Packaging foil

Abstract

The invention relates to a packaging foil comprising two aluminum foils which are adhesively bonded to one another over the full surface thereof by means of an adhesive. The aim of the invention is to obtain both an excellent dead-fold behavior and an excellent puncture resistance. This aim is achieved in that the adhesive forms, in the dried / hardened state, a layer which has a percentage elongation of at least 380%, preferably over 450% and particularly preferably over 500%. The adhesive preferably has an aqueous dispersion of a polymer based polyurethane, to which a thickening agent is added.

Description

[0001] Packaging film

[0002] The disclosure relates to a packaging film with pronounced dead-fold behavior for packaging objects, in particular foodstuffs.

[0003] Films used to wrap and package food products such as chocolate, including chocolate Easter bunnies, other figures, twisted candy wrappers, soup cubes, chocolate bars, chewing gum, or the like, must meet not only food safety regulations but also other requirements, both mechanical and, for example, optical. This includes, in particular, exhibiting pronounced dead-fold behavior, meaning that after folding or refolding, they retain the last imposed shape and do not elastically return to any previous shape.

[0004] The same applies, mutatis mutandis and, of course, even more so, to the packaging of pharmaceuticals and medical devices and instruments, such as ampoules and the like. Electronic components and precision mechanical items are also areas of application.

[0005] As an example of such films, reference is made to WO2021 / 018864 by the applicant. This document discloses a film consisting of a single layer of aluminum and a single layer of a lacquer. With such a film, the opening behavior can be significantly improved, particularly in wrap-around packaging, such as for chocolate, if the lacquer layer contains at least 49% by weight of a binder selected from the group consisting of polyurethane (PU), a thermoplastic elastomer based on polyurethane, or ethylene-vinyl acetate copolymer (EVA), polyvinyl chloride (PVC), or styrene-butadiene copolymer (SBS), and a maximum of 26% by weight of additives. For this purpose, it is also imperative that the packaging material exhibits a bond strength between the lacquer layer and the carrier layer in the range of 0.5–4 N per 15 mm.From WO2021 / 185731, the applicant discloses the surface bonding of two layers of aluminum with specific physical properties, which can be achieved through very specific heat treatments, using a pressure-sensitive adhesive, a laminating wax, a dry laminating adhesive, an exclusion lamination, or a mixed adhesive system. This packaging film is also suitable for packaging food and is intended, in particular, for wrapping (irregular) three-dimensional objects such as chocolate Easter bunnies, chocolate Santa Clauses, and the like, due to the excellent dead-fold behavior of this film.

[0006] Both of these film types fulfill their intended and desired purposes, but it has been found that both types of film exhibit poor puncture resistance. This means that when handling correspondingly wrapped items, the film is subjected to local mechanical stress and becomes perforated locally or at specific points, thus losing its integrity. Opening the packaging and "unwinding" the film, especially when it is applied to chocolate, also results in local tears and subsequent partial removal of the film from the product, making removal difficult and potentially associated with fingers becoming sticky from the chocolate.

[0007] Furthermore, there is a need for a film that can withstand the "twisting" desired in numerous applications, such as the packaging of individual candies or similar items. Current technology requires special and expensive films for this.

[0008] Similar stresses, which in the prior art can only be tolerated by special and expensive films, occur when sealing chocolate bars, for example, from whose otherwise flat surface nuts or, even more unpleasantly, pieces of broken nuts, etc., protrude, thus with relatively sharp edges and points. The pressure required during sealing and the resulting local stretching of the film cause perforations. In general, there is a need for a film that fits better than conventional films over edges, corners, and sharp-edged, pointed cereals or the like without tearing.

[0009] There is therefore a need for a film that combines the good dead-fold properties of the films described, is naturally food-safe, meets all optical, haptic, and other requirements, and also offers significantly improved perforation resistance compared to state-of-the-art films. All of this should be achieved without any technological or ecological problems.

[0010] It is the core of the present narrative to provide such a foil.

[0011] A film with such behavior is obtained by using two layers of aluminum, and by bonding these two layers together over their entire surface with an adhesive that, when dried / cured, forms a layer with an elongation (breaking elongation) of at least 380%, preferably over 450%, and particularly preferably over 500%, and whose adhesion to the aluminum surface is at least as strong as that of the two aforementioned film types. This ensures that the Al-adhesive-Al composite (which, as a composite, naturally has significantly lower elongation) achieves excellent resistance to tearing and perforation.

[0012] One possible composition for such an adhesive contains an aqueous dispersion of a polyurethane-based polymer. One such adhesive is marketed by BASF, for example, under the designation 093WTT107. However, it is crucial that a thickener is added to this dispersion / mixture, which in the prior art typically only serves to improve the application of the dispersion / mixture. It is quite surprising that the thickener also significantly improves the physical properties of the resulting layer. Furthermore, it is quite surprising that when using such a dispersion / mixture together with a thickener (short: "thickener"), the special heat treatment and mechanical processing of the aluminum foil are no longer critical, so that the complex treatment of the aluminum, as is sometimes necessary in the prior art, can be dispensed with—which is very commercially advantageous.

[0013] Regarding the thickener, it should be noted that an ASE associative thickener (i.e., an alkaline-swellable emulsion) based on an acrylic polymer is added to the above-described aqueous PU binder system. Such a thickener interacts with the water in this binder and not, or to a lesser extent, with the binders themselves. This removes unbound water from the system and increases the viscosity of the coating.

[0014] Of course, in the light of the disclosure of the subject description and the state of the art and the field of application, especially in cases where very specific properties are required, the aluminum of one or both of the foils can also be treated independently of one another using the methods described in the cited documents and especially also the methods described in the prior art cited therein, as is known to the person skilled in the art, and thus its properties can be modified and adapted.

[0015] In tests concerning the mechanical properties of the dispersion / mixture (hereinafter often simply referred to as “adhesive 4 The adhesive was applied to a corona-treated PET film or an OPP film (without aluminum foil(s)), dried at 100°C for 10 seconds to prevent deformation of the PET, and then left overnight, causing the adhesive film to lose its surface stickiness. The adhesive film was rubbed off the PET film at one corner with an eraser, whereupon it could be easily and easily peeled off by hand. The following values ​​were determined in the subsequent tensile test of the resulting film using a universal testing machine "inspekt table 5" from Hegewald & Peschke with an S2M / 200N force transducer: Adhesive alone (without thickener):

[0016] Grammage Thickness Tensile strength Elongation

[0017] [g / m2 ] [pm] [N / 15mm] [%]

[0018] 5.5 6 0.696 189.27

[0019] 7 7.5 0.914 241.32

[0020] 14.0 14 1.702 144.52*

[0021] * In this test, the application could not be carried out without bubbles, which led to cracks forming at the bubbles during the tensile test and thus to poorer elongation.

[0022] It is clear that these values, however good they are compared to the state of the art, are insufficient to solve the problems mentioned. This changes dramatically with the addition of a thickener: with thickener "A":

[0023] Grammage Thickness Tensile strength Elongation

[0024] [g / m 2 ] [pm] [N / 15mm] [%]

[0025] 5.5 6 0.556 496.826

[0026] 7 7.5 0.791 563.077

[0027] 14.0 14 1.702 530.548 with thickener “B”:

[0028] Grammage Thickness Tensile strength Elongation

[0029] [g / m 2] [pm] [N / 15mm] [%]

[0030] 5.5 6 0.458 492.685

[0031] 7 7.5 0.895 543.263

[0032] 14.0 14 1,863 509,074

[0033] Further properties of the adhesive used (often, and more correctly, called “binder”) are: pH value 8.0-9.0; dynamic viscosity at 23°C and 250 s ' 1 : 20-120MPa.s; contains between 39.0 and 41.0% solids according to DIN EN ISO 3251; has a density of 1030kg / m at 25°C 3 The glass transition temperature Tg (DSC) is -46°C. Thickener "A": It is a dispersion with an acidic environment, the pH is between 2 and 3, the solids content is about 40% (non-volatile), and the density (20°C) is about 1100 kg / m 3(DIN 51757), the viscosity at approximately 50 mPa.s (DIN EN ISO 2555(RV), spindle 1, 100 rpm, 23°C). An example is Rheovis®AS 1127 from BASF. A certain disadvantage of this composition is that a small amount of ammonia must be stirred in to establish the necessary basic environment.

[0034] Thickener "B": An aqueous solution of anionic acrylic polymers that, as mentioned, interact with water and not, or only to a limited extent, with the binders. BORCHI® Gel A LA is an example. It contains approximately 10% by weight of non-volatile components, has a pH of at least 8, and a density (20°C) of 1050 kg / m³. 3 (DIN 51757).

[0035] It is interesting to note that, for example, similar thickeners based on a zirconium complex such as BORCHI® Gel 0620 and thickeners based on HEUR, i.e. hydrophobically modified acrylate polymers such as BORCHI® Gel PN, do not produce satisfactory results.

[0036] The aluminum foils used can be made of soft or hard aluminum, independently of each other, and can have a thickness of 6 μm or more. The dead-fold behavior is excellent, as is the puncture behavior, and the problems of the state of the art mentioned above do not occur.

[0037] The adhesive can be applied in all methods known in film production; for the large quantities / areas that are usually used, roller application is the obvious choice.

[0038] It is easily possible to independently apply printing and / or a sealing layer to one or both sides of the packaging film after wrapping the item to be packaged. The fact that the two individual films are bonded together does not change the properties of the two (outer) surfaces of the packaging film. Due to the nature of the adhesive used, the film is easily recyclable.

Claims

Patent claims:

1. Packaging film, comprising two aluminum foils which are fully bonded to one another by means of an adhesive, characterized in that the adhesive, in the dried / cured state, forms a layer which in itself has an elongation of at least 380%, preferably of more than 450% and particularly preferably of more than 500%.

2. Packaging film according to claim 1, characterized in that the adhesive is an aqueous dispersion of a polymer based on polyurethane and that it contains a thickener, preferably an alkali-swellable emulsion, based on an acrylic polymer.

3. Packaging film according to claim 2, characterized in that the adhesive has a pH value between 8.0 and 9.0, that its dynamic viscosity at 23°C and 250 s ' 1: 20-120MPa.s, that it contains between 39.0 and 41.0% solids, measured according to DIN EN ISO 3251, that it has a density of 1030kg / m at 25°C 3 and that the glass transition temperature Tg (DSC) is -46°C.

4. Packaging film according to claim 2 or 3, characterized in that the thickener is present in the adhesive during application in an amount of 0.5% by weight to 8% by weight.

5. Packaging film according to one of claims 1 to 4, characterized in that the adhesive has a grammage of between 3g / m 2 and 18g / m 2 is present.

6. Packaging film according to one of claims 2 to 5, characterized in that the thickener is present as a dispersion with an acidic environment, that the pH value is between 2 and 3, that the solids content is about 40% (non-volatile), that the density (20°C) is about 1100kg / m 3(DIN 51757) and that - the viscosity is about 50mPa.s (DIN EN ISO 2555(RV), spindle 1, 100 rpm, 23 °C).

7. Packaging film according to one of claims 2 to 5, characterized in that - the thickener is an aqueous solution of anionic acrylic polymers, that it has about 10% by weight of non-volatile components, that it has a pH of at least 8 and that it has a density (20°C) of 1050 kg / m 3 (DIN 51757).