Heat shrinkable laminated film and method for its production

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The present invention relates to a heat-shrinkable film which is not very permeable to oxygen and which is particularly suitable for the packaging of food, in particular meat and cheese, as well as for its manufacture.

Thermoplastic packaging films fall into two main categories, one of which includes rigid or semi-rigid films and the other of which is flexible films. Rigid and semi-rigid films are eminently suitable for applications requiring shaping of the film hot and usually under vacuum, until a stable firm without support. Such films are normally used for packaging in cartons, tight packaging and making shallow bowls, on the contrary, the flexible film is used to simply wrap the product to be packaged. A very convenient way to achieve the result is to make a film which is heat shrinkable, to wrap the product in in the film and to heat the wrapped product to shrink the film on top. During this operation, the film tightens and adapts closely to the shape of the product. _ Heat-shrinkable films having low oxygen permeability should have high resistance to rough handling and should be capable of heat-shrinking to a reasonable extent at a reasonably low temperature. A film meeting these criteria is a laminate of an oxygen barrier film, which comprises a copolymer of vinylidene chloride and vinyl chloride, between two other layers of polymer, namely a layer of a crosslinked copolymer ethylene vinyl acetate and a layer of an ordinary, non-crosslinked eopolymer of ethylene vinyl acetate. Such a laminated film is described in US Patent No. 3,7 ^ 1,253 6e the Applicant. These laminated films comprising a layer of vinylidene chloride copolymer have an impermeability to oxygen and a shrinkability which are satisfactory, but the copolymers of vinylidene chloride expose to certain drawbacks known to the specialist. One of these drawbacks is that it has been found necessary to add a plasticizer and a stabilizer to the vinylidene chloride polymer. Plasticizers make the film easier to work₃ but their disadvantage is to increase the permeability of the film with respect to oxygen. Stabilizers inevitably increase the cost price and the complication of manufacturing. The object of the invention is therefore to provide a heat-shrinkable laminate film having low oxygen permeability which comprises another oxygen-blocking material which does not require a plasticizer or stabilizer and which is heat-shrinkable up to 'to a reasonable degree at a reasonably low temperature.

The subject of the invention is a heat-shrinkable packaging film which is a laminate.₃ i.e., a two or more layer film comprising an oxygen barrier layer which comprises a hydrolyzed copolymer of ethylene and vinyl acetate.

The preferred film which is the subject of the invention is heat-shrinkable up to a shrinkage of at least $ 10 in at least one direction and preferably both in the longitudinal direction and in the transverse direction.

Hydrolyzed copolymers of ethylene and vinyl acetate are sometimes called HEVA copolymers. These copolymers have already been described in 194-5 in United States patent No. 2,386.34-7. Films and manufactured products formed from hydrolyzed copolymers of ethylene and vinyl acetate have been described in the United States patent.

No. 3,183,203 and a laminate of hydrolyzed copolymer of ethylene and vinyl acetate and polyethylene is described in US Patent No. 3,54-0,962. U.S. Patent No. 3,595,74-Ο describes a laminate which includes an obstructing layer consisting of a hydrolyzed copolymer of ethylene and vinyl acetate, an outer base layer of thermoplastic polymer and a heat sealable layer of ethylene polymer as suitable for the manufacture of thermoformed packaging by deep drawing. However, to the knowledge of the Applicant, it has not been suggested so far, neither in the aforementioned patents nor in other publications that a layer of a hydrolyzed copolymer of ethylene and vinyl acetate would be useful in a heat shrinkable film. A heat shrinkable film is made by stretching a film which becomes thus a flexible, oriented and relatively thin film. The

4-0 oriented molecules are in a state of frozen molar eulary deformation which remains when the hot and stretched film is cooled and which is only released by reheating the film. The relaxation of the deformation results in the withdrawal.

To form a layer which effectively obstructs oxygen, the hydrolyzed copolymer of ethylene and vinyl acetate must as a general rule comprise at least 35 mol% of vinyl acetate units before the hydrolysis. In other words, the hydrolyzed polymer must as a general rule comprise in total at least

35 moles% of vinyl acetate units in both hydrolyzed and non-hydrolyzed form. The hydrolyzed form is generally called vinyl alcohol. Because the degree of hydrolysis of the copolymer is preferably at least 50% and most favorably at least 99%₅ this copolymer differs from the non-hydrolyzed copolymer of ethylene and vinyl acetate which can be used for the other laminate layers.

When the oxygen barrier layer is formed by the hydrolyzed copolymer of ethylene and vinyl acetate, according to the invention, it is easy to obtain heat shrinkable laminates having permeability to oxygen less than 30, frequently less than 5 days and sometimes even o

less than 2 ml / m. 24 hours. atmosphere at 23 ° C. These permeabilities can be measured according to standard ASTM D1434. In general, such oxygen impermeability is about equal to or greater than that of comparable laminates comprising a layer of vinylidene chloride polymer obstructing oxygen.

The preferred films of the invention are those comprising at least three layers, the oxygen barrier layer comprising the hydrolyzed copolymer of ethylene and vinyl acetate occupies the intermediate position without necessarily coming into contact with the two layers of polymer. at least one of which comprises a crosslinked polymer which is preferably a crosslinked olefin polymer. According to a preferred embodiment, the two layers of polymer are crosslinked and more advantageously are made of the same crosslinked olefin polymer, for example they are both formed by a polyethylene or by a non-hydrolyzed copolymer of ethylene and vinyl acetate. Even more advantageously, all the layers of the laminate are crosslinked and, as explained below, a peculiarity of a layer preventing oxygen formed from a hydrolyzed copolymer of ethylene and vinyl acetate is that it allows to manufacture a laminate in which all the layers are crosslinked. The two polymer layers are preferably the surface or exterior layers of the laminate, and in the tubular form, are the radially inner and outer layer.

By polymer is meant for the purposes of the invention not only homopolymers, but also copolymers, in particular block and graft copolymers, inter-polymers and terpolymers.

By olefin polymer is meant for the purposes of the invention, not only an unsaturated hydrocarbon polymer of the general formula C ^ IL ^ (where ^not represents an integer), but also copolymers of olefins with other monomers, for example ethylene and vinyl acetate. The olefin is not necessarily an α-olefin, although c-olefins are preferred. The expression copolymer of ethylene and vinyl acetate not accompanied by the indication of hydrolysis designates for the purposes of the invention a non-hydrolyzed copolymer.

The invention also relates to a process for manufacturing a heat-shrinkable packaging film according to the invention having a low permeability towards oxygen according to which ï (a) a laminate is formed comprising a layer preventing oxygen which comprises a hydrolyzed eopolymer of ethylene and vinyl acetate having, before hydrolysis, a content of vinyl acetate units of at least 35 moles between two other layers of polymer of which at least one comprises a crosslinked polymer (b) the laminate is heated to the orientation temperature of the crosslinked polymer; and (c) stretching the heated laminate to orient the crosslinked polymer and thereby form a heat shrinkable film.

Generally, the polymer of the third layer, i.e. the polymer which can be but is not necessarily crosslinked, is advantageously a polymer which is orientable at a temperature at which the crosslinked polymer of the first layer is orientable.

According to a first preferred embodiment of the invention, stage (a) of the above process is carried out by

4-0 formation of a laminate comprising an oxygen barrier layer made of a hydrolyzed eopolymer of ethylene and vinyl acetate, which satisfies the definition above, between two other layers of polymer of which at at least one comprises a polymer which can be crosslinked by the action of ionizing radiation and the laminate is irradiated by means of this radiation to crosslink the polymer. In this embodiment, at least two layers of the laminate (and three layers if the third is also made of a crosslinkable polymer) are crosslinked, which improves the mechanical strength of the laminate. Laminated films comprising a layer of vinylidene chloride copolymer have already been irradiated, but this irradiation treatment is not of general application because the vinylidene chloride copolymer sometimes undergoes degradation and discoloration when the dose radiation is high. Therefore, it has

1? Hitherto it has been customary to make a support layer and irradiate it before applying the vinylidene chloride polymer, so that only this layer of the laminate is crosslinked. A valuable advantage offered by the invention is that it is possible, according to this embodiment, to manufacture a laminate comprising a layer which obstructs oxygen in which more than one layer is crosslinked.

In the preferred method, three adjacent annular layers of polymer are simultaneously extruded into a multi-layer tube.

The degree of hydrolysis of the hydrolyzed copolymer of ethylene and vinyl acetate is preferably at least 50% and most advantageously more than 99% · In addition, the content of units of vinyl acetate before l hydrolysis must be at least 35%, since it has been established that when the content of vinyl acetate units is less than 35%, the hydrolyzed copolymer does not effectively block the passage of 'oxygen.

The preferred polymers which can be crosslinked by irradiation are olefin polymers and in particular homopolymers of ethylene and copolymers of ethylene and vinyl acetate. When the copolymer of ethylene and vinyl acetate constitutes a curable polymer for the purposes of the invention, the content of units of vinyl acetate, if it is not mixed with other polymers, may reach about 15% by weight, the preferred range being 3 to 12% by weight,

M-0 The copolymer of ethylene and vinyl acetate can be blended with polyethylene to establish the desired content of vinyl acetate units.

Preferably, the two layers of polymer between which the layer of hydrolyzed copolymer of ethylene and of vinyl acetate is disposed are layers of a homopolymer of ethylene or of a copolymer of ethylene and of vinyl acetate. Because the two layers are crosslinked in the preferred embodiment of the invention, they have a similar orientation ability and thus offer a number of well-known advantages.

In addition to olefin polymers, examples of other polymers which can be used as layers in the laminated film of the invention are poly (vinyl chloride), polyamides, ionomers, acrylic polymers, polyesters, polycarbonates, polystyrenes, polymers of vinylidene chloride and copolymers of the corresponding monomers.

After the tubular laminate is formed, it is cooled and then flattened. The flattened tube is sometimes called a ribbon and in the flattened state usually has a width of about 2.5 to 25 cm depending on the final width provided for the film.

The preferred method of crosslinking is the irradiation of the flattened tube.

By irradiation, it should be understood the exposure to ionizing radiation such as X-rays, gamma rays and electrons which directly induce molecular crosslinking. However, when chemical crosslinking agents are dispersed in the polymer, both light and heat can be considered as forms of radiant energy inducing crosslinking. Electron flux is the preferred energy radiation and is preferably maintained by an industrial type accelerator operating in the range of 0.5 to 2.0 HeV, therefore in the preferred method the flattened tube is irradiated by passage through the electron beam emitted by an electron accelerator. In a typical accelerator, the beam moves along the width of the flattened tube and it passes back and forth in the beam until the desired radiation dose is reached. The electrons must as a rule have an energy of 0.5 to 2 MeV and the preferred irradiation dose for the purposes of the invention has been found to be of the order of

2.0 to 12 megarads. The irradiation doses are expressed in rad and in million rads, or megads. The degree of molecular crosslinking is advantageously expressed by the radiation dose inducing this degree of crosslinking. it is obvious that any ionizing radiation which induces any crosslinking between the long molecular chains of olefin polymers is suitable.

The degree of crosslinking is important since it has been established that when the irradiation dose is less than about 3.0 megarads for the preferred polymer of ethylene and vinyl acetate, the tube cannot be oriented successfully following the technique of. the bubble, due to insufficient mechanical strength, so that the bubble bursts. The dose necessary for sufficient mechanical strength of the multilayer tube varies with the molecular weight, density and nature of the crosslinkable polymers and can reach barely 2.0 megarads for certain polymers such as polyethylene. On the other hand, at doses of more than about 12 megarads, most of the ethylene and vinyl acetate eopolymers are crosslinked to the point of becoming rigid and difficult to work. Therefore, for most applications, the optimal dose is 4 to 8 megarads. After irradiation, there is a laminate which comprises a layer of a hydrolyzed copolymer of ethylene and vinyl acetate between two other layers of polymer, at least one of which comprises a crosslinked polymer.

Stage (c) which is the stretching stage is preferably carried out by inflating the heated laminate in the tubular form proper rather than flattened tubular and to a degree sufficient for biaxial orientation.

The tube is heated and then inflated to a temperature falling within a range beyond the second order transition temperature, but below the crystalline melting point of the crosslinked layer (s), as appropriate. This range is the orientation temperature range in which the polymer is elastic, while its molecules tend to orient in the direction of stretching. A preferred medium for heating the flattened tape is water at boiling point or near its boiling point when the crosslinked polymer is an olefin polymer. It is well known that oriented thermoplastics generally undergo shrinkage at a temperature equal to or close to è-0 from that at which they were oriented. Since water is a readily available, relatively safe and effective heat transfer agent, it is advantageous from the industrial point of view to produce film which is heat shrinkable in water, hot or boiling. The preferred film according to the invention has this interesting feature.

After the flattened tube has been inflated into a bubble shape, it is cooled and can be flattened again and then rolled into a coil for storage. A description of the orientation according to the bubble technique is given in particular in the patent States

United States of America N® 3-022.5 ^ 3. Depending on the desired degree of orientation, the width of the film and its thickness, the tube which has been expanded from a flat width of 2.5 to 25 cm reaches a new flat width of 8.9 at 91 cm. It is obvious that these dimensions are given for illustrative purposes only.

By adjusting the orientation temperature and the inflation pressure of the tube, it is possible to control the degree of orientation at will. In practice, the tube is normally stretched at least 10% in one direction to provide the resulting film with useful heat shrinkability.

The expanded and again flattened tube can be split longitudinally and rewound in a reel or else heat sealed transversely and cut at intervals, either transversely or longitudinally, for making bags. Another advantage of the invention, when the two outer layers of the preferred laminate are formed from a crosslinked copolymer of ethylene and vinyl acetate, is that it is heat sealable and allows bags to be made at the start. tube or making envelopes by sealing sheets of film to each other. These films also have the advantageous properties from the industrial point of view of being heat sealable and of being heat shrinkable in hot or boiling water. In addition, the tear resistance and the impact resistance are excellent, so that the preferred laminated films are suitable for packaging bone-in meat which has tended to tear and puncture its packaging.

In addition, the films according to the invention have excellent resistance to delamination and although the validity of the invention is not linked to that of any particular hypothesis explaining this advantage, the Applicant is brought to

M) believe that the irradiation of the laminate results in a certain degree of crosslinking in the interfaces between the different layers.

The molecules of the layers constituting the film are, as we know, susceptible to crosslinking and especially when the layers have been assembled in the molten state by simultaneous extrusion, the molten masses constituting the layers interpenetrate to a certain extent at interfaces. It is therefore to be envisaged that the molecules of a layer are crosslinked with those of the adjacent layer. In addition, the irradiation of the complete laminate, rather than its support layer in the usual way hitherto, promotes the uniformity of properties in all layers and improves the adhesion between them.

A frequent difficulty in conventional processes for making laminated heat-shrinkable films is that even with careful precautions, small amounts of air can become trapped between the layers, so that when the film is stretched for orientation, the Air bubbles also stretch and occupy a larger area. These bubbles affect the good appearance of the film and are priming points for delamination. A particular advantage of the invention is that a layer obstructing oxygen constituted by a hydrolyzed copolymer of ethylene and vinyl acetate allows the simultaneous extrusion of the different layers of laminate. During simultaneous extrusion, the trapping of air bubbles between the layers is minimized, if not removed. The laminated film is therefore preferably obtained by simultaneous extrusion. This technique is obviously applicable only when the crosslinking is carried out on the laminate initially formed rather than on the single support layer which is then coated with one or more other layers. By simultaneous extrusion is meant a single extrusion combining at least two extruded materials, for example thermoplastic polymers, waxes or adhesives in the molten state in a mixed film dispensing with support. Simultaneous extrusion includes, among other things, the joint extrusion of at least two materials suitable for it, in the form of annular flat or coaxial streams. Each stream comprising a separate polymer specific to the extrusion is emitted by a different extruder and it is preferable, for the purposes of the invention, to perform the simultaneous exb-0 trusion of the layers in annular arrangement. A fi10

The time of simultaneous extrusion that allows such a process to be performed is described in United States Patent No. 3,802,826. However, the simultaneous extrusion of a flat film, for example as described in US Patent No.

3,865,665 is also suitable.

According to the preferred method of simultaneous extrusion, the extruded streams meet at the lips or upstream of the lips of the die, so as to remove the air which could otherwise be trapped between the layers. In addition, the simultaneous extrusion of the layer of hydrolyzed copolymer of ethylene and vinyl acetate between the two adjacent layers protects against moisture the layer of hydrolyzed copolymer of ethylene and vinyl acetate whose permeability to regard to oxygen is adversely affected by humidity.

According to a second embodiment of the method for manufacturing the film of the invention, crosslinking by ionizing radiation is replaced by chemical crosslinking.

The preferred crosslinking agents for olefin polymers are peroxides, one of which is particularly effective for polyethelene is 2,5-dimethyl-2,5 ~ di (t-butylperoxy) h'exane described in the patent of States United States of America K ° 3.201.503

According to this process, the peroxide can be added in an amount of about 0.755 by weight to the extruded mixture containing po □ low density lyethylene (specific gravity of 0.92 g / cm^J). Care should be taken to maintain the temperature of the extruder at 149 ° C or less to prevent premature crosslinking in the body of the extruder or die. When. the polymer leaves the die, the wall thickness is preferably from 356 to M) 6 microns.

For the crosslinking of the tube, it can be inflated but without expansion, then brought to pass into an oven maintained at about 260 ° C. where the fairly rapid crosslinking is caused by the high temperature. After completion of the crosslinking, the tube can be coated by sequential extrusion or by multiple extrusion, as described above, for the application of the layer of hydrolyzed copolymer of ethylene and vinyl acetate.

Alternatively, the layer comprising the crosslinking agent can be extruded simultaneously with the other layers.

In this case, stage (a), that is to say the formation of the initial laminate, is carried out by constitution of a laminate comprising an oxygen-blocking layer consisting of hydrolyzed copolymer of ethylene and vinyl acetate (as defined above) between two other layers of polymer, at least one of which comprises a crosslinkable polymer and a crosslinking agent which can be activated hot for this polymer, then by heating the laminate for the crosslinking of this polymer. The flat tube then passes into a hot water bath, then is inflated and oriented as before.

In addition to the chemical crosslinking agents used in the above process, photosensitive crosslinking agents can be incorporated into the polymer layers for crosslinking by exposure to ultraviolet light.

According to a third embodiment, the crosslinked layer called the support layer is prepared first, then coated with the other two essential layers, either by simultaneous extrusion, or by successive extrusion. Therefore, step (a) is performed by laminating a crosslinked polymer layer, an oxygen barrier layer consisting of hydrolyzed copolymer of ethylene and vinyl acetate and a third layer of polymer. The preferred polymers for the support are again olefin polymers such as polyethylene and a copolymer of ethylene and vinyl acetate comprising up to 15 / θη weight of vinyl acetate units. The extrusion is preferably carried out with an annular die producing the tube. The irradiation is preferably carried out after flattening of the extruded tube. After irradiation, the tube is swollen but not stretched and passed through a coating die. This technique, as well as a suitable coating die, is the subject of a description in US Patent No. 3,607,505 to the Applicant. When passing through the coating die, the tubular support receives a layer of hydrolyzed copolymer of ethylene and vinyl acetate, then reaches a second coating die which applies another layer of polymer. The polymer of this other layer is preferably an olefin polymer, for example a mixture of polybutene-1 and of copolymer of ethylene and propylene, as described in the patent of United States of America No. 3 · θ9ΐ · Θθδ âe the Applicant, although polyethylene or a copolymer of ethylene and vinyl acetate are also suitable. After the application of this last layer of polymer, the tube has a terminal structure which comprises a layer of hydrolyzed copolymer of ethylene and vinyl acetate and a layer of crosslinked polymer. The tube is then cooled and can be flattened. For orientation, which is preferably biaxial, as described above, the flattened tubular laminate is heated to an orientation temperature suitable for the support, which, in the case of an olefin polymer, is generally of 82-121 ° C, after which the tube is. inflated for orientation using the bubble technique.

For coating by simultaneous extrusion, in which case two or more layers are simultaneously extruded onto the crosslinked support, this passes through a co-extrusion die and preferably the layer applied directly to the support is the layer of hydrolysed copolymer of ethylene and vinyl acetate, while the outer layer is a layer of olefin polymer. If desired, three, four or five or more coats may be applied by simultaneous extrusion on a support which may itself comprise a single layer or comprise several. In addition, the sequential simultaneous extrusion coating makes it possible to form a set of multiple layers and to incorporate additional layers of hydrolyzed copolymer of ethylene and vinyl acetate into the final laminate. The tubular laminate thus produced can then be oriented according to the bubble technique, as indicated above.

The preferred methods for manufacturing the laminate according to the invention consist in forming a tube consisting of annular layers, but it is also part of the invention to extrude the layers of hydrolyzed polymer and copolymer of ethylene and d vinyl acetate by means of slotted dies using the technique of simultaneous extrusion of three or more layers, or of coating by extrusion from one layer to another. After crosslinking such a laminate, preferably by irradiation, the latter preferably heated to its orientation temperature, then stretched and oriented according to the conventional technique of working in the stretching frame.

The invention obviously also relates to various products and especially foods, in particular bone-in meat and cheese, wrapped in film of the heat-treated invention.

The invention is illustrated by the following examples.

EXAMPLE 1.

According to the simultaneous extrusion process described above, by means of concentric annular dies a laminated tube with a flat width of 10.2 cm is produced which comprises a layer with a thickness of 3% deniers of an eopolymer d ethylene and vinyl acetate, a layer of a thickness of 51 microns of hydrolyzed corolyether of ethylene and vinyl acetate and a layer of a thickness of 114 microns of copolymer of ethylene · and acetate vinyl. The first layer cited is the radially inner layer of the extruded tube and the same scoring system is applied in the other examples. The content of vinyl acetate units in the non-hydrolyzed copolymer of ethylene and vinyl acetate in the laminate is 3.5 to 5b by weight and the melt index of the eopolymer is 0 , 5 according to ASTM D123S, condition 2. The hydrolyzed eopolymer of ethylene and vinyl acetate comprises 69 moles y of units derived from vinyl acetate and its degree of hydrolysis is 99>₅ while its melt index is about 6.0 according to the Adïh 'D1238 standard, condition L. The temperature of the three polymers extruded out of the orifices of the die is about 218 ° C. The tube is cooled and flattened to its width of 10.2 cm. The flattened tube is passed twice through the area traversed by the beam of an electron accelerator with an insulated core transformer with half energy of 0.5 MeV so that it receives a total dose of irradiation. 6.5 megarads. After the irrigation, the flattened tube is passed through the boiling water, it is inflated into a bubble and it is oriented into a plain tube diameter of 4l cm having a wall thickness of 58 microns, after which it is flattened the tube again. The bubble is extremely stable and the appearance of the film is good. The resulting film shows a free line of 22 / in the longitudinal direction and of 32 / in the traaversal direction at 91 ° C. The free shrinkage is measured according to standard ASTM D2732. The hydrolyzed copolymer layer of ethylene and vinyl acetate in the film has a thickness of

5.1 microns, p

The shrinkage tension at 91 ° C. is 22 kg / cm in the longitudinal direction and 44 kg / cm in the transverse direction, to be measured according to the form ASTM D2838.

Bags are made by means of the oriented tube fabricated in accordance with the example by constituting thermi2326293 lacquer seals transverse across the width of the tube and by cutting the tube parallel and immediately downstream of these seals. We put pieces of steak in the bags, out of which we suck the air, after which we close the bags with a metal pin5 and we narrow them tightly on the pieces of meat by briefly introducing the package into a double boiler maintained at a temperature of 88 to 99 ° C. The package is then immediately cooled to 1 ° C and stored at this temperature for four weeks. The color of the meat is periodically evaluated. After this storage period, the steaks which have kept their color properly are removed from the packaging, then they are exposed to the air for 30 minutes so that they take on a bright red appearance, after which they are placed on trays and envelops them before placing them in a counter. After four days at the counter, the steaks still have an acceptable color. The color fastness of the meat at the end of the fourth week indicates the low oxygen permeability of the laminated film and its usefulness as a heat shrinkable packaging material.

EXAMPLES 2 AND 3

Under extrusion conditions similar to those chosen in Example 1, a laminated tube with a flat width of 11.4 cm is produced which comprises a layer with a thickness of è-32 microns of copolymer. ethylene and vinyl acetate, a 57 micron thick layer of hydrolyzed copolymer of ethylene and vinyl acetate and a 1 micron thick layer of ethylene and acetate copolymer vinyl. The content of vinyl acetate units in the non-hydrolyzed copolymer of ethylene and vinyl acetate in the laminate is 3.5 wt%, and the melt index of the copolymer is 0, 5 according to standard ASTM D1238 condition E. The same hydrolyzed copolymer of ethylene and vinyl acetate is used as in Example 1. An unirradiated tube of this constitution cannot be expanded into a bubble without bursting. The flattened tube is irradiated and passed through boiling water, it is oriented using the bubble technique and it is flattened again, as described in Example 1. It is possible to form a bubble in the inflated tube which has been irradiated at a dose of 3 to 6 mega-0 rads and the production can be continued continuously

1?

under satisfactory conditions with Irradiated tube at doses of 7.5 and 9.5 megarads, respectively. No attempt is made to increase the irradiation dose beyond 9.5 megarads, because experience with other laminates comprising an ethylene-vinyl acetate copolymer has shown that as a rule doses much higher than 12 megarads make the polymer too stiff and difficult to orient and seal.

The table below summarizes the properties of the laminated films of Examples 2 and 3 at the irradiation doses indicated.

example 2 exep, pl.g., ._ 3

Radiation dose (megarads)

7,5

5,7

9,5

5,7

Total thickness of laminate laid flat (microns)

Thickness of the layer of hydrolyzed copolymer, ethylene and vinyl acetate (microns)

Withdrawal force at 91 ° C: transverse direction (kg / cm2) longitudinal direction (kg / cm²)

Percentage shrinkage at 91 ° C:

transverse direction 22 -23 longitudinal direction 3¹! 3+

Oxygen permeability at 23 ° C

Q (ml / m .2¹¼ hour.atosphere) 1,¹¼ 1,6

Tear resistance, (grams) longitudinal direction 27.38 20.38 transverse direction 11.75 8.30

Impact resistance, (cm.kg) 25.8 28.0

Steak steaks are prepared using the laminate of Examples 2 and 3, operating as in Example 1. A satisfactory color fastness and a useful talk time are observed with the films of the three examples.

EXAMPLE

By applying the simultaneous extrusion method described above, under extrusion conditions similar to those chosen for Example 1, a tube is produced comprising a layer with a thickness of 6 6 microns of ethylene copolymer. and vinyl acetate, a layer with a thickness of 51 microns of hydrolyzed copolymer of ethylene and vinyl acetate and a layer with a thickness of 152 microns of copolymer of ethylene and acetate

M) vinyl. The unhydrolyzed copolymer of ethylene and vinyl acetate is a mixture of 75% by weight of a copolymer of ethylene and vinyl acetate to 3.5% by weight of vinyl acetate unit. and from 25% by weight of a copolymer of ethylene and vinyl acetate to 9% by weight of vinyl acetate units. The same hydrolyzed copolymer of ethylene and vinyl acetate is used as in Example 1. The width of the flat spread tube is 10.6 cm and the flat spread tube is irradiated to an irradiation dose of about 7.5 megarads. The tube is oriented in a boiling water bath as in Example 1, so as to obtain an oriented tube with a diameter of 41 cm having a wall thickness of> 1 to 63 microns.

EXAMPLE 5

By applying the simultaneous extrusion method described above under extrusion conditions similar to those chosen for Example 1, a tube is produced comprising a layer with a thickness of 381 microns of copolymer of ethylene and of vinyl acetate at 9% by weight of vinyl acetate units, a layer with a thickness of 63 microns of hydrolyzed copolymer of ethylene and vinyl acetate and a layer with a thickness of

228 microns of ethylene and vinyl acetate copolymer at 3.5% by weight of vinyl acetate units. The same hydrolyzed copolymer of ethylene and vinyl acetate is used as in Example 1.

The width of the flat spread tube is 11.1 cm and the spread tube is irradiated to an irradiation dose of about 6.5 megarads, after which it is inflated and oriented using the bubble technique trapped in a water bath at 99 ° C · We thus obtain a tube having after spreading flat a width of 36 cm, but causing some difficulties, the fact that the internal layer of ethylene vinyl acetate copolymer at 9% by weight of vinyl acetate units tends to adhere and to weld on itself.

EXAMPLES 6 AND 7

Whereas in the previous examples, the hydrolyzed copolymer of ethylene and vinyl acetate has a relatively low melt index, namely 6.0, we use in Examples 6 and 7 a hydrolyzed copolymer of ethylene and vinyl acetate having a melt index of approximately 19.0, to be measured according to standard ASTM D1238 condition L.

The difference in melt index is due to an increase in the content of ethylene units in the copolymer.

In Example 6, the content of vi40 nyl acetate units in the non-hydrolyzed copolymer of ethylene and vinyl acetate is 3_}5 / v by weight, while in Example 7j it is% by weight. By applying the simultaneous extrusion method described above under similar extrusion conditions e οχ-

<td> 5</td><td>the chosen ones</td><td>for 1</td>

<td></td><td>layer of a</td><td>thick</td>

<td></td><td>acetate</td><td>vinyl</td>

<td></td><td>copolymer</td><td>hydrol;</td>

<td></td><td>layer of a</td><td>thick</td>

<td> 10</td><td>acetate</td><td>vinyl</td>

Ι?

The width of the flat spread tube is 10, c cm and the flat spread tube is irradiated to an irradiation dose of about 7.5 megarads. The tube is then preheated and directed in boiling water in Example 6 and in water at 93 ° C in Example 7? following the bubble trap technique.

The width of the flat spread tube thus obtained is 42 es and its wall thickness is 46 to 63 microns. The extrusion and orientation do not cause any difficulty.

EXAMPLE 8

By applying the simultaneous extrusion method described above under extrusion conditions similar to those chosen in Example 1, a tube is produced comprising a layer with a thickness of 30? microns of low density polyethylene, a layer with a thickness of 38 microns of hydrolyzed copolymer of ethylene and vinyl acetate and a layer with a thickness of

250 microns of low density polyethelene. The copolymer of ethylene and vinyl acetate and the hydrolyzed copolymer of ethylene and vinyl acetate are the same as in Example 1. The tube spread flat is 10 cm wide and is irradiated then up to an irradiation dose of approximately 8.5 megarads. The irradiated tube is preheated and oriented in boiling water. The wall thickness of the final tube is 56 to 71 microns and the free shrinkage of the film is 26, i in the longitudinal direction and 36y in the transverse direction at 91 ° C.

The above examples demonstrate that a laminated shrinkable film comprising a layer of hydrolyzed copolymer of ethylene and vinyl acetate can be successfully produced. All the films produced in the examples have p

oxygen permeability of less than 5 ml / m. 24 hours. atmosphere at 23 ° C and have free thermal shrinkage of more than 10.0 in at least one direction. The laminates forming the object of the invention are all obtained without the plasticizers and stabilizers which are essential in thermo-shrinkable films of the conventional type having a very low permeability with respect to oxygen.

EXAMPLE 9

A tubular support made of a copolymer of ethylene and vinyl acetate at 9% by weight of vinyl acetate units is extruded, cooled, flattened and irradiated to a certain extent. '' about 5.0 megarads using electrons from an accelerator. The wall thickness of the support is approximately 45δ microns. The support is inflated without stretching it and passed through a simultaneous extrusion flask by means of which a layer 20 microns thick of the hydrolyzed copolymer of ethylene and vinyl acetate is simultaneously applied. Example 1 and a layer with a thickness of 112 microns of a mixture of 75% by weight of ethylene-propylene eopolymer and 25% by weight of polybutene-1. The content of ethylene units in the copolymer is approximately 3.1% by weight. After spreading out flat, the resulting ternary tube has a width of 10.5 cm. By applying the trapped bubble technique described above, the tube is dilated and an oriented film is produced having a flat width of 43 cm, operating in a water bath maintained at a temperature of 99 to 100 ° C.