DESCRIPTION

FIELD OF THE INVENTION

The present invention concerns a heat-shrinking, multilayer polymeric film - wherein the individual layers are co-extruded from a single annular extrusion head, having multiple input ports, and then simultaneously biaxi- ally stretched - which shows an improved barrier effect to gases and water- vapour and a reduced elastic module, is stable to delamination, has an increased low-temperature heat-shrinking and, finally, shows the opportunity to use a wider selection of PA and EVOH polymers to manufacture it. In particular the invention relates to a film of this type used for the manufacturing of bags or films for the packaging of perishable foods, which hence require to be highly gas-proof - in particular to 0 ₂ - and water-vapour-proof. Such films are manufactured in the form of planar films, to be used on machines of the flow pack type or the like, or of tubular films to be transformed into bags, or of small-diameter tubular films for the transformation into bags which are heat-sealable or closable through clips at the ends thereof.

A possible diagram of a plant for the manufacturing of multilayer film is shown, for illustration purposes only, in the attached fig. 1. In particular, in such drawing the main steps of the process are schematically shown, namely:

step 1 - extrusion of the multilayer film;

step 2 - forming of a first bubble of multilayer film;

step 3 - forming of a second bubble of multilayer film and bidimen- sional stretching of the same;

step 4 - thermal stabilisation of the multilayer film;

step 5 - winding onto the reel of the finished film.

In extrusion step 1 a plurality of screw extruders (A-I) is provided, one for each of the different plastic materials which make up the individual layers of the multilayer film. Each one of the outlets of such extruders is connected to a respective one of a plurality of annular and concentric extrusion channels, formed in an extrusion head to which the molten materials are supplied.

In step 2, immediately downstream of the extrusion head a first cylindrical bubble of multilayer film is formed, which is simultaneously cooled by cold water nebulization. The film thus formed is gathered in a planar shape, i.e. folded on itself, between a pair of rollers arranged in the lower area of the plant and then brought back through the upper area of the plant.

Step 3, for bidirectional stretching, occurs by forming a second bubble of multilayer film which goes through a primary infrared oven where it is heated up to the stretching temperature. The desired bidimensional stretching action is imparted to the film through a suitable adjustment, on the one hand, of the speed difference between the pairs of supply rollers (primary speed) and the gathering ones (stretch drawing speed), upstream and downstream, respectively, of the second bubble of multilayer film and, on the other hand, of the diameter difference between the first bubble and the second bubble. Through a procedure similar to the one described above, at the end of the stretching film, the multilayer film is gathered at the bottom of the plant in a planar shape, folded on itself in a double layer, and sent again to the upper area of the plant.

In stabilisation step 4, a third bubble of multilayer film is formed which goes through a stabilisation oven where the film is heated up to a temperature sufficient to obtain the stabilisation of the residual tensions found in the same.

In step 5, the finished multilayer film is gathered onto reels, son after any cutting and trimming operations.

STATE OF THE PRIOR ART

Heat-shrinking films made of polymer plastic materials with barrier effect to gases and water-vapour have been known for a long time, as well as the use of such films for the packaging of perishable foods.

However, the technical performances required from these films over time have become increasingly more complex and sophisticated; without going into the detail of special features of the films for special applications, the more general typical properties of the films marketed today are the fol ^¬ lowing :

- barrier to gases, and in particular to oxygen ;

- barrier to water-vapour;

- heat-shrinking in two perpendicular directions, so that, during the packaging operations, the film may adhere to the package;

- high mechanical strength, even in the sealed parts;

- even thinner thicknesses;

- visual look characterised by great transparency and brightness. The polymer initially used for the manufacture of these films - and by now on the market for decades - is a copolymer of polyvinylidene chloride (PVDC) co-extruded with other polymers such as ethylene vinyl acetate (EVA), linear low-density polyethylene (LLDPE), ionomeric resins or mixtures of the above-said polymers. In the prior art, polyvinylidene chloride is manufactured as multilayer film comprising - in addition to the intermediate PVDC layer - a layer on the side of the multilayer film intended to build the wrapper inside (referred to as "inner layer" in the following for simplicity's sake) having features of sealability and a layer on the side of the multilayer film intended to build the outside of the wrapper (referred to as "outer layer" in the following for simplicity's sake) acting as mechanical protection of the PVDC from abrasions and mechanical breaks. In some cases, the composite multilayer structure described above is also reticulated, usually through exposure of the film to a high-energy electron flow. The known PVDC-based multilayer film has high values of gas and water-vapour barrier in addition to good heat shrinking; however, it is not devoid of drawbacks and in particular it has limits both during manufacturing and during application, as it is highlighted in the following.

In the manufacture of PVDC films, the critical aspect is linked to the fact that this polymer is thermally little stable and hence susceptible of degradation during extrusion. It is hence necessary to adopt special precautions in the design of the processing plant, and in particular in that of the extrusion screws heads, the profile of which, in combination with the material flow-rates, must be suited to avoid localised overheating of the polymer being processed; nonetheless, the working conditions are in any case critical, and the forming of carbonised particles is hence frequent.

In the application of this film, the drawbacks are instead related, from a first point of view, to the poor mechanical strength to penetration by puncture and to the poor resistance to abrasion; from a second point of view, it is the very chemical composition of this film to cause application drawbacks, considering that in recent years the trend to abandon chlorinated polymers containing plasticisers - as PVDC is - is largely growing, both to avoid direct contact thereof with food products and the resulting possible migrations of chlorine and plasticisers into packaged food products, and as far as waste disposal through combustion is concerned, where the presence of chlorine causes - as known - the forming of harmful by-products in the exhaust gases.

In the sector of barrier-effect films, for the packaging of food products, are hence lately taking ever growing shares of applications and of the market some other non-chlorinated polymers, alternative to PVDC and having good gas-barrier features, such as ethylene-vinyl alcohol (EVOH) copolymers, said copolymers being the ones most used for this purpose, and some types of polyamides (PA) which, in addition to the well-known mechanical strength qualities, also have low values of gas transmission in the absence of water-vapour. As a matter of fact, in general, barrier polymers alternative to PVDC are sensitive to the action of water-vapour, which causes a reduction of the gas-barrier features thereof. For this reason hence, as well as to prevent the leaking of water-vapour from the packaging of some fresh products (typically meats), the barrier-effect polymers replacing PVDC, of which they must reproduce performances, must be protected from moisture penetration into the film by introducing in the film structure one or more water- vapour-proof layers, consisting for example of polyole- fins (polyethylene or polypropylene), which protect from the contact with water-vapour the layer/s having oxygen barrier features. Such situation implies that films based on non-chloride polymeric materials which have all the properties described in the above reported list, must necessarily have a multilayer structure comprising layers of different types of polymers which, individually, have one or more of the features listed above.

When preparing these multilayer films, it is of course necessary to also take into account the chance of the adjacent layers not having a degree of compatibility and hence an intra-layer adherence sufficient to avoid creep or even delamination between the layers during the extrusion and biaxial stretching steps. In this case it is known to insert, between layers consisting of materials not sufficiently compatible with each other, an intermediate bonding layer consisting of a polymer having adhesive features.

There have been several production and patent developments concerning the preparation of adhesive polymers which can be used for the above-said intermediate bonding layers, in multilayer film structures subject to simultaneous biaxial orientation. US-2007/0172614 (Du Pont) discloses the addition of tackifying additives to polymeric bases, generally ethylene bases. US-2009/0035594 (Equistar) discloses instead the Theological modification of ethylene-based polymers containing maleic anhydride. In both cases good adhesion between the different layers in the co-extruded structures is claimed, even after simultaneous biaxial stretching.

There are also a number of patents disclosing heat-shrinking, gas- barrier films, having multilayer structures with 5 or more layers, comprising a pack therein consisting of two or three adjacent layers, PA/EVOH or PA/EVOH/PA, respectively, which make up the structural core of the compound as far as gas-barrier effect is concerned.

EP 1513683 (Kuhne) discloses a multilayer film comprising six or seven layers wherein the layers responsible for the gas-barrier effect consist of two or three layers, PA/EVOH or PA/EVOH/PA, respectively, with which they are associated, towards the inner side of the multilayer film, a water- vapour-barrier layer consisting of polyolefins, preferably polyethylene (PE), by interposition of a first intermediate bonding layer and a final PA layer, in this case too with the arrangement in between of an intermediate bonding layer.

EP 2351644 (Kuhne) discloses a multilayer film comprising nine layers, the central gas-barrier pack of which consists of three PA/EVOH/PA lay- ers wherein the PA layers can be made of PA 6, or 66, 6.66, 11, 12 or alternatively they can be made of a mixture of each one of these types of PA with EVOH, or with polyvinyl alcohol (PVAL) or with MXD6. This central pack is flanked on both sides by water-vapour-barrier layers consisting of poly- olefins, preferably polyethylene (PE), by interposition of intermediate bonding layers. A final layer of polyethylene terephthalate (PET) is arranged on the outer side of the film, in this case too by interposition of an intermediate bonding layer, for the purpose of imparting the multilayer film mechanical strength as well as brightness and transparency features. Materials alternative to PET for the outer layer of the multilayer film are disclosed in EP-2351645 (Kuhne).

From a survey of the above-cited prior art it is immediate to note that in each one of the multilayer film structures of the prior art an inner pack of two-three layers, PA/EVOH or PA/EVOH/PA, respectively, is always present having gas-barrier function and between the EVOH and PA layers the use of intermediate bonding layers is not provided. Such intermediate bonding layers are instead always used to achieve the adhesion of said inner gas- barrier pack both to the water-vapour-barrier layers -barrier layers which often make up also the inner sealing layer contacting the product and are made of a polyolefine polymer, generally linear low-density polyethylene (LLDPE), pure or mixed with EVA - and to any outer layers having a mechanical strength function.

The direct PA/EVOH coupling is made possible due to the natural adhesion shown by these two materials, at least partly compatible with each other, and in general it is sufficient for the various applications even after the simultaneous stretching in both directions. According to the prior art, such direct coupling is desirable in order to avoid resorting to the use of intermediate bonding layers consisting of adhesive polymers.

Following the introduction on the market of new types of PA and EVOH which may show a different attitude to stretching, one with respect to the other, it has been proposed - again in order to avoid the use of intermediate bonding layers - to add a certain amount of EVOH to the adjacent PA layer, for the purpose of increasing the adhesion between the PA and EVOH layers or, vice versa, to add PA to the EVOH layer to reach the same object, as well as to vary the level of 0 ₂ transmission through the EVOH layer.

PROBLEM AND SOLUTION

In the frame of the above-described prior art, the present invention has faced the problem of supplying a gas-barrier and water-vapour-barrier multilayer film which have improved physical and mechanical features with respect to known films, in particular as concerns the central layer pack of the film responsible for the barrier effect, which pack - as seen above - consists of two or three layers, PA/EVOH or PA/EVOH/PA, respectively, directly coupled with each other without the arrangement of intermediate bonding layers.

A further problem faced by the present invention is then that of allowing a widening of the range of PA and EVOH polymers which can be used for making the multilayer film, comprising therein also the ones which, due to the different attitude to stretching thereof, have not been used as yet.

Such result has been obtained through a multilayer film having the features defined in enclosed claim 1. Further preferred features of such film are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the multilayer film according to the present invention will in any case be more evident from the following detailed description of some preferred embodiments of the same, given purely as a non-limiting example, also with reference to the attached drawings, wherein :

fig. 1 shows schematically a plant for the production of a multilayer film according to the present invention;

fig. 2A is an enlarged-scale section view of a three-layer film according to the present invention wherein the stresses between adjacent layers during the stretching step are schematically shown; and

fig. 2B is a view similar to fig. 2A of a 5-layer film according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The Applicant's studies, addressing the objects described above, started from the following observations:

- a number of PA types and relatively few EVOH types are available on the market. However, said last polymer is experiencing continuous developments and new applications;

- despite the variety of available polymers, the ones which have features suited for stretching sufficiently compatible with each other to allow the use thereof in a simultaneous bi-orientation process are still few, as appears from the compared result of such features, reported in Table 1 here below, for PA and EVOH polymers. The data reported in such Table concern both the stretching percentages in the two longitudinal (MD) and crosswise (TD) directions, and the temperatures at which said stretching operations must preferably be carried out to maintain elastic memory features of the multilayer oriented film and to prevent stretching from occurring at a viscous deformation temperature of the polymer (irreversible deformation).

Starting from these observations, the conclusion has been reached that such behaviour variability of the different types of PA and of EVOH during the steps of the manufacturing process of the multilayer film, especially as far as the attitude of such polymers to be bi-oriented, is accountable both for the instability of the bi-orientation process and for a partial or total separation of the layers.

TABLE 1

Parameters which adjust the orientability for different types of PA, EVOH and adhesives

Tg = vitreous transition temperature

Tm = melting temperature

Vicat = softening temperature Vicat

Based on such intuition - and strongly departing from the prior art teachings and from the long consolidated work conditions, by now considered as standard, which do not provide the use of any intermediate bonding layer between the PA and EVOH layers - the Applicant has hence experimented the addition of intermediate bonding layers between the layers of said central pack of polymers responsible of the barrier effect, and he could thus surprisingly notice that the interposition of layers consisting of suitable adhesive polymers between PA and EVOH, not only increased the adhesion of the composite structure of this layer pack, as was easily predictable, but unexpectedly imparts the multilayer film further improved mechanical and physical properties, such as a reduced elastic module, an improved barrier effect to gases and water-vapour and an improved shrinking at lower temperatures.

Due to the addition of said intermediate bonding layers, more stable process conditions are furthermore obtained and it is possible to widen the range of the polymers which can be used for film manufacturing. As a matter of fact, it has been ascertained that the interposition of an intermediate layer of adhesive between PA and EVOH also has an unexpected absorption effect of the different attitudes to orient themselves by the two above-said polymers, since the layer of adhesive can orient itself with different stretching percentages on the two surfaces adjacent to the layers both of PA and of EVOH, as schematically shown in Fig. 2. For this reason said intermediate bonding layer is referred to in the following also as "compensating layer" and the material of which it consists as "compensating adhesive polymer". In particular, this phenomenon is schematically shown in a strongly enlarged scale, in fig. 2A for a 3-layer film : PA (A), intermediate bonding layer (B), EVOH (C), and in fig. 2B the similar phenomenon is shown for a 5-layer film : PA (A), intermediate bonding layer (B), EVOH (C), intermediate bonding layer (D), PA (E), wherein the greatest stretching stress Fl or F3 in the PA layers with respect to the stretching stress F2 in the EVOH layer, is gradually absorbed in compensating layers B and D, as shown in the details enclosed in a circle in a further enlarged scale, hence without de- termining the above-described drawbacks in the multilayer film.

Based on these intuitions and on the Experimental evidence resulting therefrom, the present invention has been conceived.

The invention will now be further illustrated with some Examples of the preparation of the multilayer film according to the invention and comparison Examples of multilayer films manufactured according to the prior art, i.e. without layers of compensating adhesive polymer between the PA/EVOH layers, from the comparison of which the improved results obtained by the films of the present invention will be apparent.

Various types of compensating adhesive polymers have been used, arranged in between PA and EVOH in multi-layer structures, as reported in the following Examples, noticing the following variations of mechanical and physical features:

- improved adhesion between the layers after the orientation step by bi-axial stretching of the multilayer film;

- a lower elastic module of the multilayer film which is hence more flexible. The greater film flexibility is an improved aspect over the previous structures; as a matter of fact, in this case the winding of the planar or of the tubular film gives rise to a much softer spool, since a smaller winding tension may be used without creases arising. A winding with high tensile force produces spools which tend to release the tensile stress during storage, up to change their final width or shape. In case of a low elastic module value it is furthermore easier to use the tubular film for the manufacture of bags and the opening of the tubular film is easier; rigid tubular films are generally difficult to open before moving them into the sealing machine or onto the wrapping line. Finally, in the application step, the soft bags with low values of elastic module are easier to fill with the products to be wrapped;

- reduced values of water-vapour transmission due to the fact that the compensating adhesive polymers which make up the intermediate bonding layers are based on polyolefins polymers added with adhesive components and notoriously polyolefins build a layer resistant to water- vapour; - more persistent barrier effect to oxygen, due to the fact that, as a result of the water-vapour protection offered by the intermediate bonding layers, EVOH preserves its features of oxygen-barrier over time;

- higher heat-shrinking values and at a lower temperature, due to the fact that the compensating adhesive polymers building the intermediate layers may be oriented at low temperatures and have a higher stretching or blow-up ratio (BUR) than that of any other polymer generally used in barrier structures.

As far as the optical features are concerned, the multilayer film of the present invention has unchanged gloss and haze features compared to prior art films, regardless of the thicknesses used for the intermediate layers.

The Examples of 5-layer and 7-layer films have been compared with similar products having a multilayer film structure according to the prior art. In order to obtain consistent data in the comparison of the features relating to the different types of film obtained, the sum of the thicknesses of the barrier layers (PA + EVOH) has been kept constant in the different Examples, regardless of the number of layers.

MEASURING METHODS

Layer Adhesion

The adhesion of the different layers making up the multilayer film of the present invention has been assessed following an internal method which comprises two steps. In the first step film samples measuring 10 x 10 cm are dipped for 60 seconds in a bath containing a suitable diathermic oil, the temperature of which is changed in subsequent 10°C steps between +80°C and +110°C. The samples must be at least 4, taken in different areas along the circumference of the tubular film.

At the end of the test a quality judgement is expressed on layer separability: the layers separate or not, intermediate conditions being also possible wherein layer separation occurs only following the application of a mechanical stress which can be assessed from a quality point of view or through a dynamometer, with respect to a reference sample.

In case at least a partial adhesion remains between the layers, one moves on to the second step of the test, which provides the assessment of the residual adhesion force between the layers according to the standard method ASTM D1876. This second step of the test provides cutting 2 test pieces 10 mm wide from the previously dipped samples, on which samples a beginning of layer separation is evident. One test piece must be taken longitudinally (machine direction) and the other in a crosswise direction. To the effects of film use, and hence of the assessment according to this test, it is unimportant between which layers said separation occurs. The separated edges of the test piece are arranged between the clamps of a dynamometer provided with loading cell with scale bottom < 500 g. Clamp distance: 20 mm. Crossbar speed : 50 cm/min. The average force developed is measured and the ratio thereof in respect of the width of the test piece is reported value in g/cm.

For the measuring of the other properties of the films of the Examples, and in particular of the following :

Elastic module

Water-vapour transmission

Oxygen transmission

Shrinking

Gloss

Haze

the common practice of reporting data referring to the corresponding ASTM provisions has been followed.

EXAMPLES

Example 1 - 5-layer film according to the invention

Outer layer A) PA 1

Ethylene-based polymer with tackifying additives or Ethylene-based polymeric adhesive having a modi- fied rheology

C) EVOH 1

D) Ethylene-based polymer with tackifying additives or D') Ethylene-based polymeric adhesive having a modi- fied rheology

Inner layer E) Polyolefin The types and trade names of the raw materials used to manufacture the multilayer film according to Example 1 are reported below in Table 2, compared against those of the similar structure of comparison Example 3, while the type of plant used and the work conditions of the extrusion process are reported in Table 3. The characteristic properties of the multilayer films obtained according to this Example are finally reported in Table 4. Example 2 - 5-layer film according to the invention

Outer layer A) PA 2

B) Ethylene-based polymer with tackifying additives or B') Ethylene-based polymeric adhesive having a modified rheology

C) EVOH 2

D) Ethylene-based polymer with tackifying additives or D') Ethylene-based polymeric adhesive having a modified rheology

Inner layer E) Polyolefin

The types and the trade names of the raw materials used for the manufacture of the multilayer film according to Example 2 are listed below in Table 5, compared with those of the similar structure of comparison Example 3 bis, while the type of plant used and the work conditions of the Extrusion process are reported in Table 6. The features of the multilayer films obtained according to this Example are finally reported in Table 7.

Example 3 and comparison Example 3 bis - 5-layer film of the prior art

Outer layer A) PA 1

or A') PA 2

B) EVOH 1

or B') EVOH 2

C) Ethylene-based polymeric adhesive with tackifying additives

or C) Ethylene-based polymeric adhesive having a modified rheology

D) Polyolefin 1

E) Polyolefin 2 The types and the trade names of the raw materials used for the manufacture of the multilayer film according to Examples 3 and 3 bis are listed be ^¬ low in Tables 2 and 5, respectively, while the type of plant used and the work conditions of the extrusion process are reported in Tables 3 and 6. The features of the multilayer films obtained according to these comparison Examples are finally reported in Tables 4 and 7.

TABLE 2

TABLE 3

Work conditions in the extrusion plant Example 1 and comparison Example 3 Finished tubular film width 300 mm

TABLE 4

Characteristic features

Example 1 and comparison Example 3

(*) Adhesion test described above. ND = no signs of delamination. Some samples may show partial delamination of the layers. In such case the residual adhesion force according to ASTM D1876 (Peel test) is measured.

TABLE 5

TABLE 6

Work conditions in the extrusion plant

Example 2 and comparison Example 3bis

Finished tubular film width 300 mm / thickness 60 micron

TABLE 7

Characteristic features

Example 2 and comparison Example 3bis

(*) Adhesion test described above. ND = no signs of delamination. Some samples may show a partial delamination of the layers. In such case the residual adhesion force according to ASTM D1876 (Peel test) is measured.

Example 4 - 7-layer film according to the invention

Outer layer A) PA

B) Ethylene- -based polymer with tackifying additives or B') Ethylene- -based polymer having a modified rheol ogy

C) EVOH

D) Ethylene- ^■ based polymer with tackifying additives or D') Ethylene- ^■ based polymer having a modified rheol ogy

E) PA

F) Ethylene- ^• based polymer with tackifying additives or

F') Ethylene-based polymer having a modified rheol- ogy

Inner layer G) Polyolefin

The types and the trade names of the raw materials used for the manufacture of the 7-layer film according to Example 4 are listed below in Table 8, compared with those of the similar structure of comparison Example 5, while the type of plant used and the work conditions of the extrusion process are reported in Table 9. The features of the multilayer films obtained according to this Example are finally reported in Table 10.

Comparison Example 5 - 7-layer film of the prior art

Outer layer A) PA

B) PA

C) EVOH

D) PA

E) Ethylene-based polymer with tackifying additives or

E') Ethylene-based polymer having a modified rheol- ogy

F) PO

Inner layer G) PO

The types and the trade names of the raw materials used for the manufacture of the 7-layer film according to comparison Example 5 are listed below in Table 8, while the type of plant used and the work conditions of the extrusion process are reported in Table 9. The features of the multilayer films obtained according to this Example are finally reported in Table 10. TABLE 8

TABLE 9

Work conditions in the extrusion plant Example 4 and comparison Example 5 Finished tubular film width 300 mm

TABLE 10

Characteristic features

Example 4 and comparison Example 5

(*) Adhesion test described above. ND = no signs of delamination. Some samples may show partial layer delamination. In such case the residual adhesion force is measured according to ASTM D1876 (Peel test).

(**) Shrinking test described above.

(***) The gloss values measured on outer layer A are strongly influenced by the polymer nature. In the Examples, the outer layer A is always PA.

Example 6 - 9-layer film according to the invention

Outer layer A) PA

B) Ethylene-based polymer with tackifying additives or B') Ethylene-based polymeric adhesive having a modified rheology

C) PA

D) Ethylene-based polymer with tackifying additives or D') Ethylene-based polymeric adhesive having a modified rheology E) EVOH

F) Ethylene-based polymer with tackifying additives or F') Ethylene-based polymeric adhesive having a modified rheology

G) PA 2

H) Ethylene-based polymer with tackifying additives or H') Ethylene-based polymeric adhesive having a modified rheology

Inner layer I) Polyolefinic plastomer

The types and trade names of the raw materials used for the manufacture of the 9-layer film according to Example 6 are stated in Table 11 below, while the type of plant used and the work conditions of the extrusion process are reported in Table 13. The features of the multilayer films obtained according to this Example are finally reported in Table 14.

TABLE 11

Example 7 - 11-layer film according to the invention

Outer layer A) PA

B) Ethylene-based polymer with tackifying additives or B') Ethylene-based polymeric adhesive having a modified rheology

C) Polyolefin (ex. PPO)

D) Ethylene-based polymer with tackifying additives or D') Ethylene-based polymeric adhesive having a modified rheology

E) PA

F) Ethylene-based polymer with tackifying additives or F') Ethylene-based polymeric adhesive having a modified rheology

G) EVOH

H) Ethylene-based polymer with tackifying additives or H') Ethylene-based polymeric adhesive having a modified rheology

I) PA

L) Ethylene-based polymer with tackifying additives or L') Ethylene-based polymeric adhesive having a modified rheology

Inner layer M) PPO

The types and trade names of the raw materials used to manufacture the multilayer film according to Example 7 are listed in Table 12 below, while the type of plant used and the work conditions of the extrusion process are reported in Table 13. The features of the multilayer films obtained according to this Example are finally reported in Table 14. TABLE 12

TABLE 13

Work conditions in the extrusion plant

Example 6

Width of finished tubular film 300 mm

Example 6 Example 7

Temperature

240/235 240/235 extrusion head °C

Primary speed

11.5 11.5 m/min

Cooling H ₂ 0 temp.

15 14 °C

Primary oven

185/215 180/210 IR 1 °C

Drawing stretch

38.5

m/min 40

Stabilis. oven

165/175

I 2 °C 160/170

TABLE 14

Characteristic features

Examples 6 and 7

(*) Adhesion test described above. ND = no signs of delamination. Some samples may show a partial layer delamination. In such case the residual adhesion force is measured according ASTM D187 (Peel test).

(**) Shrinking test described above.

(***) The gloss values measured on outer layer A are strongly influenced by the nature of the polymer. In the Examples such value has been measured with an outer PA layer (Example 6)

As appears also from the Examples of preparation of the multilayer films of the present invention reported above, the thickness percentage of the adhesive polymer layers arranged in between the PA and EVOH layers according to the present invention, with respect to the overall thickness of the pack of PA and EVOH layers and of adhesive polymer layers arranged in between, responsible for the barrier effect, it is preferably comprised between 10% and 40%. From the description and from the Examples reported above it appears clearly how the multilayer film of the present invention has fully reached the set objects: as a matter of fact, such multilayer film shows substantial improvements of its mechanical and physical properties, in particular as regards stability to layer separation, reduction of the elastic module, reduction of the transmission of water-vapour and of oxygen and increase of the shrinking at lower temperature.

However, it is understood that the invention must not be considered limited to the particular film formulations illustrated above, which represent only exemplifying embodiments thereof, but that a number of variants are possible, all within the reach of person skilled in the field, without departing from the scope of protection of the invention, as defined by the following claims.