WO2009079676A1

WO2009079676A1 - Sterilisable, flexible composite foil

Info

Publication number: WO2009079676A1
Application number: PCT/AT2008/000459
Authority: WO
Inventors: Alfred Wegenberger; Robert Kitzberger; Martin Kornfeld
Original assignee: Teich Aktiengesellschaft
Priority date: 2007-12-20
Filing date: 2008-12-18
Publication date: 2009-07-02
Also published as: AT10572U1

Abstract

The invention relates to a sterilisable, flexible composite foil (1), comprising an aluminium foil (2), a coupling agent layer (3) and a sealing layer (4). According to the invention, the sealing layer (4) comprises a mixture of at least two polymers or copolymers that are incompatible with each other, wherein a sterilisable polypropylene polymer, polypropylene copolymer or polypropylene blend forms a matrix (5), in which at least one further incompatible polymer component (6) is embedded. Covers for closing containers comprising the composite foil (1) according to the invention are able to withstand sterilisation and at the same time are consumer-friendly, since the sealing layer can be pulled off from the rim of the container along a rupture line (7) by exerting little effort. In a variant according to the invention, the sterilisable polypropylene polymer, polypropylene copolymer or polypropylene blend is embedded in a matrix of polystyrene.

Description

Sterilizable, flexible composite foil

The invention relates to a sterilizable, flexible composite film containing an aluminum foil, a primer layer and a sealing layer and a method for producing this composite film. The invention further relates to the lid for closing containers comprising the aforementioned composite film.

It is known to use lids of the type mentioned, for example, for closing containers for perishable food or animal feed. Prior to sealing, the containers, which are usually made of polypropylene or a polypropylene inner layer, are filled with food or animal feed and then closed with the lid to form a sealed seam. In the process, the lids supplied in stacks are unstacked in a continuous process, placed on the edge of the container and frictionally connected thereto using a suitable sealing tool.

Furthermore, it is possible to use the composite film described above as masking tape, in which case the punching of the lid takes place only at the filling machine, usually after the sealing process.

In particular, in the case of perishable contents can be carried out for sterilization hot filling, in which the germ-free made by heating filling material has a temperature of for example 80-90 ⁰ C during filling. In the case of perishable food or pet food but can also be a thermal treatment of the filled and sealed containers in the form of pasteurization or sterilization.

With the sterilization, the contents can be preserved for a long time. This effect is achieved by a very high heating for a longer time than pasteurization.

While during pasteurization the contents are only heated for a few seconds, sterilization takes, for example, from about 15 minutes to one hour, depending on the product. The temperature is usually between 110 ⁰ C and 135 ⁰ C. Under such conditions, all bacteria and germs are to be killed, so that after the sterilization process of one million germs by definition, at most one germ may survive. This results in a very long shelf life, often over several years. _ ^ _

Standard used lidding material for such purposes consists for example of a composite comprising an aluminum foil and a coextrusion layer applied thereon. This usually contains a primer layer of maleic anhydride-modified polypropylene or another suitable adhesion promoter and a peelable sealing layer, which is formed from a filled with a mineral filler, such as talc or chalk, polypropylene or polypropylene blend. Using pressure and elevated temperature, a sealing of the lid and container rim takes place via the sealing layer. The filler in the sealing layer is intended essentially to ensure the necessary peelability for opening the containers by providing a partial internal weakening of the material which, on the one hand, has no effect on a tight container closure during sealing, but on the application of the tearing force (peel force) results in pleasant peeling for the consumer. As a result of the application of this force, a cohesive fracture generally occurs due to the sealing layer weakened by the fillers, so that it is possible to open the container. However, these conventionally used lids have significant disadvantages.

Basically, the described lid made of polypropylene-connected show a very tight seal, so that the manual opening is sometimes uncomfortable and, for example, difficulties for elderly people. The tearing force (peel force) required for opening can be reduced to some extent by increasing the filler content in the sealant layer or by adjusting the composition of the polypropylene blend accordingly. However, these modifications can cause a deterioration of the resistance to the contents or the sterilization resistance.

The cover described also have a limited resistance to certain Füllgutkomponenten, especially fats, fatty acids, lactic or acetic acid. This can be manifested in a defective seal seam strength, a poor peel resistance upon opening, a partial or complete delamination of partial layers or a partial or complete detachment of the entire coextrusion layer from the aluminum foil, in particular after prolonged contact with the contents. These disadvantages arise from migration of Füllgutkomponenten in the Layers of the coextrudate and weakening of these layers at the interfaces or cohesive weakening of individual or all layers, especially during the heat treatment, that is to say during pasteurization or sterilization. The adhesive layer at its interface with the aluminum foil is particularly sensitive. It is known that by applying an additional adhesion promoter layer to the aluminum surface to be coated or by a chemical surface treatment of the aluminum foil, eg a chromating of the aluminum surface, the adhesion-promoting boundary layer can be protected. For example, offer polyimide or epoxy paint protection against adhesive failure of the composite, especially in fats containing contents. Heat-sealing lacquers, for example based on vinyl chloride polymers, acrylates or epoxy resins, offer protection against salty contents or acidic contents, such as lactic acid or acetic acid, so that the contents can not reach the aluminum surface and so corrosion of the aluminum is avoided. However, these protective measures mentioned generally require an additional working step in a suitable coating machine in the production process. Furthermore, it is known from DE 102 35 584 A1 to use a relatively complex coextrusion layer with built-in barrier layer. This consists of a bonding agent layer which adjoins the aluminum foil, a barrier layer, for example of polyamide, and a further adhesion promoter layer, which is present between the barrier layer and the sealing layer. However, such structures have at least one additional material component compared with standard lid materials used, so that their preparation requires a costly expansion of existing coextrusion coating equipment, because an additional extruder, another feedblock to produce the multi-layer structure and possibly a modified extrusion tool by changing flow conditions are necessary ,

Object of the present invention is therefore to provide a composite film of the type described above, which can be produced with conventional extrusion equipment, and from which lid for closing containers for perishable goods can be made, which sterilization resistant, but at the same time consumer friendly and further at prolonged storage compared to acids and fats containing Füllgut- components are resistant. _ ^ _

According to the invention the object is achieved with a composite film of the aforementioned type, which is characterized in that the sealing layer contains a mixture of at least two mutually incompatible polymers or copolymers, wherein a sterilizable polypropylene polymer, polypropylene copolymer or polypropylene blend forms a matrix, in ¹ further comprising at least one, incompatible polymer component is embedded.

The polypropylene blend is preferably a blend of polypropylene and one or more of polyolefins and their copolymers, such as polyethylene, polybutylene, ethylene-acrylate copolymers, and the like.

As a further component, at least one incompatible with the matrix component polymer or copolymer from the group of incompatible plastics with polypropylene, preferably selected from the group of polystyrenes in the form of their individual components or blends.

The use of incompatible polymers or copolymers produces the technical effect that their blending does not result in complete mixtures in the thermodynamic sense and there are areas with reduced bond strength compared to the average, which, however, is the bond of the sealant layer to the adhesion promoter or to the container after closure only insignificantly influence, however, significantly reduce the scrubbing forces (peel forces) required for opening the container, without having to additionally admix a mineral filler component.

The effect is advantageously achieved in that complete miscibility of different polymers is given only in specific cases. However, since it is also necessary to produce stable and reproducible mixtures of their properties during the preparation of the polymer blend, the microstructure of which remains either the same during the processing steps, or the changes that are foreseeable, the mixing process requires, in addition to good mechanical dispersion preferably the use of a compatibilizer. The compatibilizer is advantageously to be selected in its structural chemical structure such that it contains chemical groups which are compatible with the matrix material and have further chemical groups which show compatibility with the mixing components. The compatibilizer therefore has three tasks to accomplish: _

1) reduction of the surface tension between the components to be mixed and thus the improvement of the dispersibility,

2) stabilization of the morphology against thermal effects and shear effects during processing and 3) ensuring adhesion between the phases in the solid state.

In the case of blending polypropylene, polypropylene copolymers and their blends with polystyrene, its copolymers and blends, styrene-butadienes are advantageous compatibilizers. The invention further relates to a process for producing the composite film according to the invention, wherein the adhesion promoter layer and the sealing layer are applied to the aluminum foil by coextrusion.

The 'invention further relates to a lid comprising the composite film according to the invention.

Depending on the enrichment of the mutually incompatible polymers in the sealing layer, the function of the matrix polymer is produced.

If the sterilizable polypropylene polymer, polypropylene copolymer or polypropylene blend is present in excess, the second subcompatible polymer, namely polystyrene, assumes the matrix function.

Further influencing factors for the structure formation are the viscosities of the individual polymers.

The invention therefore also relates to a sterilizable, flexible composite film of the aforementioned type, which is characterized in that the sealing layer contains a mixture of at least two mutually incompatible polymers or copolymers, wherein a sterilizable polypropylene polymer, polypropylene copolymer or polypropylene blend in a matrix of an incompatible Embedded polystyrene. The sterilizable polypropylene blend advantageously consists of a mixture of polypropylene and one or more materials from the group of polyolefins and their copolymers.

This variant according to the invention also advantageously contains in the sealing layer a compatibilizer, preferably a polystyrene-butadiene copolymer. The proportion of the compatibilizer also has an influence on the structure formation of the sealing layer.

A process for producing this sterilizable, flexible composite film takes place insofar as the adhesion promoter layer and the sealing layer by coextrusion onto the aluminum foil be applied.

The invention further relates to a lid comprising the composite film according to the invention, in which the sterilizable polypropylene polymer, polypropylene copolymer or polypropylene blend is embedded in a matrix of an incompatible polystyrene.

The invention will be explained in more detail below with reference to possible embodiments of the invention and with reference to FIGS. 1 and 2, wherein FIG. 1 shows the composite film 1 according to the invention with a matrix (5) made of the sterilizable polypropylene polymer, polypropylene copolymer or polypropylene blend into which at least another, incompatible polymer component (6) is embedded.

FIG. 2 shows the composite film 1 'according to the invention with a matrix (6 ") of incompatible polystyrene, in which the sterilisable polypropylene polymer, polypropylene copolymer or polypropylene blend (5') is embedded.

To produce the composite film 1,1 'according to the invention, an aluminum foil 2, 2' having a thickness of, for example, βOμm is fed to a system known per se, which is suitable for coextrusion coating. Before the actual coating takes place inline corona pretreatment of the aluminum surface to be coated, with no additional treatment of the aluminum surface, such as the order of a paint primer is required. The applied coextrusion coating consists of the adhesion promoter layer 3, 3 "and the sealing layer 4, 4. As adhesion promoter, for example, a polypropylene grafted with maleic anhydride is used, the application being controlled in such a way that a basis weight of about 8 g / m ^{2 is} achieved. The application of the sealing layer 4, 4 'is controlled in such a way that a weight per unit area of about 16 g / m ^{2 is} achieved The incompatible materials for the sealing layer 4 consist for example of a sterilisable blend of polypropylene and polyethylene, which forms the matrix 5.

As polymer incompatible with polypropylene 6, polystyrene is used according to the example. Furthermore, a compatibilizer, preferably a polystyrene-butadiene copolymer, is added.

An exemplary formulation is as follows: 48% by weight of PP-PE blend 47% by weight of polystyrene 5% by weight of polystyrene-butadiene copolymer as compatibilizer _ _η _

layered aluminum foil 2, 2 ¹ at the same system nor a floating dryer, in which by thermal activation, the adhesion of the coating is fixed on the aluminum foil. After the extrusion coating, the material present in the form of rolls can be stored or punched into lids by means of correspondingly shaped tools.

In a comparative experiment, a 60μm thick aluminum foil is prepared analogously to the aforementioned example, but with the difference that the sealing layer of a homogeneous polymer blend of polypropylene and polyethylene is formed with a mineral filler content of about 20 wt.%. Lids are also punched from this material.

To test the peelability, beaker seals are carried out against water-filled aluminum trays having a polypropylene inner layer on a laboratory sealer. For these investigations, on the one hand, the lids according to the invention made of the composite film produced according to the invention and, on the other hand, the lids from the previously described and known comparative material are used. The sealing takes place with a sealing time of 0.25 seconds at a sealing pressure of 4 bar and different sealing temperatures.

To check the sterilization resistance, the samples are sealed according to the conditions mentioned above and then subjected to sterilization in a laboratory autoclave at a temperature of 129 ° C for 30 minutes.

The tearing force, which is a measure of the peel strength or peelability of the lid, is tested with a universal tensile and pressure testing machine, type Zwick ZOlO. For this purpose, the sealed cups sterilized according to the above conditions are positioned in a suitable cup holder. The protruding corner of the sealed cup, the so-called lid flap, is provided with a clamp which is connected via a non-stretchable thread with the force transducer of the upper traverse of the tensile testing machine. The opening of the cups takes place at an angle of 45 °. At a strain rate of 400m / min, the force curve in [N] is recorded as the upper crosshead moves up. The maximum force value at the beginning of opening gives the Aufreißwert; the constant value over the further opening path gives the peel value. In the course of the comparative experiment, the following measured values were recorded: •

To investigate the product resistance is a known aggressive simulant mixture consisting of equal parts of hot ketchup, corn oil and Hesperidenessig produced. The containers used are typical deep-drawn aluminum trays with polypropylene inner film used for animal food products. These containers are filled with the simulant mixture and sealed on the one hand with the covers according to the invention described above and on the other hand with the covers of the known comparative material. The filled, sealed containers are stored so that the contents are in direct contact with the lid. The containers are stored both at room temperature as well as to accelerate the reaction in a heating cabinet at 4O ⁰ C and opened after 1,2 and 4 weeks, and in terms of a partial or complete failure of the composite by partial or complete detachment of the cover coating of the Aluminum foil or in view of a partial or complete delamination between adhesion promoter and sealing layer inspected.

While the standard material used according to the comparative example after a storage period of 4 weeks at room temperature or already after one week at elevated temperature distinct Delaminations in the boundary layer of aluminum and adhesive layer _g _

shows, the material according to the invention, which was stored at room temperature, shows no delamination in the investigation period. It was rated "in order" (i.O.). The material stored at elevated temperature according to the invention shows partial delamination in the form of strip detachment of the coating from the aluminum foil only after two weeks of storage.

Nevertheless, the tested in the comparative example, inventive lid are extremely stable, they also show consumer-friendly properties. Thus, the composite film 1 according to the invention shown in Figure 1 can be peeled (peeled) from the edge of the container when used as a lid for closing containers with little effort. This is shown by the break line 7 generated during opening. The formation of this fracture line is due to the use of incompatible polymers 5 and 6, thereby forming centers with reduced bond strength which do not affect the overall bond strength but facilitate the consumer's ability to close the sealed container by peeling the lid (peel) to open. The use of the polypropylene-containing matrix 5 further ensures adhesion to the adjoining adhesion promoter layer 3, so that, as shown in the comparative example, unwanted delamination does not occur even after prolonged storage.

This effect is also present when the proportion of PP in the mixture is increased. An exemplary formulation is as follows: 63% by weight PP-PE blend 32% by weight polystyrene 5% by weight styrene-butadiene compatibilizer

This formula achieves higher sealing forces than in the previous example. This may be necessary for long time spent on the packaged goods.

Since polystyrene is just as food-compatible as polypropylene, the following mixture was also prepared: 63% by weight of polystyrene 32% by weight of PP-PE blend

5% by weight styrene-butadiene compatibilizer

As described above, this mixture is applied to the aluminum foil 2 'in a coextrusion process together with the adhesion promoter 3'. By microscopic analysis, the formation of a matrix 6 'of polystyrene could also be observed, in which the incompatible PP polymer 5 'is embedded. This creates centers of reduced bond strength, so that when peeling a break line 7 'can be generated.

Claims

- H - Claims:

1. Sterilizable, flexible composite film (1), containing an aluminum foil (2), an adhesion promoter layer (3) and a sealing layer (4), characterized in that the sealing layer (4) is a mixture of at least two incompatible polymers or Contains copolymers, wherein a sterilizable polypropylene polymer, polypropylene copolymer or polypropylene blend forms a matrix (5) in which at least one further, incompatible polymer component (6) is embedded.

2. Composite film according to claim 1, characterized in that the polypropylene blend consists of a mixture of polypropylene and one or more materials from the group of polyolefins and their copolymers.

3. Composite film according to claim 1 or 2, characterized in that the polymer or copolymer incompatible with the matrix component is selected from the group of plastics incompatible with polypropylene, preferably from the group of polystyrenes in the form of their individual components or blends.

4. Composite film according to one of claims 1 to 3, characterized in that the sealing layer (4) additionally contains a compatibilizer.

5. Composite film according to claim 4, characterized in that the compatibilizer is a polystyrene-butadiene copolymer.

6. A method for producing a sterilizable, flexible composite film (1) according to one of claims 1 to 5, characterized in that the adhesion promoter layer (3) and the sealing layer (4) are applied to the aluminum foil (2) by coextrusion.

7. Lid for closing containers by means of a sealing seam, comprising a composite film (1) according to one of claims 1 to

5.

8. Sterilizable, flexible composite film (1'), containing an aluminum foil (2'), an adhesion promoter layer (3') and a sealing layer (4'), characterized in that the sealing layer (4') is a mixture of at least two mutually incompatible Polymers or copolymers, a sterilizable polypropylene polymer, polypropylene copolymer or polypropylene blend (5") being embedded in a matrix (6') made of an incompatible polystyrene.

9. Composite film according to claim 8, characterized in that Polypropylene blend consists of a mixture of polypropylene and one or more materials from the group of polyolefins and their copolymers.

10. Composite film according to claim 8 or 9, characterized in that the sealing layer (4 ') additionally contains a compatibilizer.

11. Composite film according to claim 10, characterized in that the compatibilizer is a polystyrene-butadiene copolymer.

12. A method for producing a sterilizable, flexible composite film (1') according to one of claims 8 to 11, characterized in that the adhesion promoter layer (3') and the sealing layer (4') are applied to the aluminum foil (2') by coextrusion. be applied.

13. Lid for closing containers by means of a sealing seam, comprising a composite film (1 ') according to one of claims 8 to 11.