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Field of the invention

The present invention relates to a composite material sheet of extremely small thickness but having high strength, to be used in the packaging field, in particular in the field of food packaging.

This application claims priory of the Italian Patent Application n. 102015000027286 (UB2015A001646) filed on June 24, 2015.

State of the art

In the packaging field the films of polymeric material are widely used, for example polyethylene, this material being provided with high impermeability characteristics and good tearing strength, but very difficult to handle when made in thicknesses in the order of few microns: how a so-made polyethylene film is able to "crumple" and "glue" on itself without the possibility of being recovered and flattened again, is well known to everybody.

In the same way films made of aluminum are used, that is a material with high barrier characteristics against UV rays and thus particularly suitable for the protection and preservation of foods. However, aluminum films currently used for packaging products suffer from poor tearing strength, even if their minimum thickness is kept at least equal to 6-10 microns.

It is also to be considered that aluminum is a relatively expensive material and thus it is desired to reduce as much as possible the amount used for the preparation of the packing sheet.

Furthermore, aluminum foils with thickness lower than 50-60 μπι, because of stresses generating in production processes at the rolling mill, have a plurality of micro- holes that, even if not visible to the naked eye, indeed are able to limit the impermeability of the package and the protection of foods against the light.

If the surface density of the micro-holes is excessive, the sheet is not usable for food use.

Also, the normal wrapping operations of the products with aluminum foils can easily cause rips and tears of the sheet itself during its folding to shape to the product, thus affecting its preservation capability.

Another well-known solution, widely and long used for making containers for the transport and preservation of liquids for food use, is that conceived by Tetra Pak, providing for packages made starting from a paper or cardboard substrate intended to give consistency and structural stiffness to the package itself, on which various aluminum and polyethylene layers are coupled.

In any case, the paper or cardboard substrate has relatively high thickness (in addition to thicknesses of the aluminum and polyethylene layers, which are in the order of 6-20 μπι) , so that such a material de facto is only suitable to make containers, but not for example to wrap products .

In any case, at present it doesn't look possible to produce and use single and thin aluminum foils, with thickness lower than 40 μπι, for packing food products, since they are too fragile. The International Patent Application WO 2012/156082 describes a package for foods, such as cream cheese, obtained by using a sheet of multilayer material consisting in turn of an aluminum foil coated with a polymer; the thickness of the polymer is between 6 μπι and 60 μπι. The coating is obtained by extrusion. The so obtained multilayer sheet has high stiffness and can further be laminated with another aluminum foil.

Object and Summary of the Invention

General object of the present invention is to overcome the drawbacks mentioned above by providing a sheet material for packaging food products, both in the form of continuous film and single sheets, that is thin enough to allow saving aluminum material, still being sufficiently sturdy and resistant to guarantee optimal preservation of food products and easy to be handled without possible tear of the sheet.

In view of such an object, a multilayer sheet material according to the invention has been designed, constituted by:

- one or more aluminum layers each having thickness equal or less than 10 μπι,

one or more layers of thermoplastic polymeric material having flexural stiffness in the range of 3000- 14000 kg/cm ² , i.e. 294-1373 MPa, and each having thickness equal or less than 10 μπι.

The aluminum layer/s and the layer/s of thermoplastic polymeric material are coupled alternately one to another, and the multilayer sheet material is embossed.

The multilayer material according to the present invention not only exceeds the limits of traditional solutions, as its mechanical strength is sufficient to allow packaging common use products, for example foods, thus avoiding the drawbacks aluminum shows when is directly in contact with foods (such as the closure of the micro- holes inevitably opening in the aluminum following the thinning process upon lamination) , but also the multilayer material offers an advantage well beyond its mechanical properties. In fact, by taking care to make the aluminum layer with a composition equal to 94-99% pure aluminum and 1-6% ferromagnetic or ferrimagnetic metals, for example in dusts dispersed in aluminum, the just described multilayer material generates heat when subjected to time-varying magnetic fields, such as for example those produced by conventional induction cookers.

In other words, the multilayer material object of the present Patent Application, provided with an aluminum layer doped with ferromagnetic or ferrimagnetic metals with weight not higher than 6% of the overall weight of the aluminum layer - and preferably 3% -, combines the advantages of high mechanical strength offered by the layer or layers of thermoplastic polymeric material that are coupled to the aluminum layer or layers, to the advantages offered by the systems generating heat by induction and offered by ferromagnetic or ferrimagnetic metals.

At this point it could be argued that aluminum is not a ferromagnetic or ferrimagnetic metal; however, the Applicant also found that aluminum behaves as ferromagnetic or ferrimagnetic metals when subjected to time-varying magnetic fields, as those induced by an induction cooker, provided that aluminum is a foil with a thickness lower than 10 μπι and doped as described above.

It is important to underline that very often the aluminum produced, intended to the field of the food packaging, per se is not 100% pure, but has impurities, such as for example small percentages of other, also ferromagnetic or ferrimagnetic, metals.

The multilayer material is thus a candidate to become an innovative system for packaging products, in particular food products, able to guarantee at the same time the correct packaging perfectly air-tight, gas-tight, liquid- tight and solid-tight and, when subjected to magnetic field of an inductor, also the heating and cooking.

For example, the packages made by the multilayer material can be disposable food bags or films, that can be integrated to other primary or secondary packages. For example, the multilayer material can be in turn coupled to a rigid plastic polymer, such as that of paint cans, to make such a package susceptible to heating to cook or heat the material inside them. Such a coupling possibility can be extended to each solid material the already existing packages could be made of, thus allowing to give them heating capability and perfect impermeability (e.g. cardboard, paper, plastic) .

In a first embodiment, the thermoplastic polymeric material is made by a homopolymer or a copolymer of at least one olefin.

In a second embodiment, the thermoplastic polymeric material is made by an ethylene homopolymer or ethylene copolymer with at least one alpha-olefin C3-C8. In a third embodiment, the thermoplastic polymeric material is made by high density polyethylene (HDPE) .

Preferably, the thickness of each aluminum layer is in the range 1-5 μπι.

Preferably, the thickness of each layer of thermoplastic polymeric material is in the range 2-10 μπι.

In the preferred embodiment, there are exclusively two aluminum layers sandwiching between each other one single layer of thermoplastic polymeric material.

The multilayer material can be produced, for example, by spray-coating the thermoplastic polymeric material on an aluminum foil. If there are two aluminum layers, the first sheet on which the thermoplastic polymeric material has been sprayed is laminated with a second aluminum foil.

In an embodiment, the multilayer material comprises, in addition to the layers described above, one further reinforcing layer of waterglass. For example, it is possible to apply one further layer of sodium silicate by spreading it to the above described multilayer, and then subjecting the assembly to heat treatment until obtaining the crystallization or drying of the sodium silicate conferring further stiffness, anyway resulting flexible and impermeable. The thickness of the layer of sodium silicate can be of some microns.

Brief Description Of The Drawings

In order to make clearer the explanation of the innovative principles of the present invention and its advantages compared to the known art, with the aid of the attached drawings a possible exemplary embodiment applying such principles is described below. In the drawings: - figure 1 depicts a schematic and partial view of a multilayer sheet material according to the invention.

figure 2 depicts a sectional view of a sheet according to the invention, composed of three layers.

Detailed Description of the Invention

With reference to the figures, in figure 1 a portion is depicted of a sheet 10 of multilayer material composed of, in the preferred embodiment, one aluminum layer 11 and one layer 12 constituted by a film of thermoplastic polymeric material whose flexural stiffness is in the range of 3000-14000 kg/cm ² , corresponding to 294-1373 MPa; the two layers 11 and 12 are coupled one to another. The aluminum layer 11 has thickness equal or less than 10 μπι, preferably in the range of 1-5 μπι.

The layer 12 of thermoplastic polymeric material has thickness equal or less than 10 μπι, preferably between 2 μπι and 5 μπι.

For the purposes of the present description, by the term "aluminum" is referred to a material composed of aluminum at least 97% by weight, the remaining percentage being made by one or more metallic elements (up to 2.85% by weight) or other elements (up to 0.15% by weight, each of said other elements could be present up to 0.005% by weight) in order to impart the desired physical and/or mechanical characteristics to the sheet.

Preferably, the thermoplastic polymeric material of the layer 12 is made by a homopolymer or a copolymer of at least one olefin (polyolefin) , such as for example polyethylene (PE) , polypropylene (PP) , polyamide (PA) , polyethylene terephthalate (PET) , polyvinyl chloride (PVC) , polyvinylidene chloride or ethylene/vinyl alcohol copolymer (EVOH) .

In a particularly preferred embodiment, the thermoplastic polymeric material of the layer 12 is made by an ethylene homopolymer or ethylene copolymer with at least one alpha-olefin C3-C8, i.e. having overall 3 to 8 carbon atoms (such as polypropylene, 1-butene, 1-pentene, 1-hexene or 1-octene) .

More preferably, said ethylene homopolymer or copolymer with at least one alpha-olefin C3-C8 is a high density one (HDPE) .

Preferably, HDPE has density in the range of 0.945 - 0.965 g/cm ³ (ASTM D 1505 T) .

The use of a thermoplastic polymeric material of HDPE has the advantage to confer greater heat resistance to the multilayer material, thus allowing its use for example for packaging hot food products or food products that, once packaged, must be subjected to moderate heating (for example up to about 100-120°C) .

Preferably, the thermoplastic polymeric material has

Melt Flow Rate in the range of 0.3-10 g/10', more preferably in the range of 0.3-5 g/10' (ASTM D 1238 T) .

Preferably, the thermoplastic polymeric material has softening temperature (ASTM D 1525) in the range of 80- 140°C, more preferably in the range of 90-130°C.

The thermoplastic polymeric material can further comprise appropriate amounts of substances (for example, antibacterial compounds, antioxidants compounds, etc.) adapted to improve the characteristics of the sheet material, in connection with its function of packaging food products .

The layer of polymeric material can be coupled to one of the two sides of the aluminum layer according to the following methods.

For example, the layer 12 of thermoplastic polymeric material can be produced separately, with the desired thickness, and then coupled by lamination process to the aluminum layer 11.

Advantageously, the layer 12 of polymeric material is spray-coated (technique called in English "Thermal Spray Coating"), in the desired grammage, to one of the two sides of the aluminum layer 11, as well known to the technician. As well visible in figure 1, the sheet 10 shows an embossing 13 obtained by passing the sheet between a pair of rollers, at least one of which having a relief surface with a pattern corresponding to the desired embossing pattern, as well known to the skilled person.

For the use purposes of the sheet 10, it is basically indifferent that the relief part of the embossing is on the sheet surface constituted by the aluminum layer 11 and the recessed part of the embossing is on the surface constituted by the layer 12 of thermoplastic polymeric material, or vice versa.

The coupling of the layer 12 of thermoplastic polymeric material with the aluminum layer 11 allows compensating for the problem of the presence of micro-holes that, as reported above, are inevitably generated during the aluminum lamination process to reduce it to a sheet with a thickness of few tens of microns or few microns. In fact, the layer of thermoplastic polymeric material allows restoring the complete impermeability of the aluminum foil against air and liquids.

The layer of thermoplastic polymeric material also allows protecting more the aluminum layer 11 against oxidation phenomena, situation to be avoided as much as possible when food products are packaged.

At the same time, the layer 12 of thermoplastic polymeric material provides the sheet 10 with an increased flexibility, increasing the tearing strength of the simple aluminum layer. Such an effect is particularly pronounced in case the thermoplastic polymeric material is made by HDPE .

The embossing 13 (thanks to its "wavy" pattern, involving both the aluminum layer and the layer of polymeric material) further provides for a material reserve cooperating with the flexibility given by the layer 12 of polymeric material in order to facilitate the foldability of the sheet 10 during the wrapping of the product, following all the shape irregularities, aiding to greatly increase the tearing strength of the sheet 10 and thus keeping the protective characteristics towards the wrapped food product.

For practical use, the multilayer sheet material according to the invention can be provided both in the form of continuous film wrapped around a cardboard or plastic core, and in the form of single sheets pre-cut in the desired shape and size. The multilayer material can be cut in single sheets by the embossing treatment the material itself is subjected to.

In use, the sheet 10 can advantageously be used with the layer 12 of polymeric material in contact with the food product to be packaged, however it is clear that it can also be used with the aluminum layer 11 in contact with the product if the circumstances make it preferable.

In figure 2 a section of a possible further embodiment of the multilayer sheet material according to the invention is schematically depicted.

In this case, there are two aluminum layers 11, 11' sandwiching between a layer 12 of thermoplastic polymeric material having the same characteristics described above. The second aluminum layer 11' can advantageously be coupled, by lamination process, to the surface of the polymeric material 12 on the side opposite to the first aluminum layer 11.

This sandwich configuration allows further increasing the strength of the sheet 10 and, if the two aluminum layers 11, 11' are selected with the same thickness characteristics, they provide to the sheet 10 uniformity of behavior in any direction it is folded in wrapping the products.

As an alternative, the two aluminum layers could be expressly selected of different thickness, in order to obtain specific characteristics of behavior in use.

Obviously, the description given above of an embodiment applying the innovative principles of the present invention is reported as an example of such innovative principles and should not therefore be understood as a limitation of the scope of the herein claimed Patent.

For example, there could be more layers of aluminum and polymeric material with respect to what shown herein; also a composition with one aluminum layer sandwiched between two layers of polymeric material could be provided for, according to the innovative concepts of the present invention.

If there are several aluminum layers and/or several layers of polymeric material, the upper limits of the thickness ranges reported above (i.e. 10 μπι per each of the two materials) could be understood as overall thickness of the layers of the respective material.

Thus, in the embodiment of figure 2, each of the two aluminum layers 11, 11' could for example have thickness equal to 3-5 μπι.

In a possible embodiment, as a material for said one or more aluminum layers, an aluminum alloy comprising 1-2% by weight, based on the weight of the alloy, iron or other ferromagnetic or ferrimagnetic material (as described for example in the Italian Patent Application MI2014A000801 ) could be used, in order to provide adequate thermo- inductive capabilities to the sheet 10 if the latter is subjected to time-varying electromagnetic fields.

The multilayer material where aluminum:

- has thickness lower than 10 μπι, and

- comprises 1-6% by weight of dispersed ferromagnetic or ferrimagnetic powders,

boasts unexpected properties: it heats and generates heat if subjected to time-varying magnetic fields, as those generated by the induction cookers in the kitchens. Therefore, the so made multilayer material is well suited to the realization of food bags, for wrapping and directly cooking foods stored inside it on induction cookers.

Two experiments have been carried out.

Example 1 - Cooking of Fennels

A bag has been made with a multilayer sheet having overall thickness of 10 μπι, embossed as described above, folded on itself and heat-sealed on two open sides. A fennel of 250 about grams, divided in two halves, and a pinch of salt were put in the bag. Subsequently, the left- open third side is heat-sealed. At this point the obtained bag, completely closed around the two fennel halves, has been placed on an induction plate powered at 200 watts for the duration of 15 minutes. The bag swelled thus creating an inner microclimate allowing a complete and uniform cooking of the fennel similar to steam cooking, with final browning of the surface in contact with the embossed multilayer bag and with the plate.

Example 2 - Cooking of Frozen Chicken Thighs

A bag has been made, starting from a multilayer sheet of 10 μπι embossed as described above, folded on itself and heat-sealed on two open sides. In the forming bag, a frozen chicken thigh of about 150 grams and a pinch of salt were put in. At this point, the bag has been completely closed, by heat-sealing the third side that remained open until that moment. The closed bag has been placed on an induction plate in contact therewith, powered at 200 watt for the duration of 25 minutes. The bag swelled thus creating an inner microclimate allowing the defreezing of the chicken thigh and its complete and uniform cooking similar to steam cooking, with final browning of the surface in contact with the embossed multilayer bag and with the plate.