
In FIG. 2, there is shown an exploded cross-section through the foregoing 
embodiments of the multiply structure 20 making up the laminate film. The 
layflat tube 11 has a mirror image multiply structure about interface 13, 
shown in exploded configuration, which is the interface between the 
interior layflat surfaces of layflat tube 11. Layers 21 and 21a are 
interior tacky coatings that promote self-welding of the interior layflat 
surfaces of tube 11. A preferred composition is EVA (ethylenevinyl acetate 
copolymer) having 10-40% VA (vinyl acetate) content by weight. Layers 22 
and 22a are structural layers and preferably are composed of polyethylene, 
most preferably low density or linear low density. Optionally, in layflat 
tubular film 11 layers 21 and 21a may be deleted with the further 
requirement that layers 22 and 22a be of a self-welding polymeric material 
such as polyethylene. Layers 24 and 24a are barrier layers of a high 
oxygen barrier material such as VDC (vinylidene chloride) copolymer having 
a relatively high VDC content, representatively at least about 85% by 
weight, most preferably about 90%. As discussed above, the invention 
provides for imparting substantial heat shrinkable properties to an 
otherwise relatively brittle high barrier film. Layers 23 and 23a are 
adhesive interlayers that tie the barrier layers to adjacent layers, 
respectively. A preferred adhesive is EVA having 5-40% VA content or 
ethylene-methacrylate copolymer. Layers 25 and 25a form the exterior 
layflat surfaces of layflat tube 11 and preferably are composed of 
polyethylene or EVA having a VA content in the range of about 4-10%. 
Optionally, a conventional opacifying pigment may be added to this layer. 
Heat shrinkable base film 12 is joined to one face of the layflat tube at 
interface 14. Thus, layer 26 is preferably of a material that is rendered 
pressure sensitive adherable to layer 25 by corona conditioning and 
preferably is composed of EVA having 4-10% VA content. Layer 27 is a 
selectively oriented shrink controlling layer and is preferably 
polyethylene, most preferably low or medium density, that has been 
cross-linked with ionizing radiation prior to orientation. Exposed surface 
layer 28 of heat shrinkable base film 12 is heat sealable to layer 25a at 
surface 15 of layflat tube 11. Preferably, layer 28 is preferably composed 
of the same polymeric material as layer 26. Optionally, layers 26 and 28 
may be deleted so that component film 12 is composed simply of a single 
layer of heat shrinkable material 27, such as polyethylene, so long as 
film 12 is adherable to the layflat tube at interface 14 and is heat 
sealable to the exterior surface of the layflat tube at surface 15. The 
discussion of FIG. 2 thus far describes the multilayer structure making up 
the embodiment shown in FIG. 1. 
The multilayer structure making up the embodiment of FIG. 1A further 
includes a second heat shrinkable film 12a which is substantially 
identical to heat shrinkable film 12 and is joined in the structure at 
surface 15 and is disposed so that film 12a lies in mirror image symmetry 
relative to film 12 insofar as layer 26a corresponds to 26, layer 27a 
corresponds to 27, and layer 28a corresponds to 28. The basic multiply 
structure of the embodiment of FIG. 1A may be represented in minimal terms 
by the notation S//C/B/A//A/B/C//S wherein A is the self-welding interior 
surface layer of the layflat tube, B is an intermediate barrier layer, C 
is a heat sealable and corona treatable exterior layer of the layflat 
tube, and S is a single ply heat shrinkable planar film. 
Key properties of the material of the invention are determinable by the 
widely accepted testing procedures of ASTM. Specifically, the orientation 
and shrink tension properties are determinable by ASTM D2838. The ultimate 
elongation property is determinable by ASTM D882. The oxygen barrier 
property is determinable by ASTM D3985. The impact strength property is 
determinable by ASTM D3420. The extent of cross-linking property is 
determinable by ASTM D2765. 
In FIG. 3, there is depicted a preferred process line 30 for making the 
embodiment of FIG. 1. The process begins with the provision of roll 31 of 
layflat tubular film of the type described above. The layflat film is fed 
out at 32 and passes through the nip of heated press rollers 33, the 
heated pressing causing self-welding of the interior layflat surfaces of 
the layflat tube. Optionally, the starting material may be provided 
already in this condition when the hot blowing formative film process is 
conducted as part of the overall process by passing the nascent hot blown 
film through nip rollers while the film is still hot. One exterior surface 
of the layflat tube is then treated by corona discharge with conventional 
corona discharge unit 34 to render one face of the layflat tube pressure 
sensitive adherable. Roll 35 provides a supply of heat shrinkable base 
film which is fed out at 36 to conventional corona discharge unit 37 which 
treats one face of the heat shrinkable film to render it pressure 
sensitive adherable. The two films are then merged in the nip of press 
rollers 38 with their corona treated surfaces contacting one another so 
that at 39 a laminated film results of the type shown in FIG. 1. Takeup 
roll 40 then accumulates the laminate film. The embodiment shown in FIG. 
1A may be made in similar fashion except that both exterior layflat 
surfaces of the layflat tube are corona treated and components 35 through 
37 are duplicated on the other side of the process line to sandwich the 
layflat tube between mirror image heat shrinkable films, the sandwiching 
being completed in the nip of rollers 38. 
In use, the laminate film of the invention may be incorporated in chub 
packaging of sausage. Chub packaging is essentially conventional 
form/fill/seal packaging wherein film is first formed into a tube with a 
longitudinal overlap seal made to close the side of the tube. One end of 
the formed tube is then clipped, a flowable meat product is then stuffed 
into the tube, and the remaining end of the tube is clipped thereby 
completing the food log. Then the chub package is heat shrunk by immersion 
in a hot water bath at a temperature within the shrink temperature range 
of the film, whereupon the film shrinks snugly about the contained food 
product. 
Although the present invention has been described in conjunction with 
preferred embodiments, it is to be understood that modifications and 
variations may be utilized without departing from the principles and scope 
of the invention, as those skilled in the art will readily understand. 
Accordingly, such modifications and variations may be practiced within the 
scope of the following claims.