The multi-layer polyolefin shrink film of this invention provides a unique combination of shrink tension, optical clarity, cuttability, sealability, shrink temperature range, and tear resistance which is highly desirable for many packaging applications. The preferred film has three layers in which the core layer comprises an ethylene-vinyl acetate copolymer blended with an ethylene-propylene copolymer and each surface layer comprises an ethylene-propylene copolymer.

FIELD OF THE INVENTION 
This invention relates to heat shrinkable, thermoplastic packaging films; 
and in particular, this invention relates to a multilayer, pololefin 
shrink film. 
BACKGROUND OF THE INVENTION 
The polyolefins and polyvinyl chlorides can be considered to be the two 
major families of plastic resins from which the bulk of commercially 
available shrink films for wrapping purposes are made. Other resin 
families from which shrink films can be made include the ionomers, 
polyesters, polystyrenes, and polyvinylidene chlorides. The shrinkable 
polyolefins currently on the market are mainly monolayer films which 
include both cross-linked and uncross-linked oriented polyethylene, 
oriented polypropylene, and oriented ethylene-propylene copolymers. The 
polyvinyl chloride (hereinafter "PVC") shrink films are monolayer films 
consisting of a variety of formulations of polyvinyl chloride. 
A shrink film's distinguishing characteristic is its ability upon exposure 
to some level of heat to shrink or, if restrained, to create shrink 
tension within the film. This ability is activated by the packager when 
the wrapped product is passed through a hot air or hot water shrink 
tunnel. This process causes the film to shrink around the product 
producing a tight, transparent wrapping that conforms to the countour of 
the product and which is aesthetically pleasing while providing the useful 
functions required of packaging materials such as protection of the 
product from loss of components, pilferage, or damage due to handling and 
shipment. Typical items wrapped in PVC or polyolefin shrink films are toys 
games, sporting goods, stationery, greeting cards, hardware and household 
products, office supplies and forms, foods, phonograph records, and 
industrial parts. 
The manufacture of shrink films requires sophisticated equipment including 
extrusion lines with "racking" capability, irradiation units when 
cross-linking is desired, tenter frames, mechanical centerfolders, and 
slitters. "Racking" or "tenter framing" are orientation processes which 
cause the material to be stretched in the cross or transverse direction 
and in the longitudinal or machine direction. The films are usually heated 
to their orientation temperature range which varies with the different 
polymers but is usually above room temperature and below the polymer's 
melting temperature. After being stretched, the film is rapidly cooled to 
quench it thus freezing the molecules of the film in their oriented state. 
Upon heating, the orientation stresses are relaxed and the film will begin 
to shrink back to its original, unoriented dimension. 
The PVC and polyolefin families of shrink films provide a wide range of 
physical and performance characteristics such as shrink force (the amount 
of force that a film exerts per unit area of its cross-section during 
shrinkage), the degree of free shrink (the reduction in surface area a 
material undergoes when unrestrained), tensile strength (the highest force 
that can be applied to a unit area of film before it begins to tear 
apart), sealability, shrink temperature curve (the relationship of shrink 
to temperature), tear initiation and resistance (the force at which a film 
will begin to tear and continue to tear), optics (gloss, haze and 
transparency of material), and dimensional stability (the ability of the 
film to retain its original dimensions under all types of storage 
conditions). Film characteristics play an important role in the selection 
of a particular film and may differ for each type of packaging application 
and for each packager. Consideration must be given to the product's size, 
weight, shape, rigidity, number of product components, other packaging 
materials which may be used along with the film, and the type of packaging 
equipment available. 
Polyolefins have been most successful with applications where moderate to 
high shrink tensions are preferred; and, on new, automatic, high speed 
shrink wrapping equipment where shrink and sealing temperature ranges are 
more closely controlled. The polyolefins tend to be cleaner leaving fewer 
deposits and less residue thereby extending the life of the equipment and 
reducing equipment maintenance. The PVC films generally have better 
optics, lower shrink tensions, and will seal and shrink over much broader 
temperature ranges than the polyolefins. Polyolefins usually do not emit 
gases upon sealing, and in this respect, are also cleaner than PVC films. 
Heretofore, polyolefins have not been able to penetrate PVC film packaging 
applications where the products to be packaged require the lower shrink 
tensions of the PVC film because the products are too fragile for use with 
polyolefins which possess shrink tensions up to four times those of the 
PVC films. PVC film is also the shrink film of choice for older, manually 
operated sealers and semi-automatic wrappers where temperatures are highly 
variable. Older poorly maintained wrapping equipment of any type usually 
runs PVC better than present monolayer polyolefins due to the combination 
of the generally broader shrink and sealing temperature ranges of the PVC 
films. In addition, products with sharp or pointed extensions will often 
require PVC as a wrapping film due to the high initial tear resistance of 
the PVC film relative to that of the polyolefins, i.e. it takes about 7 
grams of force to propagate a tear in PVC whereas only 2 to 3.5 grams of 
force are necessary to propagate a tear in a typical monolayer polyolefin 
shrink film. 
Accordingly, it is a general object of the present invention to provide a 
shrink polyolefin film that has many of the desirable qualities of PVC 
films and overcomes many of PVC's limitations. 
Specifically, it is an object of the present invention to provide a 
polyolefin film having shrink tensions approximating those of PVC films 
and also having good optical qualities, a wide shrink temperature range, 
sealability, and resistance to tear propagation. 
In addition, it is an object of the present invention to provide a 
polyolefin film which has none of the undesirable qualities of PVC films 
such as noxious odors and corrosive by-products. 
Furthermore, it is an object of this invention to produce a multi-layer 
film having very thin layers of oriented propylene homopolymers or 
copolymers. 
These and other objects are achieved by the multi-layer polyolefin shrink 
film which is disclosed herein. 
SUMMARY OF THE INVENTION 
It has been surprisingly discovered that a flexible thermoplastic packaging 
film having a unique combination of shrink tension, optical clarity, 
cuttability, sealability, shrink temperature range, and tear resistance 
heretofore unobtainable in a monolayer polyolefin film is achieved by the 
multi-layer, flexible, thermoplastic, packaging film of the present 
invention. This multi-layer film has a "core" layer that comprises an 
ethylene-vinyl acetate copolymer blended with an ethylenepropylene 
copolymer and a "surface" or "skin" layer, each surface layer comprising a 
copolymer of propylene. The multi-layer film is oriented so that it is 
heat shrinkable in at least one direction, the preferred stretching range 
ratios being from 4:1 (transverse).times.4:1 (longitudinal) to 7:1 
(transverse).times.7:1 (longitudinal). 
The vinyl acetate content of the ethylene-vinyl acetate copolymer in the 
core layer is preferably between 4% and 18% by weight and comprises about 
50 to 95% by weight of the blend. The core layer thickness is 80% to 30% 
of the total thickness of the multi-layer film whose total thickness 
ranges from 0.5 to 1.5 mils. 
The preferred material for the balance of the core blend and for the 
surface layer is ethylene-propylene copolymer with an ethylene content of 
1% to 6% by weight. 
DEFINITIONS 
Unless specifically set forth and defined or limited, the term "polymer" as 
used herein generally includes homopolymers, copolymers, terpolymers, 
block, graft polymers, random, and alternating polymers. 
The term "melt flow" as used herein or "melt flow index" is the amount, in 
grams, of a thermoplastic resin which can be forced through a given 
orifice under a specified pressure and temperature within 10 minutes as 
described in ASTM D 1238. 
The term "oriented" or "orientation" as used herein means the alignment of 
the molecules of a polymer predominately in a particular direction. 
"Orientation" is used interchangeably with "heat shrinkability" herein and 
designates a material which has been stretched and set at its stretched 
dimensions and which will tend to return to its original dimensions when 
heated to a specific temperature below its melting temperature range. 
The term "core" or "core layer" as used herein means a layer in a 
multi-layer film which is enclosed on both sides by additional or 
auxiliary layers. The core may be either "hot blown" or "oriented". 
The term "hot blown" as used herein means that the material referred to has 
been stretched at or above its melting temperature range so that the 
stretching has induced a minimum of stresses and molecular orientation. 
Such a material is not considered to be "heat shrinkable" as it will have 
very low shrink tension. 
"Skin" or "surface" layers are outer layers which are oriented and 
contribute to the shrink properties of the film. 
DISCLOSURE STATEMENT 
Closely related patents are listed and discussed briefly in the paragraphs 
which follow: 
(1) U.S. Pat. No. 3,381,717 issued on May 7, 1968 to Frederick S. Tyrrel 
and discloses a blown polypropylene tubular film wherein the core or 
center layer comprises an ethylene vinyl acetate copolymer and the outer 
layers constitute a block copolymer of propylene and butylene. U.S. Pat. 
No. 3,595,735 which issued on July 27, 1971 also to Frederick S. Tyrrel 
discloses a similar multi-layer structure but the outer layers constitute 
linear polyethylene. 
(2) U.S. Pat. No. 3,620,825 issued on Nov. 16, 1971 to Harold Lohman et al 
and discloses a biaxially oriented film of isotactic polypropylene with at 
least one surface coated with a blend of isotactic and non-isotactic 
polypropylene or a propylene-ethylene copolymer. 
(3) U.S. Pat. No. 3,817,821 which issued on June 18, 1974 to J. B. Gallini 
shows a three layer laminar, sealable, packaging film wherein the first 
layer is a blend of ethylene vinyl acetate copolymer with a second 
ethylene vinyl acetate copolymer or polybutene-1; the next or core layer 
consists of high density polyethylene; and the third layer is a blend of 
high density polyethylene and ethylene vinyl acetate copolymers. 
(4) U.S. Pat. No. 3,821,182 issued on June 28, 1974 to William G. Baird, 
Jr. and discloses a method of extruding a three ply material from a die 
wherein saran is the center layer and the outer layers are polyethylene. 
This three ply material may be irradiated, biaxially oriented, and then 
the outer polyethylene layers stripped away to provide a saran film with a 
smooth surface. 
(5) In tables entitled "Properties of specialty films" and "Shrink and 
stretch film properties" on Pages 37 and 39 of Modern Packaging 
Encyclopedia, December 1977 a coextruded ethylene-vinyl 
acetate/polypropylene laminate and a coextruded 
polyethylene/polypropylene/low density polyethylene/polypropylene/low 
density polyethylene laminate is disclosed as having been stretched but as 
having no heat shrinking characteristics. Shrink characteristics of 
monolayer polyolefins such as polyethylene, polypropylene, and 
ethylene-vinyl acetate copolymer are listed as well as the properties of 
polyvinyl chloride. 
(6) On page 38 of Modern Plastics Magazine for February 1981 a shrink film 
having layers of ethylene-propylene copolymer blended with polystyrene 
coextruded with ethylene-propylene copolymer layers is disclosed. 
(7) U.S. Pat. No. 4,194,039 issued to Walter B. Mueller on Mar. 18, 1980 
discloses a film structure in which skin layers comprise 
ethylene-propylene copolymer and the core layer comprises ethylene-vinyl 
acetate copolymer alone or with a blend of ethylene-butylene copolymer.

PREFERRED EMBODIMENT 
The preferred embodiment of the subject invention is a three layer, 
coextruded polyolefin packaging film having a core layer and skin or 
surface layers which can be illustrated simply and schematically as 
follows: 
skin/core/skin 
The preferred core layer comprises an ethylene-vinyl acetate copolymer 
(hereinafter designated "EVA") having a vinyl acetate content of about 12% 
by weight having a melt index of 0.3 blended with an ethylene-propylene 
copolymer (hereinafter designated "EP") having about 2.7 to 3.0% ethylene 
by weight with a melt index of 2.3. About 90% by weight of the blend is 
EVA and the remaining portion is EP. 
The skin layer may comprise the same EP copolymer as the core but slip and 
anti-block agents which are well known in the art can be added in minor 
amounts to enhance machineability and handling. 
The core blend of 90% EVA with 10% EP has been found to give the best 
combination of shrink properties, machineability and processability. If 
the EP content is dropped, particularly if it is less than 5%, the film 
becomes too soft to extrude and rack (see process description below) at 
commercially acceptable rates and thin films, especially, those films that 
approach total thicknesses of 0.5 mil or less cannot be produced for all 
practical purposes. On the other hand, if the core blend approaches and 
exceeds 50% EP, optical problems occur and the film loses its clarity. In 
addition, in preparing the film it has been found that a layer thickness 
ratio of 1/3/1 provides the most satisfactory combination of the 
properties of the core material with that of the skin layer for the core 
with 5% to 50% EP. 
In the preferred process for making the multi-layer, polyolefin shrink film 
of the present invention the basic steps are blending the polymers for the 
layers, coextruding the layers to form multi-layer film, and then 
stretching the film to biaxially orient it. 
The process begins by blending the raw materials or polymeric resins in the 
proportions desired, namely, for the core layer, 90% by weight of 
ethylene-vinyl acetate copolymer is blended with 10% by weight of 
ethylene-propylene copolymer. The resin is usually purchased from a 
supplier in pelletized form and can be blended in any one of a number of 
commercially available blenders as are well known in the art. In the 
blending process any additives necessary for special properties may be 
added such as plasticizers, slip agents, anti-block agents, or anti-static 
compound. 
The blended resins are fed into the hoppers of extruders which feed 
coextrusion dies. For the three layer film, three extruders are employed 
to feed the coextrusion die. Two extruders are fed ethylene-propylene 
copolymer for the two skin layers and the other extruder is fed the blend 
of ethylene-vinyl acetate copolymer with ethylene-propylene copolymer. 
Preferably the materials are coextruded as concentric tubing having a 
diameter which is dependent on the racking ratio and desired final 
diameter. This coextruded tube is relatively thick and is referred to as 
the "tape." Circular coextrusion dies are well known in the art and can be 
purchased from a number of manufacturers. In addition to tubular 
coextrusion, slot dies could be used to coextrude the material in sheet 
form; or single or multi-layer extrusion coating could be employed. 
Following coextrusion the extruded tape is heated and is continuously 
inflated by air pressure into a bubble thereby transforming the arrow tape 
with thick walls into wide tubing with thin walls of the desired film 
thickness. This process is sometimes referred to as the "trapped bubble 
technique" of orientation or as "racking." After stretching, the bubble is 
then deflated and the film is wound onto semifinished rolls called "mill 
rolls." The racking process orients the film, stretching it transversely 
and longitudinally thereby rearranging the molecules, to impart shrink 
capabilities to the film and to modify physical characteristics. In the 
present invention the racking temperature is above the melting temperature 
of the EVA in the core as the oriented layers are the ethylene-butylene 
copolymer layers which form the skin layers. Thus, in the racking process 
the core layer is hot stretched or hot blown and the skin layers are 
biaxially oriented. It is believed that the hot blown core layer provides 
a moderating or damping effect on the rather strong shrink properties of 
the ethylene-propylene layers. In addition, by this process 
ethylene-propylene copolymer layers that are very thin are oriented. 
It is desirable that the preferred embodiment be prepared by a coextrusion 
process as described above wherein the layers are melt joined without 
adhesive materials interposed between the layers. 
In Table I below properties of the preferred embodiment are listed for 
comparison with properties of similar films without the EP blended in the 
core layer and for comparison with PVC: 
TABLE I 
______________________________________ 
Preferred 
Embodi- EP/EVA/ EP/EVA/ 
Example ment EP EP PVC 
______________________________________ 
Layer ratio 
1/3/1 1/3.5/1 1/4/1 Mono- 
layer 
Tensile strength.sup.1 
.times. 100 (PSI) 
MD 92 90 103 113 
TD 70 30 98 121 
Tear Propagation 
(gms).sup.2 
MD 3.66 3.79 4.21 2.84 
TD 5.13 3.62 5.01 2.45 
Optics:.sup.3 
Haze (%) 2.9 1.8 1.8 2.0 
Gloss (%) 85 87 87 90 
Total 
Transmission (%) 
92.6 92.4 92.3 91.9 
Shrink 
Tension.sup.4 
215-285 215-315 270-365 160-250 
Range (PSI) 
##STR1## 
______________________________________ 
.sup.1 ASTM D882 
.sup.2 ASTM D1938 
.sup.3 ASTM D1003 
.sup.4 ASTM D2838 
In Table I "MD" is for machine direction and "TD" is for transverse 
direction, machine direction being the direction in which the material 
flows as it leaves the extruder. 
As can be seen the properties of the preferred embodiment compare well with 
those of PVC and the inclusion of EP in the core blend significantly and 
surprisingly improves properties. Most important, however, is the fact the 
EP is necessary for adequate processability and machineability. 
In addition to the three layer construction of the preferred embodiment, 
other multi-layer constructions may be extruded as schematically 
represented below: 
______________________________________ 
(a) Preferred 
Embodiment: EP/Blend/EP 
(three layers) 
(b) Five layers: 
EP/Blend/EP/Blend/EP 
(c) Seven layers: 
EP/Blend/EP/Blend/EP/Blend/EP 
______________________________________ 
In the representation above "Blend" is for the blend of EVA with EP. In the 
five and seven layer structures the layer ratios should remain close to 
the preferred or 1/3/1 or 3:2 for Total Blend to Total EP thicknesses, 
i.e., the core is about 60% of the Total thickness. However, this can vary 
from 80% to 30% of the total thickness within the scope of the invention, 
the core thickness being less as the proportion of EP in the core 
increases and the vinyl acetate content of the EVA increases. 
EP copolymer is required as the skin layer material rather than propylene 
homopolymer because the homopolymer has too high a melting point with too 
high a shrink temperature range to be a satisfactory material for the 
packaging applications for which the subject invention is useful. 
As the percentage of ethylene in the EP copolymer is increased, from 
2.7-3.0% to 3.5-4.0%, the proportion of EP in the blend may be increased 
with satisfactory results. Below 3.0% ethylene content EP and EVA exhibit 
incompatibility. At about 3.5% ethylene content in the EP miscibility 
appears to occur with a significant increase in optical properties. For 
example, a multi-layer film according to this invention having a core 
blend of 50% EVA and 50% EP with 3.5-4.0% ethylene content could be 
expected to have commercially acceptable optical properties. Thus, the use 
of EP having 3.5 to 4.0% ethylene in the core blend and the use of EP 
having 2.7% to 3.0% ethylene as the skin layer will produce an excellent 
multi-layer shrink film. Other desirable combinations can be projected 
within the scope of the invention.