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Field of the invention

The present invention relates to a tray container of composite material to be used for containing and packaging food products.

The present application claims priory of the Italian Patent Application n. 102015000027285 (UB2015A001593) filed on June 24, 2015.

State of the art

In the field of food packaging, aluminum trays made starting from an aluminum foil generally having thickness greater than 80 μπι, typically between 100 μπι and 200 μπι, deep-drawn in the desired shape and sizes, are widely used.

The above mentioned thickness is obviously required to provide the best possible structural stiffness to the tray and to avoid tears of the sheet during molding operations .

The aluminum trays have a number of advantages, such as for example small sizes allowing an easy transport of foods therein contained, in addition with the possibility to subject foods to heating and/or cooking while they are directly contained in the tray itself.

Moreover, aluminum is a relatively expensive material and thus, although it is completely recyclable, it is desired to reduce as much as possible the amount used for the preparation of the trays. The reduction of the aluminum amount constituting the tray is also desirable for generally reducing the weight of the container and, thus, the transport costs.

Thus, it would be desirable being able to make aluminum trays with foils of small thickness, preferably lower than 60-70 μπι.

However, the 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 de facto, even if not visible to the naked eye, being able to reduce the impermeability of the walls of the tray and, thus, prevent a correct protection and preservation of foods .

Furthermore, aluminum foils of such thickness would easily be subjected to rips and tears during the molding operations of the trays, thus generating important amounts of processing wastes.

FR-A-2607110 describes a multilayer container for foods. A first layer is constituted by an aluminum foil, which in turn is preferably coated with a polyester film that boasts heat-sealing properties. The second layer is made by the technique of the injection molding: the aluminum foil, suitably deep-drawn and pleated to form a tray, is put in the mold and at the outer surface a thermoplastic material is injected that, by solidifying, externally adheres to the tray, thus making it rigid and sturdy. In this solution the polyester film remains in contact with the food and withstand temperatures of about 130°C. The container structure allows for sterilizing the food without container deformation.

The solution described in FR-A-2607110 overcomes the problem of rips, but is affected by the following drawbacks .

At temperatures higher than 80°C, the polyesters generally show hydrolysis phenomena in the presence of water. This phenomenon creates a negative synergy with the aluminum porosity described above. In other words, by using an aluminum foil with thickness lower than 60 μπι - thus affected by the presence of micro-holes - and a polyester film, the resulting multilayer could easily be permeable to water and to the liquids released by the foods. Therefore in the solution described in FR-A-2607110 , the outer rigid layer, obtained by molding a thermoplastic material, is mandatory, i.e. it must be present to guarantee the isolation of food. In other words, the only aluminum portion coated with polyester would not be sufficient by itself to guarantee the correct preservation of foods.

Further, by increasing the temperature and much before reaching the softening point, polyesters show a decline of relative physical characteristics. Therefore, the heating up to 130°C could easily make the container not reusable for a second time.

Another drawback ascribable to the solution described in FR-A-2607110 is related to the thickness of the polyester film coating the aluminum layer. By increasing the thickness, the polyesters show an increase of superlinear type of the structural fragility of their semi- crystalline matrix, i.e. an increase greater than a linear increase. Therefore, when a layer of polyester is coupled to a layer of a different material having different thickness, such as aluminum described in the document, the mechanical behavior of the two coupled layers is not the same; in other words, the response to mechanical stress, such as for example the bending, would be different between the polyester film and the aluminum layer. For example, the thermoformed full-matrix polyesters having thicknesses higher than 50-100 micrometers demonstrated being very fragile against impacts, thus not suitable for the use in the containers described above because a number of micro- cracks would form and frustrate the impermeability effect initially created by coupling the two materials.

Other solutions known in the art are described in US

5,199,595, EP 2145759, EP 1048438, EP 0140282, EP 0132468, WO 89/08549, JP 2000/109129 and US 2011/120993.

Summary of the Invention

General object of the present invention is to compensate for the drawbacks mentioned above by providing a tray container made of aluminum material for containing and packaging food products, and the relative production method, that allow an appreciable reduction of the aluminum amount needed to produce it but, at the same time, that has a sufficiently sturdy structure.

Further object of the invention is to provide a tray container for containing and packaging food products that, still with a small thickness of aluminum, guarantees food protection and the possibility of cooking them comparably to the aluminum trays currently present on the market.

In view of such an object it has been thought to implement, according to the invention, a tray container for containing and packaging food products, comprising a bottom and side walls, made for example by deep-drawing from a sheet material, characterized in that said bottom and said side walls are made by a multilayer sheet material comprising at least one aluminum or aluminum alloy layer having thickness equal or less than 60 μπι, and at least one layer of thermoplastic polyolefin material having thickness equal or less than 10 um, said at least one aluminum or aluminum alloy layer and at least one layer of thermoplastic polyolefin material being coupled one to another and the multilayer sheet material being embossed.

The just described tray container has a plurality of advantages compared to the known art.

The thermoplastic polyolefin material closes possible micro-holes being in the aluminum layer, thus guaranteeing complete impermeability of the multilayer to air and liquids also when the aluminum has thicknesses lower or equal to 60 μπι. Furthermore, the layer of thermoplastic polyolefin material protects the aluminum layer against oxidation phenomena and provides an increased flexibility and tearing strength to the multilayer compared to the simple aluminum layer.

These results are obtained by compensating for the drawbacks described above related to the known art, and in particular related to the polyester.

In fact, polyolefins are subjected to decline at temperatures higher than 115°C, by far higher than what described for the polyester and in any case compatible with the cooking of foods.

Furthermore, polyolefins have better mechanical behavior than the polyester for what concerns the stress response. In general, polyolefins are less fragile at the same thickness and thus are less subjected to the formation of rifts and cracks when a thick layer of a thermoplastic polyolefin material is coupled to a thin aluminum layer.

Ultimately, the multilayer according to the present invention is particularly suitable to preservation and cooking of foods without the need of coupling other materials and/or other layers to the multilayer itself, as instead provided in FR-A-2607110. The multilayer is effective per se, and further layers are optionally applied .

Preferably, the thermoplastic polyolefin material 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. More preferably, the ethylene homopolymer or ethylene copolymer with at least one alpha-olefin C3-C8 is a high density one (HDPE) , for example in the range 0.945 - 0.965 g/cm ³ (ASTM D 1505 T) .

A second aspect of the present invention relates to a method according to claim 11 for producing a tray container for packaging food products.

The method comprises the steps of:

a) providing an aluminum or aluminum alloy film having thickness equal or less than 60 μπι;

b) providing a film of thermoplastic polyolefin material, having thickness equal or less than 10 μπι;

c) coupling, by lamination, the film of thermoplastic polyolefin material to the aluminum or aluminum alloy film, to obtain a multilayer film;

d) embossing the multilayer film, and

e) deep-drawing the embossed multilayer film to obtain the tray for packaging food products. Preferably, step e) is carried out cold, i.e. at room temperature .

Step c) can also be carried out with the aid of an adhesive material, for example a resin interposed between the two films.

As an alternative to steps b) and c) , the method according to the present invention comprises the steps of: b' ) providing a thermoplastic polyolefin material at the liquid state, for example at the relative melting temperature, and

c' ) spray-coating the thermoplastic polyolefin material on at least one side of the aluminum or aluminum alloy film until making a layer solidifying on cooling and having thickness equal or less than 10 μπι.

In both modes, the method allows to overcome the problem of the micro-holes present in the aluminum or aluminum alloy film. The layer of thermoplastic polyolefin material, whether is coupled by lamination or sprayed, restores the complete impermeability against air and liquids of the aluminum or aluminum alloy film.

Advantageously, it is not needed to provide for the molding step described in the document FR-A-2607110.

The tray container obtained with the just described method doesn' t need the rigid bottom layer described in the document FR-A-2607110.

In an alternative implementation of the described method, the layer of thermoplastic polyolefin material is in turn protected with a layer of an insulating polymeric material resistant to high temperatures, that for example can be spray-coated or laminated. For example organopolysiloxanes , commonly known as silicones, and the relative derivatives such as the silicone rubbers are all suitable to the purpose. The layer of insulating polymeric material is between 2 microns and 20 microns and can be applied simultaneously or immediately after step c) or c' ) · The layer of insulating polymeric material allows withstanding even higher temperatures, up to 250-300°C, avoiding the release of volatile polyolefin components and allowing an extreme cooking in oven.

The present invention, in a third aspect thereof, relates to a tray container directly obtained with the just described method.

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 shows a tray container for containing and packaging food products to which the innovative principles of the present invention are applied;

- figure 2 shows a schematic and partial view of a multilayer sheet material for implementing the tray container of figure 1 ;

- figure 3 shows a sectional view of a multilayer sheet composed of three layers, for implementing the tray container of figure 1.

Detailed Description of the Invention

With reference to the figures, in figure 1 it is depicted a tray container 10 for containing and packaging food products, constituted by a bottom 11 and side walls 12. It is, as it is well clear by the figure, a container structure of the open-top type, that might be closed if desired with a specific lid or aluminum/polymeric material film suitable for food use.

For example, according to the invention, the tray 10 is made by deep-drawing a sheet 13 (a portion of which is shown in figure 2, on a greatly magnified scale) being of multilayer material composed, in the preferred embodiment, of an aluminum layer 14 and a layer 15 constituted by a film of thermoplastic polyolefin material, the two layers being coupled one to another.

The aluminum layer 14 has thickness equal or less than 60 μπι, preferably between 10 μπι and 40 μπι. The layer 15 of thermoplastic polyolefin material has thickness equal or less than 10 μπι, preferably between 3 μπι and 8 μπι.

For the purposes of the present description, by the terms "aluminum" and "aluminum layer" is respectively made reference to a material or material layer composed of aluminum at least 99% by weight.

Furthermore, still for the purposed of the present description, by the terms "aluminum alloy" and "aluminum alloy layer" is respectively made reference to a material or material layer composed of aluminum at least 96% by weight, the remaining percentage being made by one or more metallic elements (up to 3% by weight) or other elements (up to 1% by weight, each of said other elements being possibly present up to 0.005% by weight) in order to impart to the tray the desired physical and/or mechanical characteristics. Furthermore, according to the present invention, the verb "comprising" and all the terms derived from this, as used herein in the description and claims, also include the meaning of the verb "consisting of" and of the terms derived from this.

According to the present invention, the thermoplastic polyolefin material of the layer 15 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 polyolefin material of the layer 15 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 ethylene copolymer with at least one alpha- olefin C ₃ -C ₈ 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) .

In another preferred embodiment, said thermoplastic polyolefin material is made by a propylene homopolymer or propylene copolymer with at least one alpha-olefin C3-C8.

The use of a thermoplastic polymeric material, such as HDPE or polypropylene (homopolymer or copolymer) , has the advantage to give greater heat resistance to the multilayer material, thus allowing its use for example to package hot food products or food products that, once packaged, must be subjected to heating and/or cooking (for example, up to at least about 100-120°C or even up to 150- 180°C) .

Preferably, the thermoplastic polyolefin material has a Melt Flow Rate in the range of 0.3-10 g/10', more preferably in the range of 0.3-5 g/10' (ASTM D1238 T) .

The thermoplastic polyolefin 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 thermoplastic polyolefin material can be coupled to one of the two sides of the aluminum or aluminum alloy layer, according to the following methods. For example, the layer of thermoplastic polyolefin material can be produced separately to the desired thickness, and then coupled by lamination process to the aluminum or aluminum alloy layer 14, possibly with the aid of an adhesive material (for example, a polyurethane, polyester or acrylic resin) . Advantageously, the layer 15 of thermoplastic polyolefin material is spray-coated (technique called in English "Thermal Spray Coating"), in the desired grammage, to one of the two sides of the aluminum or aluminum alloy layer 14.

As well visible in figure 2, the sheet 13 shows an embossing 16 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 technician.

For the use purposes of the sheet 13, it is basically indifferent that the relief part of the embossing is on the sheet surface constituted by the aluminum layer 14 and the recessed part of the embossing is on the surface constituted by the layer 15 of thermoplastic polyolefin material, or vice versa. The coupling of the layer 15 of thermoplastic polyolefin material with the aluminum layer 14 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 thickness of few tens of microns or few microns. In fact, the layer of thermoplastic polyolefin material allows restoring the complete impermeability of the aluminum foil against air and liquids.

The layer of thermoplastic polyolefin material also allows protecting more the aluminum layer 14 against oxidation phenomena, situation to be avoided as much as possible when food products are packaged.

At the same time, the layer 15 of thermoplastic polyolefin material provides the sheet 13 with an increased flexibility, increasing the tearing strength of the simple aluminum layer. Such an effect is particularly pronounced in case the thermoplastic polyolefin material is made by HDPE or polypropylene.

The embossing 16 (thanks to its "wavy" pattern, involving both the aluminum layer and the layer of thermoplastic polyolefin material) further provides a material reserve cooperating with the flexibility given by the layer 15 of thermoplastic polyolefin material, in order to facilitate the processability of the sheet 13 during the molding operation of the tray by deep-drawing it, aiding to greatly increase the tearing strength of the sheet and thus keeping the barrier characteristics towards the food product contained in the tray.

The operation of forming the tray 10 starting from the sheet 13 is advantageously made by keeping the layer 15 of thermoplastic polyolefin material at the inside of the tray, i.e. on the side intended to be in contact with the food product. The thermoplastic polyolefin material is thus avoided from directly contacting possible heating surfaces on which the tray should be leant for cooking the foods contained therein.

In figure 3 a section of a possible further embodiment of the multilayer sheet intended for the tray forming according to the invention, is schematically depicted .

In this case, there are two aluminum or aluminum alloy layers 14, 14' sandwiching a layer 15 of thermoplastic polyolefin material, having the same characteristics described above. The second aluminum or aluminum alloy layer 14' can advantageously be coupled, by lamination process, to the surface 15 of the thermoplastic polyolefin material opposite to the first aluminum or aluminum alloy layer 14.

This "sandwich" configuration allows further increasing the strength of the sheet 13 and, if the two aluminum layers 14, 14' are selected with the same thickness characteristics, also provides to the sheet 13 uniformity of behavior in any direction the operation of deep-drawing is made. 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 thermoplastic polyolefin material than what shown herein .

If there are more aluminum layers and/or more layers of thermoplastic polyolefin material, the aforementioned upper limits of the thickness ranges (i.e. respectively 60 μπι and 10 μπι) have to be understood as overall thickness of the layers of the respective material.

Thus for example, in the embodiment of figure 3, each of the two aluminum layers 14, 14' could have maximum thickness equal to 30 μπι.

In order to increase the structural strength of the tray, obviously it could be made, during the deep-drawing operation, with suitable stiffening ribs, as well known to the technician.

Finally, as mentioned above, in the present description reference was made to aluminum layers, however meaning that such layers could be made of an aluminum alloy comprising an appropriate percentage of other metallic and non-metallic elements. In a preferred embodiment, for example, such an aluminum alloy comprises 1-3% by weight, based on the weight of the alloy, iron or other ferromagnetic (as described for example in the Italian Patent Application MI2014A000801 ) or ferrimagnetic material (as described for example in the Italian Patent Application 102015000066016 filed on 27.10.2015), in order to provide to the sheet 13 adequate ferromagnetic characteristics making the tray suitable to allow direct heating of the food product contained therein, on induction cookers.