DE102022005098A1 - Retort-sterilizable layer material made predominantly from polypropylene - Google Patents

Abstract

The present invention relates to a retort-sterilizable layer material (10) for forming packaging, comprising a first polypropylene layer (12) and a second polypropylene layer (26), wherein a barrier layer arrangement (19) is arranged between the first polypropylene layer (12) and the second polypropylene layer (26), wherein the barrier layer arrangement (19) has a first polymer barrier layer (14) and a second polymer barrier layer (22), wherein the second polymer barrier layer (22) is part of a coextrusion layer arrangement (21) which, as an extrusion laminated layer arrangement (23), connects the first (12) and the second polypropylene layer (26) to one another.

Description

The present invention relates to a retort-sterilizable sheet material which is suitable as a flexible sheet material for forming packaging, such as packaging bags, in particular for packaging foodstuffs.

Common layer materials or layer composites for the production of retort-sterilizable packaging are composites made of polyethylene terephthalate (PET), aluminum foil and polypropylene (PP). The aluminum foil provides the necessary barrier against the migration of oxygen and water vapor or moisture through the layer composite. The PP layer, as a sealable layer, can create the possibility of bonding to itself or to another compatible layer. The PET layer gives the layer composite the necessary robustness against external mechanical influences and increases the bursting strength of packaging made from the known layer composite.

The disadvantage of the well-known layer composite made of PET, aluminum foil and PP is that it is difficult to recycle, since the three layers mentioned are made of materials from different material classes, which have to be recycled in separate recycling streams. With the increasing focus on sustainability in packaging technology, the well-known layer composites are becoming less and less preferred, although they have the advantage of being retort-sterilizable. For perishable goods, such as human and animal food, packaging is retort-sterilized, whereby the packaging and thus the layer material forming it are exposed to a high temperature of, depending on the specific process, 110 °C to 135 °C in order to achieve a sterilization effect. The aluminum foil can easily withstand this thermal sterilization, especially by steam. Polymer barrier layers, on the other hand, are sensitive to the thermal stress of retort sterilization and often lose their barrier effect due to sterilization.

From the EP 3 991 967 A1 A tube laminate free of aluminium foil with a layer of aluminium oxide or silicon oxide arranged between two vinyl alcohol layers is known.

Another disadvantage of known packaging films with barrier properties is the adhesive lamination of layers to one another. Isocyanates are usually used as adhesives, which represent further foreign substances in relation to the polymers used to produce the layers and make the recycling of the known layer materials more difficult.

It is an object of the present invention to provide a retort-sterilizable layer material with a highly effective barrier against migration of oxygen and water vapor, which can undergo recycling processes more easily than the known layer material with aluminum foil and can thus be reused.

The present invention solves this problem by means of a layer material which comprises a first polypropylene layer and a second polypropylene layer, wherein a barrier layer arrangement is arranged between the first polypropylene layer and the second polypropylene layer. The barrier layer arrangement has a first polymer barrier layer and a second polymer barrier layer. The second polymer barrier layer is part of a coextrusion layer arrangement which, as an extrusion laminate layer arrangement, connects the first and second polypropylene layers, in particular the first polymer barrier layer and the second polypropylene layer, to one another.

With the above-mentioned structure, the layer material of the present application does not require metal foil and even metallization. By forming the second polymer barrier layer as part of an extrusion lamination layer arrangement, the second polymer barrier layer can not only act as a barrier against the migration of undesirable substances through the layer material, compared to a cover layer of the same thickness made of unstretched conventional polypropylene, but can also be used as a connecting layer arrangement for connecting the first and second polypropylene layers to one another. This means that the use of a lamination adhesive layer can be avoided.

In the present application, the terms “layer material” and “layer composite” are used synonymously.

When the present application refers to a barrier effect against the migration of oxygen, this means a barrier effect which influences the oxygen permeation through the layer material, as measured according to ISO 15105-2 (Annex A) under test conditions at 23 °C and 50 % relative humidity.

When the present application refers to a barrier effect against the migration of water vapor, this means a barrier effect which influences the water vapor permeation through the layer material, as measured in accordance with DIN EN ISO 12572 under test conditions at 23 °C and 85 % relative humidity.

The layer material discussed here achieves an oxygen permeation according to the ISO standard mentioned of no more than 2 cm ³ /(d·m ² ), preferably no more than 1 cm ³ /(d·m ² ). It achieves a water vapor permeation according to the ISO standard mentioned of no more than 2 g/(d·m ² ), preferably no more than 1 g/(d·m ² ). Due to its ability to be retort sterilized, it is therefore ideally suited for the production of packaging for perishable goods, such as ready meals and pet food.

In principle, the first and second polypropylene layers can be any polypropylene layer. The barrier effect and mechanical strength of the layer material can be increased by making the first polypropylene layer a monoaxially or biaxially stretched polypropylene layer.

Furthermore, the first polypropylene layer can support the bonding of the layer material by having a layer of maleic anhydride-grafted polypropylene on its side facing the second polypropylene layer. The layer of maleic anhydride-grafted polypropylene has an adhesion-enhancing effect in a layer composite made predominantly of polypropylene. The layer material discussed here is preferably made predominantly of polypropylene material.

In principle, the second polypropylene layer can also be stretched monoaxially or biaxially in order to increase its mechanical resistance and its barrier properties against the migration of undesirable substances through the layer composite. However, to form packaging with an enclosed packaging space from the layer composite, it is preferred that the second polypropylene layer is a sealable polypropylene layer. As a sealable, i.e. heat-sealable, polypropylene layer, the second polypropylene layer can comprise a conventional polypropylene layer, on the side of which facing away from the barrier layer arrangement a thin polypropylene layer with a lower sealing initiation temperature is arranged, which provides the sealability.

In this respect, the first and second polypropylene layers can be structurally similar in such a way that each of the polypropylene layers comprises a first polypropylene base layer and has a polypropylene functional layer arranged on one side thereof. In the case of the first polypropylene layer, the polypropylene functional layer is the above-mentioned maleic anhydride-grafted polypropylene layer and in the case of the second polypropylene layer, the polypropylene functional layer is the above-mentioned sealable polypropylene layer. In both cases, the polypropylene base layer is thicker, preferably at least twice, particularly preferably at least three times, even more preferably at least four times thicker, than the polypropylene functional layer.

Preferably, the two polymer barrier layers are specialized in such a way that the first polymer barrier layer prevents a different substance from migrating through the layer material more than the second polymer barrier layer. The migrating substances with the greatest potential for problems, especially for the packaging of perishable goods, are oxygen and water vapor. Oxygen promotes the oxidation of packaged goods. Water vapor promotes moistening and thus, on the one hand, soaking and, on the other hand, microbial growth in packaged goods.

Preferably, the first polymer barrier layer impedes migration of oxygen therethrough more than the second polymer barrier layer. Preferably, the second polymer barrier layer impedes migration of water vapor therethrough more than the first polymer barrier layer.

A very good barrier effect against oxygen and also against fat can preferably be obtained on the first polymer barrier layer by the first polymer barrier layer comprising polyvinyl alcohol or being formed from polyvinyl alcohol. A polyvinyl alcohol layer as the first polymer barrier layer can advantageously be applied in a solvent-based wet roller application to the first polypropylene layer, more precisely to its preferred exposed maleic anhydride-grafted layer.

In order to convey consumer information, the first polypropylene layer, which on a package formed from the layer material preferably forms the outside of the package or is at least located closer to the outside than to the inside of the package, can be printed with printing inks.

To protect the printing ink application from external influences, it is preferred that the first polypropylene layer, particularly preferably the first polymer barrier layer, is printed with printing inks in reverse printing. The first polypropylene layer can then protect not only the first polymer barrier layer, but also the printing inks from external influences. Preferably, the first polypropylene layer and the first polymer barrier layer are transparent, so that the printing ink application on a packaging made from the layer material is clearly visible from the outside.

A particularly good adhesion of the printing inks to the first polymer barrier layer can be achieved by using polyurethane-based printing inks.

In order to achieve the best possible bond between the printing inks and the immediately adjacent layer of polypropylene material, according to a preferred development of the present invention, a primer layer can be applied to the side of the printing ink application facing away from the first polymer barrier layer, which has an adhesion-enhancing effect for polypropylene material compared to a state without the primer layer. The extrusion lamination layer arrangement can be applied directly to this primer layer or can adjoin this primer layer.

The second polymer barrier layer may comprise or be a cyclic olefin copolymer (COC). This COC layer can not only be very well integrated into a coextrusion layer arrangement with at least one other polypropylene layer, but can also be processed in a polypropylene recycling stream for reuse.

In principle, the second polymer barrier layer can be applied directly to the printing ink layer or to the primer layer or can be adjacent to it. An even higher bond strength and thus an even higher delamination resistance of the layer material can be achieved by the coextrusion layer arrangement having, in addition to the second polymer barrier layer, a polypropylene connecting layer which is arranged between the first polymer barrier layer and the second polymer barrier layer.

The polypropylene connecting layer can, on the one hand, be directly adjacent to the printing ink layer or, if present, to the primer layer applied to the printing ink layer and, on the other hand, can be directly adjacent to the COC layer by coextrusion. To increase the bonding in the area of the polypropylene connecting layer, according to a preferred development of the invention, the polypropylene connecting layer can comprise a maleic anhydride-grafted polypropylene or be formed from a maleic anhydride-grafted polypropylene.

In principle, it can again be provided that the second polymer barrier layer, in particular comprising COC or designed as a COC layer, is directly connected to the second polypropylene layer. However, tests have shown that a higher bond strength and thus delamination safety can be achieved if the coextrusion layer arrangement has a polypropylene coupling layer in addition to the second polymer barrier layer, which is arranged between the second polymer barrier layer and the second polypropylene layer. The polypropylene coupling layer is merely a linguistic distinction from the polypropylene connecting layer. Nevertheless, the polypropylene coupling layer is preferably a normal polypropylene layer without maleic anhydride grafting. However, within the scope of the present invention, it should not be ruled out that the polypropylene coupling layer also comprises a maleic anhydride-grafted polypropylene or is formed from such a layer in order to further increase adhesion.

In order to ensure that the layer material is in a single recycling stream for polypropylene, it is preferably provided that the layer material is formed of at least 90 wt.%, particularly preferably at least 95 wt.%, of polypropylene material.

Preferably, the layer material comprises only the layers and materials mentioned above. Particularly preferably, the layer material is free of metal foils and/or metallization.

In a preferred embodiment, the first polypropylene layer can have a thickness of at least 15 µm, particularly preferably at least 20 µm. In the preferred embodiment, the first polypropylene layer can have a thickness of no more than 30 µm, particularly preferably no more than 25 µm. The same applies to the second polypropylene layer.

The first polymer barrier layer can have a thickness of at least 0.4 µm, preferably at least 0.5 µm. Likewise, the first polymer barrier layer can have a thickness of no more than 1 µm, preferably no more than 0.8 µm and particularly preferably no more than 0.7 µm. In a preferred embodiment, the first polymer barrier layer has a thickness of 0.6 µm. If the first polymer barrier layer is preferably made of polyvinyl alcohol, the thickness of the first polymer barrier layer in micrometers preferably corresponds in amount to the application weight in grams per square meter.

The second polymer barrier layer can have a thickness of at least 5 µm, preferably at least 7 µm. Likewise, the second polymer barrier layer can have a thickness of no more than 15 µm, preferably no more than 12 µm and particularly preferably no more than 10 µm. In a preferred embodiment, the second polymer barrier layer has a thickness of 8 µm.

The entire coextrusion layer arrangement, including the second polymer barrier layer, can have a thickness of at least 15 µm, preferably at least 18 µm. Likewise, the entire coextrusion layer arrangement can have a thickness of no more than 30 µm, preferably no more than 26 µm and particularly preferably no more than 23 µm. In a preferred embodiment, the second polymer barrier layer has a thickness of 20 µm.

The printing ink application preferably has a thickness of at least 2.2 µm, particularly preferably at least 2.5 µm. Likewise, the printing ink application preferably has a thickness of no more than 5 µm, particularly preferably no more than 3.5 µm.

The primer layer, if present, between the printing ink application and the adjacent extrusion lamination layer arrangement preferably has a thickness of at least 0.6 µm, particularly preferably of at least 0.9 µm. Likewise, the primer layer preferably has a thickness of no more than 1.6 µm, particularly preferably of no more than 1.2 µm.

• 1 eine grobschematische Querschnittansicht einer erfindungsgemäßen Ausführungsform eines Lagenmaterials.

The present invention will be explained in more detail below with reference to the accompanying drawings. It shows:

• 1 a roughly schematic cross-sectional view of an embodiment of a layer material according to the invention.

In 1 an embodiment of a previously described layer material according to the invention is shown in a roughly schematic cross-sectional view and is generally designated 10. An exposed surface 10a of the layer material 10 forms a later inner surface of a package made from the layer material 10, facing a packaged product. The opposite exposed surface 10b forms an outer surface of a package made from the layer material 10, facing the consumer. Due to the orientation on the package made from the layer material 10, the free surface 10b of the layer material 10 is also referred to as the outer surface of the layer material 10.

The outer surface 10b of the layer material 10 is also an outer surface 12b of a first polypropylene layer 12, which as a monoaxially or biaxially stretched polypropylene layer 12 has a higher barrier effect with respect to the migration of oxygen and water vapor through the layer material 10 compared to an unstretched polypropylene layer of the same thickness and which, due to its stretching, gives the layer material 10 an increased basic strength and basic stability.

On its opposite surface 12a, which faces the inner surface 10a of the layer material 10, the first polypropylene layer 12 is provided with a layer 14 of polyvinyl alcohol, preferably applied wet by roller application. The layer 14 of polyvinyl alcohol forms a first polymer barrier layer. Due to its polyvinyl alcohol material, it has an increased barrier effect or a lower permeation value with regard to the migration of oxygen and fat through the layer 14 compared to a comparable layer of the same thickness made of unstretched polypropylene.

The first polymer barrier layer 14 is part of a barrier layer arrangement 19.

For better adhesion of the layer 14 to the first polypropylene layer 12, the first polypropylene layer 12 is provided on its side facing the inner surface 10a with a thin layer 12' which comprises maleic anhydride-grafted polypropylene or is formed from maleic anhydride-grafted polypropylene. The thin layer 12' forms the surface 12a of the first polypropylene layer 12 facing the inner surface 10a.

The first polymer barrier layer 14 is printed with a printing ink application 16 in reverse printing, so that when the layer material 10 is viewed from the outside, the printed image can be seen through the first polypropylene layer 12. A thin primer layer 18 is applied to the side of the printing ink application 16 facing away from the first polymer barrier layer 14, for example again wet in a roller application, in order to increase the adhesion of the laminate layer arrangement 21 to the layer arrangement made up of the first polypropylene layer 12, first polymer barrier layer 14, printing ink application layer 16 and primer layer 18.

In the illustrated embodiment, the coextrusion layer arrangement 21 comprises three coextruded layers 20, 22 and 24.

Of the coextruded layers 20, 22 and 24, layer 22 is a layer of cyclic olefin copolymer (COC) which, due to its material properties, serves as a water vapor barrier layer since it has a higher water vapor barrier effect or a lower water vapor permeation compared to a comparable layer of the same thickness made of unstretched polypropylene. Layer 22 is a second polymer barrier layer.

In order to bond the second polymer barrier layer 22 to the layer arrangement of the first polypropylene layer 12, first polymer barrier layer 14, printing ink application layer 16 and primer layer 18 in a delamination-proof manner, the coextrusion layer arrangement The layer 21 has, on the side facing the primer layer 18, a polypropylene layer 20 which, as a polypropylene connecting layer 20, comprises maleic anhydride-grafted polypropylene or is formed from such.

On the side of the second polymer barrier layer 22 opposite the polypropylene connecting layer 20, the coextrusion layer arrangement 21 has a polypropylene coupling layer 24 which connects the coextrusion layer arrangement 21 to a second polypropylene layer 26. Due to the material identity of the polypropylene coupling layer 24 and the second polypropylene layer 26, it is not necessary to use grafted polypropylene in the polypropylene coupling layer 24. The polypropylene coupling layer 24 can thus be formed from conventional polypropylene in the same way as the second polypropylene layer 26.

The coextruded layer arrangement 21, which is part of the barrier layer arrangement 19, thus serves as an extrusion laminating layer arrangement 23 in order to connect the first polypropylene layer 12 together with the first polymer barrier layer 14, and optionally with the printing ink layer 16 and the primer layer 18, to the second polypropylene layer 26 by extrusion laminating.

The second polypropylene layer 26 preferably has a thin layer 26' of polypropylene with a lower sealing initiation temperature at its end region in the thickness direction forming the inner surface 10a of the layer material 10 or the inner surface 26a of the second polypropylene layer 26. The second polypropylene layer 26 is thus a sealable polypropylene layer which can be sealed to itself or to a similar polypropylene layer to form a package.

The layer material 10 can be provided as a roll and unwound from a supply roll and cut to size in order to produce packaging, such as packaging bags, stand-up pouches, four-edge seal bags, three-edge seal bags and the like, from the blanks obtained by cutting.

The layer material 10 can have an oxygen permeability of less than 1 cm ³ /(d·m ² ) and a water vapor permeability of less than 1 g/(d·m ² ) according to the ISO standards mentioned above. The layer material 10 is thus a high-barrier film which advantageously does not lose its barrier effect even after carrying out retort sterilization processes customary for food packaging.

Not only is the layer material 10 retort-sterilizable without loss of barrier properties, it also consists of at least 90% by weight, particularly preferably at least 95% by weight, of material that can be recycled in a single polypropylene recycling stream. The layer material 10 is therefore particularly easy and advantageous to recycle after use.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• EP 3991967 A1 [0004]

Claims (14)

Retort-sterilizable layer material (10) for forming packaging, comprising a first polypropylene layer (12) and a second polypropylene layer (26), wherein a barrier layer arrangement (19) is arranged between the first polypropylene layer (12) and the second polypropylene layer (26), wherein the barrier layer arrangement (19) has a first polymer barrier layer (14) and a second polymer barrier layer (22), wherein the second polymer barrier layer (22) is part of a coextrusion layer arrangement (21) which, as an extrusion laminate layer arrangement (23), connects the first (12) and the second polypropylene layer (26) to one another. Retort-sterilizable layer material (10) according to Claim 1 , characterized in that the first polypropylene layer (12) is a monoaxially or biaxially stretched polypropylene layer. Retort-sterilizable layer material (10) according to Claim 1 or 2 , characterized in that the first polypropylene layer (12) has a layer (12') with maleic anhydride-grafted polypropylene on its side facing the second polypropylene layer (26). Retort-sterilizable layer material (10) according to one of the preceding claims, characterized in that the second polypropylene layer (26) is a sealable polypropylene layer. Retort-sterilizable layer material (10) according to one of the preceding claims, characterized in that the first polymer barrier layer (14) comprises or is a polyvinyl alcohol layer. Retort-sterilizable layer material (10) according to one of the preceding claims, characterized in that the first polymer barrier layer (14) is printed with printing inks in reverse printing Retort-sterilizable layer material (10) according to Claim 6 , characterized in that the printing inks are polyurethane-based inks. Retort-sterilizable sheet material (10) according to Claims 6 and 7 , characterized in that on the side of the printing ink application (16) facing away from the first polymer barrier layer (14), a primer layer (18) is applied, which, compared to a state without the primer layer (18), has an adhesion-enhancing effect for polypropylene material. Retort-sterilizable layer material (10) according to one of the preceding claims, characterized in that the second polymer barrier layer (22) comprises or is a cyclic olefin copolymer. Retort-sterilizable layer material (10) according to one of the preceding claims, characterized in that the coextrusion layer arrangement (21) has, in addition to the second polymer barrier layer (22), a polypropylene connecting layer (20) which is arranged between the first polymer barrier layer (14) and the second polymer barrier layer (22). Retort-sterilizable layer material (10) according to Claim 10 , characterized in that the polypropylene connecting layer (20) comprises a maleic anhydride-grafted polypropylene or is formed from a maleic anhydride-grafted polypropylene. Retort-sterilizable layer material (10) according to one of the preceding claims, characterized in that the coextrusion layer arrangement (21) has, in addition to the second polymer barrier layer (22), a polypropylene coupling layer (24) which is arranged between the second polymer barrier layer (22) and the second polypropylene layer (26). Retort-sterilizable sheet material (10) according to one of the preceding claims, characterized in that the sheet material (10) is formed of at least 95% by weight of polypropylene material. Retort-sterilizable sheet material (10) according to one of the preceding claims, characterized in that the sheet material (10) is free of metal foils.