Heat shrinkable laminated film and method for its production

(74) Agent

A / S Oslo Patent Office Dr.ing. KO Berg, Oslo.

(56) Cited publications

No.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for producing heat shrinkable packaging film by extruding a film of hydrolyzed ethylene-vinyl acetate copolymer which, prior to hydrolysis, has at least 35 mole percent units of vinyl acetate and heats and stretches the film to make it heat shrinkable. low oxygen permeability and is particularly suitable for food packaging including meat and cheese.

Thermoplastic packaging films can be grouped into two major categories, one being rigid or semi-rigid films and the other being flexible films. The semi-rigid and rigid films are quite useful for applications that require a heat forming operation to mold the film by heat and usually a vacuum in a solid, self-supporting form. Such films are often used in bladder packaging, leather packaging and for making shallow boxes. In contrast, a flexible film is only wrapped around the article to be packaged. A very common way to do this is to make a film that is shrinkable by heating, wrapping the item in the film and heating the wrapped item so that the film shrinks on it. During this operation, the film becomes tighter and fits tightly to the shape of the object.

Heat shrinkable film with low oxygen permeability should be very durable, and capable of heat shrinkage sufficiently at sufficiently low temperatures. Films meeting these requirements have been prepared by a laminate of an oxygen barrier layer consisting of a copolymer of vinylidene chloride with vinyl chloride interposed between two other polymeric layers, namely a layer or a cross-linked ethylene vinyl acetate copolymer and a layer of common, non-crosslinked ethylene vinyl ethylene copolymer.

Such a laminate film is disclosed in US Patent No. 3,741,253.

Although these laminate films with a vinylidene chloride copolymer layer really provide satisfactory oxygen permeability and shrinkage, the use of vinylidene chloride polymer has certain disadvantages well known by those working in this field of technology.

Such a disadvantage is that it has been found necessary to include a plastic plasticizer and stabilizer in the vinylidene chloride polymer. Although the iridescence serves its purpose in making the film more workable, they have the undesirable characteristic of increasing the permeability of the film. The use of stabilizers necessarily increases costs and complicates manufacturing. Accordingly, it is an object of the present invention to provide a wettable shrinkable laminate film having low oxygen permeability using another oxygen barrier material which does not require use. of a plastic softener or stabilizer, yet heat shrinkable to a reasonable extent and at reasonably low temperatures.

DE-AS 1,127,077 discloses a process for preparing a heat shrinkable film by heating and stretching a non-plasticizing copolymer with from 95 to 30% by weight of ethylene and 5 to 70% by weight of vinyl acetate, under which the copolymer may be fully or partially hydrolyzed. However, it does not disclose a method of incorporating such a film into a laminate with films of other polymers to provide a stronger packaging film.

A hydrolyzed ethylene vinyl acetate copolymer, hereinafter referred to as HEVA for short-term sake, could be incorporated into a laminate by the method of U.S. Patent 3,741,253 in which a substrate of non-hydrolyzed ethylene vinyl acetate copolymer is irradiated to cross-link it and then coated with an oxygen barrier layer and then with a second layer. In this process, only the substrate layer is irradiated to avoid degradation of the barrier layer of the vinylidene chloride polymer. This often causes discoloration which makes the appearance of the packaging film unacceptable.

Through the present invention, it has now been found that with an HEVA oxygen barrier layer, the entire laminate can be irradiated, ie all layers simultaneously without breaking down HEVA. This method has several advantages.

Thus, the laminate can be prepared by co-extrusion (simultaneous extrusion of the layers from openings and bringing the layers together downwards from the openings in a constantly soft state). Furthermore, it appears that the irradiation of the entire laminate strengthens it more than similar irradiation of the substrate only.

The present invention provides a process of the kind mentioned above in which a laminate of an oxygen barrier layer of the hydrolyzed ethylene vinyl acetate copolymer arranged between two other polymer layers comprises at least one of which comprises a polymer which is crosslinkable by ionizing irradiator. to crosslink this polymer and heat and stretch the irradiated laminate. An appropriate orientation temperature for olefin polymers will be in the range of 82-121^uC. In practice, the tube will normally be stretched at least 10% in one direction to provide the resulting film with commercially exploitable heat shrinkage capabilities.

A heat shrinkable film is made by a method in which a film is stretched to provide a relatively thin, flexible oriented film. The oriented molecules have an inherent molecular stress which is retained when the heated and stretched film is cooled and which is eliminated only by reheating the film. Releasing the tension manifests itself as shrinking the film.

To form an effective oxygen barrier layer, the hydrolyzed ethylene vinyl acetate copolymer will generally contain at least 35 mole percent vinyl acetate-derived hydrolysis units. That is, the hydrolyzed polymer will generally have at least 35 mole percent vinyl acetate units in hydrolyzed or. non-hydrolyzed form. The hydrolyzed form is generally considered to be vinyl alcohol. Since the hydrolyzed polymer preferably has a degree of hydrolysis of at least 50% and most preferably at least 9%, the polymer is different in character from the non-hydrolyzed ethylene vinyl acetate copolymer, which can be used in other layers of the laminate.

Using HEVA as the oxygen barrier layer of the present invention, it is readily possible to make heat shrinkable laminates with an oxygen permeability rate less than 30, frequently

2 less than 5 and sometimes less than 2.0 cm / m. 24 hours.

atmosphere, measured at 23 ° C. These throughput rates can be measured by ASTM method D1434. In general, the characteristics of these hypoxia barriers are substantially equal to or better than those obtained from comparable laminates with a vinylidene chloride polymer barrier layer.

The preferred films which can be prepared according to the invention have at least three layers, wherein the oxygen barrier layer consisting of HEVA is placed between, but not necessarily in direct contact with two polymer layers, at least one consisting of a cross-linked polymer, preferably a cross-linked olefin polymer. A preferred feature of the invention is that both of these two polymeric layers are cross-linked and preferably are of the same cross-linked olefin polymer, e.g. both of polyethylene or non-hydrolyzed ethylene vinyl acetate copolymer. Further preferably, all the layers of the laminate are crosslinked and, as explained later, a particular feature of the use of HEVA as an oxygen barrier material is that it is possible to produce such a laminate in which all the layers are crosslinked. The two polymeric layers are preferably surface or outer layers of the laminate and in their rudder form they will be the radially inner and outermost layers.

The term olefin polymer as used herein includes not only homopolymers, but copolymers comprising block and grafted, interpolymers or terpolymers.

The term olefin polymer as used herein does not only comprise a polymer of an unsaturated hydrocarbon of the general formula ^Cn^H2n (where n represents an integer) but copolymers of olefins with other monomers such as ethylene with vinyl acetate. The olefin is not necessarily an alpha-olefin although this is a preferred group. When the term ethylene vinyl acetate copolymer is used in this specification without indicating that it is hydrolyzed, it is meant a non-hydrolyzed polymer.

The process of producing a heat shrinkable packaging film according to the invention with a low oxygen permeability is carried out by:

(a) forming a laminate consisting of an oxygen barrier layer of a hydrolyzed ethylene vinyl acetate copolymer which, for hydrolysis, has at least 35 mole percent units of vinyl acetate derived between two polymer layers, at least one of which is a cross-linked polymer;

(b) heating the laminate structure to the orientation temperature of said cross-linked polymer; and (c) stretching the heated laminate to orient the cross-linked polymer, thereby producing a heat shrunk film.

The polymeric material of the third layer, i.e., the polymeric material which may be but not necessarily cross-linked, is generally one which is bizarre at a temperature at which the cross-linked polymer in the first layer is orientable.

In a first and preferred embodiment of the invention, step (a) of the above method is carried out by forming a laminate consisting of an oxygen barrier layer of a hydrolyzed ethylene vinyl acetate copolymer. wherein the copolymer is as indicated above between two other polymer layers, at least one of which consists of a polymer which is cross-linkable by the action of ionizing radiation, and irradiating the laminate with this radiation to cross-link this polymer.

In this embodiment, at least two layers of the laminate (three if the third layer is also of crosslinkable polymer) are crosslinked, thereby improving the strength of the laminate. The film laminate with a vinyl chloride copolymer layer has been irradiated, but such irradiation is not generally carried out, as the vinylidene chloride copolymer is sometimes degraded and discolored upon exposure to excessive radiation doses. Thus, it has been customary in the past to produce:

and irradiating the substrate material for the application of the vinylidene chloride polymer coating, with the result that only this layer of the laminate is crosslinked. It is a valuable advantage of the present invention that it is possible in this embodiment to produce a laminate having an oxygen barrier layer when more than one layer is cross-linked.

In the preferred method, three annular layers are juxtaposed together of polymeric material to form a round multilayer structure.

It is preferred that HEVA be hydrolyzed to at least 50%, but the most preferred degree of hydrolysis is greater than 99%. Also, mole percent of vinyl acetate-derived units for hydrolysis should be at least 35%, since it has been found that if it is lower than 35%, the hydrolyzed copolymer is not an effective oxygen barrier.

Preferred polymers that can be crosslinked by irradiation are olefin polymers, in particular, polyethylene and ethylene vinyl acetate copolymers.

When ethylene vinyl acetate copolymer is used as a crosslinkable copolymer in the present invention, the vinyl acetate unit content of the copolymer, if mixed, may be up to about 15% by weight, but the preferred range is from 3 to 12% by weight. Ethylene vinyl acetate copolymer can be mixed with polyethylene to give an effective vinyl acetate content as desired.

Preferably, both of these polymeric layers between which the HEVA layer is applied are made of polyethylene or an ethylene vinyl acetate copolymer.

Since in the preferred method both are cross-linked, they would have a similar orientation, which has well-known advantages.

In addition to the olefin polymers, examples of other polymeric materials which may be included as layers in the laminate film made herein include polyvinyl chloride, polyamides, ionomers, acrylic polymers, polyesters, polycarbonates, polystyrenes. vinylidene chloride polymers and copolymers of their monomers.

After the tubular laminate is made, it gets the dress, and then flatleages. The flattened tube structure is sometimes called a band, and in its flattened state will usually be from

2.5 - 25 cm wide depending on the desired end width of the film.

The preferred method of cross-linking is by irradiating the flattened tubular laminate.

The term irradiation here generally means the influence of ionizing radiation such as X-rays, gamma rays, and electrons that directly induce molecular cross-linking. (However, used in conjunction with chemical cross-linking reagents distributed within a material, both heat and light can be considered as forms of radiant energy that induce cross-linking). Electrons are the preferred form of radiant energy and are preferably produced by commercially available accelerators in the range 0.5 to 2.0 MEV. (million electron volts).

Accordingly, in the preferred process, the flattened tape is irradiated by lining through an electron beam emanating from an electron accelerator. In a typical accelerator, the jet will spread across the rudder and the rudder will be fed back and forth through the jet until the desired radiation dosage is achieved. The electrons will generally be in the energy range 0.5 - 2.0 MEV (million electron volts) and it has been found that for the present invention, the preferred dosage level is 2.0 - 12.0 megarads (MRI). Radiation dosages are referred to herein by the term radiation unit, row, where 1 millinoder of rads or a megarad is referred to as MR. The degree of molecular cross-linking is usually expressed by radiation dosage comprising the degree of cross-linking. Of course, any ionization radiation that will induce any cross-linking between the long molecular chains of olefin polymers is suitable.

The crosslinking step is crucial as it has been found that when the radiation dosage is less than about 3.0 MRI for the preferred ethylene vinyl acetate copolymer, the helm cannot be properly oriented by the bubble technique because there is insufficient strength in the structure to hold a bubble without smoking. The required dosage to adequately strengthen the multilayer structure will vary depending on the molecular weight, density and constituents of the crosslinkable material and will be as low as 2.0 MRI for some structures such as polyethylene. On the other hand, at the dosage level greater than 12 MRI most ethylene vinyl acetate copolymers are crosslinked to such a degree that they become rigid and difficult to work with. For most purposes, it has therefore been found that the optimal dosage level is between 4 and 8 MR ·. After the irradiation step, a laminate consisting of a HEVA layer between two other polymeric layers, at least one of which consists of a cross-linked material, has been formed.

The stretching step (c) is preferably carried out by inflating the heated laminate in its rudder shape as opposed to its surface shape to a sufficient degree to provide biaxial orientation.

The tube is then heated and inflated to a temperature range above the temperature at which the first crystallizers begin to melt, but below the crystalline melting point of the cross-linked, layer or layers, respectively. This range is the orientation temperature range where the material has elasticity, but the molecules within the material will tend to orient in the direction the material is stretched. A preferred medium for heating the flat tape is water at or near the boiling point when the cross-linked material is an olefin polymer. It is well known that oriented thermoplastic materials will generally shrink at or near the temperature at which they were oriented. Since water is readily available and is a relatively safe and effective heat transfer medium, it is commercially advantageous to prepare a film structure which will be heat shrinkable in hot or boiling water. The preferred film of the present invention has this desirable property.

After the flat rudder has expanded into a bubble, it boils and can be flattened and won for storage. A description of the orientation of the bubble technique can be found in e.g. U.S. Patent No. 3,022,543. Depending on the desired orientation, film width and film thickness, rudders that have been expanded from a 2.5 - 25 cm bandwidth will have an expanded bandwidth in the range of 8.9 - 91 cm.

These widths are, of course, merely illustrative.

By regulating the orientation temperature and the pressure used to inflate the rudder, the degree of orientation or distance can be controlled as desired.

The flat expanded tube can be split longitudinally and wound onto a roll, or they can be heat-sealed transversely and then spaced longitudinally or transversely to provide bags on request. Another advantage of the preferred laminate according to the invention wherein the two surface or outer layers of the laminate are of cross-linked ethylene vinyl acetate copolymer, is that it is heat sealable and allows the manufacture of bags from the rudder or to make liquids by sealing film surfaces together. It also has the commercially desirable properties of being heat sealable and heat shrinkable in hot or boiling water. Tear strength and impact strength are also excellent, making the preferred laminate films suitable for packaging bone with a tendency to cut or tear the film.

Furthermore, the films made according to the present invention exhibit excellent resistance to lamination and since the present invention does not limit itself to any particular theory to account for this effect, it is believed that by irradiating the laminated structure some degree of cross-linking between the surfaces of the various layers. The molecules of these layers in the present film are known for cross-linking, and especially when the layers have been fused in the co-extrusion process, there is a certain degree of mixing of material. _ved the layers over.

surfaces. Thus, it can theoretically be assumed that the molecules from a layer are cross-linked with those in a neighboring layer. In addition, irradiation of the entire multilayer structure produces a single substrate layer, which, as in previous practice, produces smoother characteristics throughout all the layers and improves the bonding between the layers.

A frequently encountered problem in previous processes for making heat-shrinkable laminate films is that even with the utmost care, small amounts of air can be trapped between the layers; And when the film is stretched for orientation, these small air bubbles are also stretched so that the area they cover increases. These bubbles reduce the appearance of the film and provide starting points from which separation of the layers can occur. It is a particularly valuable advantage of the present invention that the use of HEVA as an oxygen barrier layer material allows one to co-extrude the layers of the laminate. It has been found that by co-extrusion, trapping of air between the layers is either eliminated or minimized. Thus, the laminate film is preferably prepared as by extrusion. Of course, this technique is only applicable in the embodiments in which the crosslinking is to be carried out on a pre-formed laminate, as opposed to crosslinking the substrate and then coating it with other layers. The term co-extrusion used herein means a single extrusion process combining two or more extrudable materials, e.g. thermoplastic resins, waxes or adhesives in a molten state to a self-supporting composite film. The co-extrusion comprises co-extrusion of at least two extrudable materials, either as flat streams or as coaxial circular streams. Each stream consisting of a separate extrudable polymer will come from a different extruder and it is preferred in the present invention to use the co-extrusion process in which the layers are co-extruded annularly. A co-extrusion matrix capable of performing such a process is disclosed in U.S. Patent No. 3,802,826. However, the flat film co-extrusion technique, e.g. as shown in

US Patent No. 3,865,665 is used.

In a preferred co-extrusion method, the extrudate streams are joined on or for the matrix lip, thereby eliminating air that could otherwise be trapped between the layers. In addition, co-extrusion of the HEVA layers between the two adjacent layers keeps moisture away from

The HEVA layer, whose oxygen permeability is adversely affected by moisture.

For co-extrusion coatings, wherein two or more layers of cow^_is extruded onto the crosslinked substrate, the substrate is passed through a co-extrusion matrix and the layer applied to the substrate directly is the HEVA layer and the outermost layer is an olefin polymer layer.

If desired, three, four, five or more layers can be added by co-extrusion onto a substrate and the substrate may have one or more layers. In addition, subsequent co-extrusion coatings can be used to build the desired multilayer structure and additional HEVA layers are used in the structure. A rudder-forming laminate produced in this way can then be oriented by the blasting technique as before.

Although the preferred methods of preparing the multilayer laminate of the present invention include the construction of a multilayered, cylindrical, multilayered structure, it is also within the scope of the present invention to extrude the polymeric and HEVA layers from slit matrices either by co-extruding three or more layers. or by extrusion coating one layer on another. After crosslinking such a multilayer structure, preferably by irradiation, the structure is heated to its orientation temperature and then stretched by the well-known stretching frame technique to orient it.

The following examples illustrate the invention.

Example 1

A 10.2 cm wide laminated band consisting of the following layers was prepared by the co-extrusion process described above with concentric annular matrices: 356 micron thick layer of ethylene vinyl acetate copolymer / 51 micron thick layer of HEVA / 114 micron thick layer of ethylene vinyl copolymer. The aforementioned layer is that which becomes the radially innermost layer of the extruded rudder, and the same designation applies in the other examples hereinafter. The content of vinyl acetate-derived units in the (non-hydrolyzed) ethylene vinyl acetate copolymer in this laminate was from 3.5 to 5% by weight and its melt flow index was 0.5 determined by ASTM method D 1238, condition E. HEVA was the product of a 99% hydrolysis of an ethylene vinyl acetate copolymer with 69 mol% vinyl acetate-derived units. It had melt flow index of around 6.0 determined by ASTM method D 1238 condition L.

The temperature of all three extrudates as they left the matrix openings was around 218 ° C. The tape was cooled and rolled a width of 10 cm. This band was rapidly passed twice through the scattered beam of a 0.5 MEV isolated nuclear transformer electron accelerator where it received a total dose of 6.5 MR. After irradiation, the rolled strip was rapidly passed through boiling water, inflated to a bubble and oriented to form rudders with a 41 cm diameter and 58 microns thick, which was laid flat. The bubble was extremely stable and the look of the film was good. The resulting film had a 22% free shrinkage in the longitudinal direction and a 32% fresh shrinkage in the transverse direction at 91 ° C. Free shrinkage is determined by ASTM Method D 2732. The HEVA layer of the film was 5.1 microns thick.

The shrinkage at 91 ° C was 22 kg / cm² in the longitudinal direction and 44 kg / cm²in the transverse direction measured by ASTM method D 2838.

Bags were made from the oriented fabricated rudder in this example by making a transverse heat seal over the width of the rudder and then dividing the rudder parallel to, and immediately behind, the seal. Steak steaks were tucked into the bags, air was sucked from the seals, the bags were closed with a metal pinch also shrunk tightly around the steaks by placing the package briefly in a hot water bath maintained at a temperature of from 88 - 99 ° C. The package was then immediately cooled to 1 ° C and stored at this temperature for four weeks. The color of the roast was examined periodically. After storing, the roasts had a proper color removed from the oven, allowed to lie for 30 minutes in the air to regain their clear-cut appearance, and then placed in boxes and ten decks, and. so put in the exhibition counter. After four days in the exhibition counter, the steaks were still usable in color. Maintaining color in the roast at the end of the four week period showed the low oxygen permeation of the laminate film and its utility as a shrinkable packaging material.

Examples 2 and 3

Using extrusion conditions similar to those of Example 1, a laminated strip of width 11.4 cm and consisting of the following layers was prepared:

432 micron thick layer of ethylene vinyl acetate copolymer / 57 micron thick layer of HEVA / 140 micron thick layer of ethylene vinyl acetate copolymer. The content of vinyl acetate derived units in the (nonhydrolyzed) ethylene vinyl acetate copolymer in this laminate was

3.5 wt% and its melt index was 0.5 determined by ASTM method

D 1238, Condition E. The same HEVA polymer was used as an example

1. An irradiated band with this composition would not expand into a bladder without cracking. The rolled strip was irradiated, rapidly through boiling water, oriented by blistering technique and laid flat, completely as described in Example 1. Blisters could not be produced by inflated rudder which had been irradiated to dosage levels in the 3 - 6 MR range and satisfactory continuous production could be achieved. with bands irradiated with irradiations of respectively

7.5 MR and 9.5 MR. Increasing the dosage past the 0.5 MR level was not attempted as experience with other ethylene vinyl acetate copolymer laminates has shown that in general dosage levels are much lower than 12 MR material for rigid and difficult to orient and seal.

The table below summarizes the characteristics of the film laminates in Examples 2 and 3, the radiation dosages being as shown.

Example 2 Example 3

<td>Radiation dose (MR)</td><td> 7,5</td><td> 9,5</td>

<td>Total measured thickness of</td><td>flat-added</td><td></td>

<td>laminate, micron</td><td> 58</td><td> 58</td>

<td colspan="2">Thickness of HEVA layer micron 5.7</td><td> 5,7</td>

<td>Shrink energy at 91 ° C:</td><td></td><td></td>

<td>β 2 Pa across, kg / cm</td><td> 19</td><td> 18</td>

<td>o 2 longitudinal, kg / cm</td><td> 36</td><td> 40</td>

<td>Shrinkage% at 91 ° C;</td><td></td><td></td>

<td>across,</td><td> 22</td><td> 23</td>

<td>lengthwise</td><td> 34</td><td> 34</td>

<td>Oxygen review by</td><td>23 ° C</td><td></td>

<td>2 2 (cm / m 24 h)</td><td> 1,4</td><td> 1, 6</td>

<td>Tear strength, grams</td><td></td><td></td>

<td>lengthwise</td><td> 27.38</td><td> 20.38</td>

<td>across</td><td> 11,75</td><td> 8.30</td>

<td>Impact resistance, cm.kg</td><td> 25,8</td><td> 28,0</td>

<td>Packages with tiffen steak were</td><td>made using the laminate</td><td>in the examples</td>

<td colspan="2">2 and 3 as described in Example 1. In the experiment with</td><td>movies from everyone</td>

three examples achieved satisfactory color retention and retention time.

Example 4

Using the above-described co-extrusion process and extrusion conditions as in Example 1, a band was formed which consisted of the following layers: 406 micron thick layer of ethylene vinyl acetate copolymer / 51 micron thick layer of HEVA / 152 micron thick layer of ethylene vinyl acetate . The (non-hydrolyzed) ethylene vinyl acetate copolymer used consisted of a mixture of 75 wt% of an ethylene vinyl acetate copolymer with 3.5 wt% acetate-derived units and 25 wt% of an ethylene vinyl acetate copolymer with 9 wt% vinyl acetate derived devices. The same HEVA polymer was used as in Example 1. The width of the flat tube was 10.6 cm and the rolled tube was irradiated with a dosage of about 7.5 MR. The rudder was oriented from a boiling water bath as described in Example 1, where the formed oriented rudder was 41 cm in diameter and had a thickness of from 51 to 63 microns.

Example 5

Using the above-described co-extrusion process and extrusion conditions as in Example 1, a band was formed which consisted of the following layers: 381 micron thick layer of ethylene vinyl acetate copolymer with 9 'wt% vinyl acetate-derived units / 63.5 microns thick layer HEVA / 229 micron thick layer ethylene vinyl acetate copolymer with 3.5 wt% vinyl acetate derived units. The same HEVA polymer was used as in Example 1. The rolled band was 11.1 cm wide and was irradiated with a dosage of approximately 6.5 MR, after which the inflated band was oriented by the trapped bladder technique from a hot water bath maintained at 99 ° C. The resulting flat layered rudder had a width of 36 cm, but difficulties arose because the inner layer of ethylene vinyl acetate copolymer with 9 wt% vinyl acetate derived units tended to adhere and adhere to itself.

Examples 6 and 7

While in the previous examples, HEVA had a relatively low melt index (of 6.0), HEVA in Examples 6 and 7 had a melt index of around 19.0 determined by ASTM method D 1238 condition L. The melt change comes from the increased ethylene unit content of the copolymer.

In Example 6, the content of vinyl acetate-derived units in the non-hydrolyzed ethylene vinyl acetate copolymer was 3.5% by weight, and in Example 7 it was 9% by weight. Using the above-described cooling extrusion process and extrusion conditions similar to those of Example 1, a strip was made with the following layers: 406 micron thick layer of ethylene vinyl acetate copolymer / 51 micron thick layer of HEVA / 153 micron thick layer of ethylene vinyl acetate . The rolled band width was 10.6 cm and this rolled band was irradiated to a dosage level close to 7.5 MR. The tape was then preheated and oriented by boiling water in Example 6 and hot water at 93 ° C in Example 7, by the trapped blistering technique. The flattened width of the thus obtained tubular film was 42 cm and its thickness ranged from 46 - 63 microns. No extrusion or orientation problems occurred.

Example 8

Using the co-extrusion process described above and extrusion conditions similar to those of Example 1, a strip was made with the following layers: 305 micron thick layer of low density polyethylene / 38 micron thick layer of HEVA / 250 micron thick layer of low density polyethylene. The ethylene vinyl acetate copolymer and HEVA polymer used were as described in Example 1. This rolled strip was 10 cm wide and was then irradiated to a dosage level of close to 8.5 MR. The irradiated band was preheated and oriented from a boiling hot water bath. The final stirring target was in the range of 56 - 71 microns and the film had a free shrinkage of 26% longitudinally and 36% transversely at 91 ° C.

The foregoing examples demonstrate that a shrinkable laminate film with a HEVA layer can be successfully manufactured. All the films prepared in the examples had oxygen permeability of less than 5.0 cm / m. pr. 24 hours per atm. at 23 ° C and free heat shrinkage of more than 10% in at least one direction. The laminates prepared in accordance with the present invention were all without plasticizers and stabilizers which had to be used for commercial shrinkable films according to the state of the art with very low oxygen permeability.